WEBVTT 00:00:00.000 --> 00:00:01.940 align:middle line:90% [SQUEAKING] 00:00:01.940 --> 00:00:03.860 align:middle line:90% [RUSTLING] 00:00:03.860 --> 00:00:05.300 align:middle line:90% [CLICKING] 00:00:05.300 --> 00:00:10.850 align:middle line:90% 00:00:10.850 --> 00:00:12.600 align:middle line:84% PROFESSOR: So this class is for people who 00:00:12.600 --> 00:00:14.780 align:middle line:90% have no background in nuclear. 00:00:14.780 --> 00:00:16.680 align:middle line:84% And I think some of you are probably here 00:00:16.680 --> 00:00:19.200 align:middle line:84% and have a background in nuclear. 00:00:19.200 --> 00:00:21.220 align:middle line:84% So I apologize if you find this boring. 00:00:21.220 --> 00:00:22.840 align:middle line:90% You don't have to stay. 00:00:22.840 --> 00:00:25.440 align:middle line:84% If you see people leaving, that might be why. 00:00:25.440 --> 00:00:28.120 align:middle line:84% But we certainly have people in this class who 00:00:28.120 --> 00:00:34.360 align:middle line:84% are lawyers, who are young undergrads who have not yet 00:00:34.360 --> 00:00:39.840 align:middle line:84% taken 22.101, people who are joining 00:00:39.840 --> 00:00:41.840 align:middle line:84% nuclear engineering as grad students, 00:00:41.840 --> 00:00:44.740 align:middle line:84% but from a different discipline and don't really 00:00:44.740 --> 00:00:46.240 align:middle line:84% know what nuclear reactors do, maybe 00:00:46.240 --> 00:00:48.160 align:middle line:90% from chemistry or from physics. 00:00:48.160 --> 00:00:52.240 align:middle line:84% And so this class is to put everyone on the same page. 00:00:52.240 --> 00:00:54.560 align:middle line:84% The material that I'm going to cover in this class 00:00:54.560 --> 00:00:56.520 align:middle line:84% is not material that you need to know 00:00:56.520 --> 00:00:59.270 align:middle line:84% to do the rest of the class, but it's material 00:00:59.270 --> 00:01:04.069 align:middle line:84% that I hope will give you some kind of grounding so what 00:01:04.069 --> 00:01:06.390 align:middle line:90% the hell we're talking about. 00:01:06.390 --> 00:01:09.990 align:middle line:84% And so we're just going to start with how nuclear energy works 00:01:09.990 --> 00:01:13.370 align:middle line:84% and then talk a little bit about how reactors work. 00:01:13.370 --> 00:01:17.710 align:middle line:84% So we'll start with physics, reactors, and energy extraction, 00:01:17.710 --> 00:01:20.470 align:middle line:84% and fusion reactors, fission reactors, and then 00:01:20.470 --> 00:01:23.030 align:middle line:90% fusion reactors at the end. 00:01:23.030 --> 00:01:23.530 align:middle line:90% All right. 00:01:23.530 --> 00:01:26.190 align:middle line:84% Any questions about the paper or anything else before I get 00:01:26.190 --> 00:01:27.150 align:middle line:90% started? 00:01:27.150 --> 00:01:27.830 align:middle line:90% Super. 00:01:27.830 --> 00:01:28.410 align:middle line:90% Yes. 00:01:28.410 --> 00:01:31.530 align:middle line:84% STUDENT: Does it have to include some quantitative analysis? 00:01:31.530 --> 00:01:33.710 align:middle line:84% PROFESSOR: It does not have to include. 00:01:33.710 --> 00:01:36.910 align:middle line:84% However, I generally encourage it to include. 00:01:36.910 --> 00:01:40.910 align:middle line:84% And the reason I encourage it is because, as engineering 00:01:40.910 --> 00:01:43.310 align:middle line:84% students, most people are comfortable 00:01:43.310 --> 00:01:45.390 align:middle line:90% with quantitative methods. 00:01:45.390 --> 00:01:49.230 align:middle line:84% And so it's a way-- it's an easy toolkit that you have 00:01:49.230 --> 00:01:52.310 align:middle line:90% access to answer a question. 00:01:52.310 --> 00:01:54.550 align:middle line:84% And often you see if you can drill down something 00:01:54.550 --> 00:01:58.190 align:middle line:84% to something where you can do a calculation, that's nice. 00:01:58.190 --> 00:02:02.710 align:middle line:84% There are other acceptable methodologies, qualitative, 00:02:02.710 --> 00:02:05.430 align:middle line:90% political science, legal. 00:02:05.430 --> 00:02:06.810 align:middle line:90% All of those are acceptable. 00:02:06.810 --> 00:02:10.110 align:middle line:84% But if you are not well practiced in those methods, 00:02:10.110 --> 00:02:12.010 align:middle line:84% you are probably going to make a mistake. 00:02:12.010 --> 00:02:15.410 align:middle line:84% So I encourage quantitative, but it is not required. 00:02:15.410 --> 00:02:15.910 align:middle line:90% Yeah. 00:02:15.910 --> 00:02:19.390 align:middle line:90% 00:02:19.390 --> 00:02:25.590 align:middle line:84% So let's look at energy at the atomic scale. 00:02:25.590 --> 00:02:30.390 align:middle line:84% So energy is this measure of motive power 00:02:30.390 --> 00:02:32.710 align:middle line:90% that you can use to do things. 00:02:32.710 --> 00:02:38.110 align:middle line:84% If you took physics, you know of the unit of joule. 00:02:38.110 --> 00:02:42.310 align:middle line:84% But when we work at the scale of atoms, 00:02:42.310 --> 00:02:45.830 align:middle line:84% we prefer a different unit because the joule 00:02:45.830 --> 00:02:46.970 align:middle line:90% is just way too big. 00:02:46.970 --> 00:02:49.630 align:middle line:84% So we use this unit called the electron volt. 00:02:49.630 --> 00:02:53.390 align:middle line:84% And it is defined as the energy required 00:02:53.390 --> 00:02:58.140 align:middle line:84% to move one electron up 1 volt of potential. 00:02:58.140 --> 00:03:03.360 align:middle line:84% So if you would think in your AA battery, which is a 1.5 volt, 00:03:03.360 --> 00:03:05.900 align:middle line:84% and you need to move one electron from one electrode 00:03:05.900 --> 00:03:09.420 align:middle line:84% to the other electrode, the amount of energy it would take 00:03:09.420 --> 00:03:13.180 align:middle line:90% would be 1.5 electron volts. 00:03:13.180 --> 00:03:15.600 align:middle line:90% So that is the energy. 00:03:15.600 --> 00:03:23.300 align:middle line:84% Now, to put that in context, our macroscopic unit, the joule, 00:03:23.300 --> 00:03:26.820 align:middle line:84% is the energy to move one Coulomb. 00:03:26.820 --> 00:03:31.660 align:middle line:84% The Coulomb is a count, total number of electrons 00:03:31.660 --> 00:03:33.860 align:middle line:90% up 1 volt in potential. 00:03:33.860 --> 00:03:37.140 align:middle line:84% Wow, this thing is jiggling around. 00:03:37.140 --> 00:03:43.420 align:middle line:84% So it turns out the conversion is 1.6 times 10 to the minus 19. 00:03:43.420 --> 00:03:47.200 align:middle line:84% This is a very, very small amount of energy, 00:03:47.200 --> 00:03:50.240 align:middle line:84% and that's why we just, instead of having these huge exponents, 00:03:50.240 --> 00:03:53.142 align:middle line:90% we just use this unit. 00:03:53.142 --> 00:03:55.600 align:middle line:84% All right, so we'll talk about electron volts all the time. 00:03:55.600 --> 00:03:56.892 align:middle line:90% Electron volts, electron volts. 00:03:56.892 --> 00:03:59.100 align:middle line:90% You hear me say it a lot. 00:03:59.100 --> 00:04:00.820 align:middle line:84% This gives you a sense of the scale. 00:04:00.820 --> 00:04:01.320 align:middle line:90% All right. 00:04:01.320 --> 00:04:04.420 align:middle line:84% So let's talk a little bit about nomenclature around the atom. 00:04:04.420 --> 00:04:06.860 align:middle line:84% Many of you who come from physics domains 00:04:06.860 --> 00:04:08.820 align:middle line:90% will, of course, know this. 00:04:08.820 --> 00:04:11.140 align:middle line:84% You know that the nucleus is made 00:04:11.140 --> 00:04:13.300 align:middle line:84% of protons, which are positively charged, 00:04:13.300 --> 00:04:16.420 align:middle line:90% neutrons, which are neutral. 00:04:16.420 --> 00:04:20.500 align:middle line:84% And we call together these things nucleons. 00:04:20.500 --> 00:04:24.580 align:middle line:84% So when we count the total number of balls 00:04:24.580 --> 00:04:27.860 align:middle line:84% in the nucleus of an atom, we say they have so many nucleons 00:04:27.860 --> 00:04:30.480 align:middle line:84% and we add them all up, neutrons plus protons. 00:04:30.480 --> 00:04:32.860 align:middle line:90% We use the letter Z for protons. 00:04:32.860 --> 00:04:35.840 align:middle line:84% And we get a number that we call a mass number. 00:04:35.840 --> 00:04:39.600 align:middle line:90% So this is uranium 238. 00:04:39.600 --> 00:04:43.340 align:middle line:84% So named by the mass number, it means it has-- 00:04:43.340 --> 00:04:46.300 align:middle line:90% uranium means it has 92 protons. 00:04:46.300 --> 00:04:50.440 align:middle line:84% 238 means it has 238 protons and neutrons. 00:04:50.440 --> 00:04:53.190 align:middle line:84% And so what can change between other forms of uranium? 00:04:53.190 --> 00:04:56.230 align:middle line:84% The number of neutrons, this part. 00:04:56.230 --> 00:04:57.512 align:middle line:90% That can change. 00:04:57.512 --> 00:04:58.970 align:middle line:84% But we never really talk-- we never 00:04:58.970 --> 00:05:02.830 align:middle line:84% index atoms for whatever nuclei by the number of neutrons, 00:05:02.830 --> 00:05:04.770 align:middle line:90% which happens to be 146 here. 00:05:04.770 --> 00:05:06.410 align:middle line:84% But we just talk about it in terms 00:05:06.410 --> 00:05:08.710 align:middle line:84% of the number of protons, which is the element, 00:05:08.710 --> 00:05:13.010 align:middle line:84% and the number of nucleons, which gives us its weight. 00:05:13.010 --> 00:05:16.970 align:middle line:84% Roughly, a proton and a neutron weigh the same thing. 00:05:16.970 --> 00:05:19.230 align:middle line:90% So what are the protons doing? 00:05:19.230 --> 00:05:24.210 align:middle line:84% Well, they are contributing coulombic repulsive forces. 00:05:24.210 --> 00:05:28.970 align:middle line:84% This is electrostatic forces, just like what causes your hair 00:05:28.970 --> 00:05:30.390 align:middle line:90% to stand up on end. 00:05:30.390 --> 00:05:32.890 align:middle line:84% They are contributing something called the weak force, which 00:05:32.890 --> 00:05:35.490 align:middle line:84% we're not going to talk about, but has to do with beta decay. 00:05:35.490 --> 00:05:38.930 align:middle line:84% And both of these are contributing something 00:05:38.930 --> 00:05:40.027 align:middle line:90% called the strong force. 00:05:40.027 --> 00:05:41.610 align:middle line:84% And the strong force is what is really 00:05:41.610 --> 00:05:43.770 align:middle line:90% gluing the nucleus together. 00:05:43.770 --> 00:05:46.730 align:middle line:84% And so let's talk a little bit about the strong force. 00:05:46.730 --> 00:05:52.290 align:middle line:84% The strong force is a little weird. 00:05:52.290 --> 00:05:57.050 align:middle line:84% So this is a plot of the potential energy 00:05:57.050 --> 00:06:02.410 align:middle line:84% on the left versus distance between two nucleons. 00:06:02.410 --> 00:06:06.770 align:middle line:84% And this is a plot of the strong force. 00:06:06.770 --> 00:06:09.430 align:middle line:84% I didn't plot here the coulombic repulsion, 00:06:09.430 --> 00:06:10.770 align:middle line:90% which is also in play. 00:06:10.770 --> 00:06:13.890 align:middle line:84% Coulombic repulsion just goes as 1 over r squared. 00:06:13.890 --> 00:06:19.890 align:middle line:84% So we would see that this thing would have a kind of curve 00:06:19.890 --> 00:06:22.930 align:middle line:90% like that. 00:06:22.930 --> 00:06:27.970 align:middle line:84% The strong nuclear force doesn't just 00:06:27.970 --> 00:06:30.470 align:middle line:84% pull or push uniformly like the coulombic force. 00:06:30.470 --> 00:06:32.955 align:middle line:90% It has this minima here. 00:06:32.955 --> 00:06:34.330 align:middle line:84% So what this is saying is there's 00:06:34.330 --> 00:06:39.530 align:middle line:84% an energy minimum, which is to say a favorable plot right 00:06:39.530 --> 00:06:42.770 align:middle line:90% here below 1 fm. 00:06:42.770 --> 00:06:45.003 align:middle line:90% Does anyone know what an fm is? 00:06:45.003 --> 00:06:45.836 align:middle line:90% STUDENT: Femtometer? 00:06:45.836 --> 00:06:47.130 align:middle line:90% PROFESSOR: A femtometer. 00:06:47.130 --> 00:06:49.480 align:middle line:90% 10 to the minus 15 meters. 00:06:49.480 --> 00:06:50.493 align:middle line:90% So a very tiny distance. 00:06:50.493 --> 00:06:54.280 align:middle line:84% It's the approximate distance between nucleons. 00:06:54.280 --> 00:07:00.480 align:middle line:84% So you can see that as you go to shorter distances, suddenly, 00:07:00.480 --> 00:07:03.160 align:middle line:84% this force becomes repulsive, which 00:07:03.160 --> 00:07:07.480 align:middle line:84% is to say its potential energy becomes positive. 00:07:07.480 --> 00:07:09.860 align:middle line:84% And so if you try to cram these nucleons together, 00:07:09.860 --> 00:07:11.480 align:middle line:90% they push back. 00:07:11.480 --> 00:07:14.960 align:middle line:84% If you go to longer distances, it never becomes positive, 00:07:14.960 --> 00:07:18.020 align:middle line:84% but just falls off and becomes weaker, and weaker, and weaker. 00:07:18.020 --> 00:07:19.920 align:middle line:90% And it quickly approaches zero. 00:07:19.920 --> 00:07:22.040 align:middle line:84% It's right out here around 3 femtometers, 00:07:22.040 --> 00:07:24.040 align:middle line:84% which is not very far, smaller than the diameter 00:07:24.040 --> 00:07:26.280 align:middle line:90% of the nucleus. 00:07:26.280 --> 00:07:28.800 align:middle line:84% Already this thing is very close to zero. 00:07:28.800 --> 00:07:32.520 align:middle line:84% So typically, it falls off as 1 r to the fourth. 00:07:32.520 --> 00:07:36.040 align:middle line:84% And so the effect is that it doesn't just 00:07:36.040 --> 00:07:38.720 align:middle line:84% suck up other nucleons in the environment 00:07:38.720 --> 00:07:40.900 align:middle line:84% and build bigger and bigger molecules. 00:07:40.900 --> 00:07:45.160 align:middle line:84% It only works if you get the nucleon really, really 00:07:45.160 --> 00:07:46.760 align:middle line:84% close to another nucleon, and then it 00:07:46.760 --> 00:07:48.560 align:middle line:90% goes poof and sticks together. 00:07:48.560 --> 00:07:53.000 align:middle line:90% And that's an important feature. 00:07:53.000 --> 00:07:57.600 align:middle line:84% So you can see here this is why the average distance 00:07:57.600 --> 00:08:00.400 align:middle line:84% between nucleons in an atom will be 00:08:00.400 --> 00:08:04.360 align:middle line:90% around 0.8 femtometers in size. 00:08:04.360 --> 00:08:05.600 align:middle line:90% Now, we can compare this. 00:08:05.600 --> 00:08:07.600 align:middle line:84% You can see this is potential energy is measured 00:08:07.600 --> 00:08:10.040 align:middle line:90% in millions of electron volts. 00:08:10.040 --> 00:08:12.080 align:middle line:84% And you can see here that it's something 00:08:12.080 --> 00:08:14.180 align:middle line:90% around minus 100 million. 00:08:14.180 --> 00:08:16.800 align:middle line:84% That's means you need to add 100 million electron 00:08:16.800 --> 00:08:21.360 align:middle line:84% volts to get it out of this well to separate the two things. 00:08:21.360 --> 00:08:23.140 align:middle line:90% How big is the coulombic force? 00:08:23.140 --> 00:08:26.420 align:middle line:84% The coulombic force, remember, is electrostatic repulsion, say, 00:08:26.420 --> 00:08:29.240 align:middle line:90% between two protons. 00:08:29.240 --> 00:08:31.980 align:middle line:84% There's not really any more space, but you could do it. 00:08:31.980 --> 00:08:35.159 align:middle line:84% It's 1 over 4 pi epsilon naught if you remember your physics. 00:08:35.159 --> 00:08:39.240 align:middle line:84% You would integrate the force from infinity 00:08:39.240 --> 00:08:45.990 align:middle line:84% to zero as a function of the distance, where the force is 00:08:45.990 --> 00:08:53.110 align:middle line:84% equal to 1 over 4 pi epsilon naught, q1, q2, which 00:08:53.110 --> 00:08:55.870 align:middle line:90% are the charges in this case. 00:08:55.870 --> 00:08:58.950 align:middle line:84% So we can call this e2, the electron charge, 00:08:58.950 --> 00:09:00.750 align:middle line:90% over the distance squared. 00:09:00.750 --> 00:09:03.270 align:middle line:84% And if you do this integral, you'll 00:09:03.270 --> 00:09:07.830 align:middle line:84% find out that the repulsive energy that-- actually, we 00:09:07.830 --> 00:09:09.350 align:middle line:84% don't want to integrate to zero we 00:09:09.350 --> 00:09:21.230 align:middle line:84% want to integrate to, well, 0.8 fm. 00:09:21.230 --> 00:09:26.350 align:middle line:84% 8 times 10 to the minus 60 meters. 00:09:26.350 --> 00:09:31.350 align:middle line:84% If we do this integral, we'll find out that the repulsive 00:09:31.350 --> 00:09:39.990 align:middle line:84% force is about 1.4-ish MeV, which is 1% of the attractive 00:09:39.990 --> 00:09:42.110 align:middle line:90% force from the strong force. 00:09:42.110 --> 00:09:45.370 align:middle line:84% So that's why this attractive force glues it together. 00:09:45.370 --> 00:09:49.350 align:middle line:84% It overcomes the repulsive force from electrostatic charges 00:09:49.350 --> 00:09:51.630 align:middle line:90% from the two positive charges. 00:09:51.630 --> 00:09:59.310 align:middle line:84% So this is the picture in play for two nucleons. 00:09:59.310 --> 00:10:03.190 align:middle line:84% But it's actually a lot more complicated than this. 00:10:03.190 --> 00:10:05.710 align:middle line:84% There are a few other things that count here. 00:10:05.710 --> 00:10:08.070 align:middle line:90% There is something called spin. 00:10:08.070 --> 00:10:13.110 align:middle line:84% And these exclusion rules, which force 00:10:13.110 --> 00:10:14.870 align:middle line:84% every nucleon to have a different spin 00:10:14.870 --> 00:10:16.370 align:middle line:90% and a different energy state. 00:10:16.370 --> 00:10:21.570 align:middle line:84% But that is why nuclei have specific configurations. 00:10:21.570 --> 00:10:28.230 align:middle line:84% That's why you can't take one proton and 37 neutrons 00:10:28.230 --> 00:10:29.583 align:middle line:90% and stick them together. 00:10:29.583 --> 00:10:31.750 align:middle line:84% Despite the fact that the strong force would suggest 00:10:31.750 --> 00:10:33.750 align:middle line:84% they would all stick together, they 00:10:33.750 --> 00:10:37.670 align:middle line:84% won't because of all these other rules related to the spin. 00:10:37.670 --> 00:10:42.460 align:middle line:84% So yes, you can't make helium 39, if you will. 00:10:42.460 --> 00:10:47.780 align:middle line:84% So this is the table of isotopes. 00:10:47.780 --> 00:10:51.680 align:middle line:84% This is showing you along the bottom the number of protons. 00:10:51.680 --> 00:10:54.260 align:middle line:84% So in other words, this is the periodic table 00:10:54.260 --> 00:10:56.900 align:middle line:84% starting over here with hydrogen and going up 00:10:56.900 --> 00:10:58.700 align:middle line:90% through all the elements. 00:10:58.700 --> 00:11:01.820 align:middle line:84% 92 of course, is uranium-- number of protons. 00:11:01.820 --> 00:11:03.640 align:middle line:84% And this is the number of neutrons. 00:11:03.640 --> 00:11:05.640 align:middle line:90% We talked about this earlier. 00:11:05.640 --> 00:11:09.620 align:middle line:90% So it's the mass minus z number. 00:11:09.620 --> 00:11:11.900 align:middle line:84% And you'll see that these are basically the ones that 00:11:11.900 --> 00:11:13.320 align:middle line:90% are allowed by nature. 00:11:13.320 --> 00:11:16.140 align:middle line:90% 00:11:16.140 --> 00:11:19.460 align:middle line:84% They're color coded by their lifetime. 00:11:19.460 --> 00:11:24.460 align:middle line:84% So the light blue ones have lifetimes 00:11:24.460 --> 00:11:26.900 align:middle line:84% of around 10 nanoseconds, and they go all the way 00:11:26.900 --> 00:11:28.980 align:middle line:84% to black, which are stable, which means 00:11:28.980 --> 00:11:31.340 align:middle line:90% they do not radioactive decay. 00:11:31.340 --> 00:11:36.740 align:middle line:84% So this sort of suggests that these things could be created, 00:11:36.740 --> 00:11:39.900 align:middle line:84% but they would have such short lifetimes, 00:11:39.900 --> 00:11:41.560 align:middle line:84% they don't bother to color code them. 00:11:41.560 --> 00:11:43.620 align:middle line:84% They just instantly disintegrate. 00:11:43.620 --> 00:11:45.340 align:middle line:84% And then they become more and more stable 00:11:45.340 --> 00:11:47.080 align:middle line:90% as you get towards this line. 00:11:47.080 --> 00:11:51.220 align:middle line:90% 00:11:51.220 --> 00:11:54.650 align:middle line:84% So this is all related to that color force, color charge, spin, 00:11:54.650 --> 00:11:58.580 align:middle line:90% et cetera, and so on, and so on. 00:11:58.580 --> 00:12:03.620 align:middle line:84% Now, it's some combination of these. 00:12:03.620 --> 00:12:07.680 align:middle line:84% If it takes energy to assemble all these nuclei together, 00:12:07.680 --> 00:12:10.340 align:middle line:84% it also suggests that when we disassemble them, 00:12:10.340 --> 00:12:11.980 align:middle line:90% we get some energy back. 00:12:11.980 --> 00:12:15.900 align:middle line:84% And so you could ask the question, when one of these-- 00:12:15.900 --> 00:12:19.140 align:middle line:84% when one of these atoms decays, do 00:12:19.140 --> 00:12:21.300 align:middle line:90% we get useful energy out of it. 00:12:21.300 --> 00:12:22.280 align:middle line:90% And the answer is yes. 00:12:22.280 --> 00:12:25.960 align:middle line:84% We can get useful energy out of just radioactive decay. 00:12:25.960 --> 00:12:29.020 align:middle line:84% So here is a picture of plutonium 00:12:29.020 --> 00:12:34.300 align:middle line:84% 238, a form of plutonium that is kind of rare. 00:12:34.300 --> 00:12:38.930 align:middle line:84% But you can, through lots of effort, 00:12:38.930 --> 00:12:41.850 align:middle line:84% assemble macroscopic quantities of it. 00:12:41.850 --> 00:12:44.290 align:middle line:84% This thing has a half life of 87 years, which 00:12:44.290 --> 00:12:46.490 align:middle line:84% means half of the plutonium atoms 00:12:46.490 --> 00:12:49.790 align:middle line:84% undergo radioactive decay every 87 years. 00:12:49.790 --> 00:12:53.810 align:middle line:90% 00:12:53.810 --> 00:12:55.250 align:middle line:90% This is not in air. 00:12:55.250 --> 00:12:58.330 align:middle line:84% This is in either a vacuum environment 00:12:58.330 --> 00:13:01.550 align:middle line:84% or a low-pressure inert gas environment, 00:13:01.550 --> 00:13:04.130 align:middle line:84% because plutonium is extremely chemically reactive, 00:13:04.130 --> 00:13:06.290 align:middle line:90% thanks to its D orbitals. 00:13:06.290 --> 00:13:09.610 align:middle line:84% But you can see that it generates so much heat 00:13:09.610 --> 00:13:11.610 align:middle line:90% that it turns red hot. 00:13:11.610 --> 00:13:15.090 align:middle line:84% The natural color of this metal is silver, 00:13:15.090 --> 00:13:18.810 align:middle line:84% but it is so hot that it is just glowing 00:13:18.810 --> 00:13:20.770 align:middle line:90% because it is self-heating. 00:13:20.770 --> 00:13:23.770 align:middle line:84% So you can take a chunk of stuff that 00:13:23.770 --> 00:13:26.930 align:middle line:84% just from radioactive decay, produces a useful amount of heat 00:13:26.930 --> 00:13:28.990 align:middle line:84% that you could then extract at that. 00:13:28.990 --> 00:13:31.830 align:middle line:84% And we indeed use this isotope, plutonium 38, 00:13:31.830 --> 00:13:37.330 align:middle line:84% to power what we call RTGS, radio isotope thermoelectric 00:13:37.330 --> 00:13:41.690 align:middle line:84% generators, which are what power spacecraft, like the Voyager 00:13:41.690 --> 00:13:44.690 align:middle line:84% spacecraft that goes into deep space. 00:13:44.690 --> 00:13:46.330 align:middle line:84% There are also other elements that 00:13:46.330 --> 00:13:48.430 align:middle line:84% are less exotic than this one that do this. 00:13:48.430 --> 00:13:50.950 align:middle line:84% They don't typically produce as much energy. 00:13:50.950 --> 00:13:53.130 align:middle line:84% They don't necessarily glow red hot. 00:13:53.130 --> 00:13:56.250 align:middle line:84% But for example, strontium 90 and cesium 137 00:13:56.250 --> 00:13:59.570 align:middle line:84% were isotopes that were used by the Soviet Union 00:13:59.570 --> 00:14:02.070 align:middle line:90% to make these devices. 00:14:02.070 --> 00:14:03.650 align:middle line:84% You can see they have cooling fins 00:14:03.650 --> 00:14:07.930 align:middle line:84% and they are filled with radioactive stuff that 00:14:07.930 --> 00:14:11.210 align:middle line:84% makes heat, that then drives a thermocouple. 00:14:11.210 --> 00:14:15.330 align:middle line:84% And then that produces electricity. 00:14:15.330 --> 00:14:17.630 align:middle line:84% These were basically nuclear batteries. 00:14:17.630 --> 00:14:20.190 align:middle line:84% They had working lines of about 10 to 20 years, 00:14:20.190 --> 00:14:27.850 align:middle line:84% and they were used to power these lighthouses on the Arctic 00:14:27.850 --> 00:14:31.890 align:middle line:84% sea, where people basically couldn't live there 00:14:31.890 --> 00:14:33.620 align:middle line:90% because it was too remote. 00:14:33.620 --> 00:14:36.320 align:middle line:84% But they needed some kind of lighthouse 00:14:36.320 --> 00:14:39.960 align:middle line:84% to protect ships that were transiting in the region. 00:14:39.960 --> 00:14:42.400 align:middle line:84% The beta decay, which is the primary source of energy 00:14:42.400 --> 00:14:44.420 align:middle line:84% is, of course, all trapped by this metal casing. 00:14:44.420 --> 00:14:48.300 align:middle line:84% But it's the gammas that you have to worry about. 00:14:48.300 --> 00:14:54.040 align:middle line:84% So I said some isotopes, those marked in black, are stable. 00:14:54.040 --> 00:14:57.220 align:middle line:84% What I mean by stable means that they just will last forever. 00:14:57.220 --> 00:15:02.560 align:middle line:84% There's no natural perturbations of their nucleus, which 00:15:02.560 --> 00:15:06.320 align:middle line:84% will lead to the elements spontaneously decaying 00:15:06.320 --> 00:15:09.760 align:middle line:84% or the isotope spontaneously decaying into another isotope. 00:15:09.760 --> 00:15:14.440 align:middle line:84% But even so, some isotopes can be 00:15:14.440 --> 00:15:17.560 align:middle line:84% said to be more stable than others, 00:15:17.560 --> 00:15:21.420 align:middle line:84% which is to say they are further down that potential energy well. 00:15:21.420 --> 00:15:25.120 align:middle line:90% They're more stable. 00:15:25.120 --> 00:15:28.100 align:middle line:84% So they're more tightly bound together. 00:15:28.100 --> 00:15:31.000 align:middle line:84% So this gets us to this chart, which 00:15:31.000 --> 00:15:34.160 align:middle line:84% is called the curve of binding energy, which is also 00:15:34.160 --> 00:15:36.700 align:middle line:84% the name of a book by John McPhee, 00:15:36.700 --> 00:15:40.280 align:middle line:84% if you're interested, that has got 00:15:40.280 --> 00:15:44.260 align:middle line:84% interesting stuff about the history of the atomic age, 00:15:44.260 --> 00:15:45.360 align:middle line:90% if you are-- 00:15:45.360 --> 00:15:46.860 align:middle line:84% So let me just explain what this is. 00:15:46.860 --> 00:15:50.560 align:middle line:84% These are different isotopes now plotted. 00:15:50.560 --> 00:15:53.900 align:middle line:84% It's kind of mixed up because this is not element. 00:15:53.900 --> 00:15:58.280 align:middle line:84% This is now number of nucleons in the-- 00:15:58.280 --> 00:16:01.840 align:middle line:84% so you have different elements right next to each other. 00:16:01.840 --> 00:16:04.740 align:middle line:84% But typically, the lighter elements will be over here, 00:16:04.740 --> 00:16:07.600 align:middle line:84% and the heavier elements will be over here. 00:16:07.600 --> 00:16:11.420 align:middle line:84% And on the y-axis is the binding energy, 00:16:11.420 --> 00:16:15.560 align:middle line:84% which is how deep are you into the well of stability 00:16:15.560 --> 00:16:17.520 align:middle line:90% per nucleon. 00:16:17.520 --> 00:16:21.920 align:middle line:84% So you divide by the number of nucleons in the atom. 00:16:21.920 --> 00:16:25.200 align:middle line:84% And you'll notice this overall trend. 00:16:25.200 --> 00:16:28.240 align:middle line:84% When nuclei are very light or very heavy, 00:16:28.240 --> 00:16:32.470 align:middle line:84% the binding energy per nucleon is lower. 00:16:32.470 --> 00:16:36.750 align:middle line:84% And so those isotopes are not as tightly bound, if you will, 00:16:36.750 --> 00:16:38.450 align:middle line:90% as these here in the middle. 00:16:38.450 --> 00:16:44.590 align:middle line:84% So you see iron is basically as tight as it gets. 00:16:44.590 --> 00:16:47.390 align:middle line:84% And that's why everything decays to iron. 00:16:47.390 --> 00:16:50.630 align:middle line:84% And it's why the center of the Earth is full of iron, 00:16:50.630 --> 00:16:53.230 align:middle line:84% because there's nowhere else to go after you get iron. 00:16:53.230 --> 00:16:59.070 align:middle line:84% That is the most stable element that you can make. 00:16:59.070 --> 00:17:06.609 align:middle line:84% So for radioactive elements, the starting isotope is, of course, 00:17:06.609 --> 00:17:07.990 align:middle line:90% unstable. 00:17:07.990 --> 00:17:09.329 align:middle line:90% What does that mean? 00:17:09.329 --> 00:17:12.190 align:middle line:84% It just means that there's a simple transition. 00:17:12.190 --> 00:17:14.550 align:middle line:84% You see if I have a little-- yes, here I go. 00:17:14.550 --> 00:17:16.670 align:middle line:84% There's a simple transition in which 00:17:16.670 --> 00:17:18.430 align:middle line:84% it can do something with a nucleon 00:17:18.430 --> 00:17:21.990 align:middle line:84% and move up to a more stable state. 00:17:21.990 --> 00:17:23.869 align:middle line:90% That is radioactive decay. 00:17:23.869 --> 00:17:28.150 align:middle line:84% It's just like going from this state 00:17:28.150 --> 00:17:30.950 align:middle line:84% into a slightly more stable state. 00:17:30.950 --> 00:17:32.530 align:middle line:84% So let's attach some numbers to this. 00:17:32.530 --> 00:17:34.510 align:middle line:84% You can see the size of the change that 00:17:34.510 --> 00:17:37.630 align:middle line:84% is on the left are in units of MeV. 00:17:37.630 --> 00:17:41.710 align:middle line:84% And you can see that this little tiny change 00:17:41.710 --> 00:17:48.230 align:middle line:84% from radioactive decay was pretty small, much less 00:17:48.230 --> 00:17:51.950 align:middle line:84% than an MeV, maybe a tenth of an MeV or something like that. 00:17:51.950 --> 00:17:54.570 align:middle line:84% And that's kind of typical of radioactive decay. 00:17:54.570 --> 00:18:01.470 align:middle line:84% They're usually in the 100 KeV to 1 MeV range. 00:18:01.470 --> 00:18:03.130 align:middle line:90% That's, of course, per nucleon. 00:18:03.130 --> 00:18:06.910 align:middle line:84% So this tiny little change has to be 00:18:06.910 --> 00:18:09.450 align:middle line:90% multiplied by a factor of 120. 00:18:09.450 --> 00:18:12.630 align:middle line:84% So it's tiny, tiny little change, but then multiplied by-- 00:18:12.630 --> 00:18:15.670 align:middle line:84% I'm trying to get something of order MeV. 00:18:15.670 --> 00:18:22.390 align:middle line:84% Recall that 1 MeV is still 1.6 times 10 to the minus 13 joules. 00:18:22.390 --> 00:18:26.020 align:middle line:84% It's a very tiny amount of energy. 00:18:26.020 --> 00:18:31.900 align:middle line:84% You need 4 joules to heat about that much water one degree. 00:18:31.900 --> 00:18:37.740 align:middle line:84% So this is just a teensy tiny bit of energy. 00:18:37.740 --> 00:18:39.220 align:middle line:84% You need trillions of these things 00:18:39.220 --> 00:18:41.860 align:middle line:84% to heat this a quarter of a degree, 10 trillion 00:18:41.860 --> 00:18:44.820 align:middle line:84% to heat this amount of water a quarter of a degree. 00:18:44.820 --> 00:18:48.080 align:middle line:84% But the key point is that atoms, of course, 00:18:48.080 --> 00:18:50.240 align:middle line:90% come in large numbers. 00:18:50.240 --> 00:18:54.540 align:middle line:84% So you all know from high school that Avogadro's number is 6.022 00:18:54.540 --> 00:18:57.760 align:middle line:84% times 10 to the 23rd, so is a mole, 00:18:57.760 --> 00:19:02.340 align:middle line:84% which is kind of a reasonable number of atoms or this amount 00:19:02.340 --> 00:19:04.060 align:middle line:90% of atoms kind of thing. 00:19:04.060 --> 00:19:08.460 align:middle line:84% And so when you have a whole lot of these things decaying, 00:19:08.460 --> 00:19:10.540 align:middle line:84% you can indeed get useful quantities 00:19:10.540 --> 00:19:13.180 align:middle line:84% like that regular electric thermal generators I showed you 00:19:13.180 --> 00:19:16.220 align:middle line:90% on the previous page. 00:19:16.220 --> 00:19:17.060 align:middle line:90% All right. 00:19:17.060 --> 00:19:19.820 align:middle line:84% The problem with using radioactive decay 00:19:19.820 --> 00:19:25.540 align:middle line:84% as a source of energy is that you wait for this process 00:19:25.540 --> 00:19:27.500 align:middle line:90% to happen on its own. 00:19:27.500 --> 00:19:31.060 align:middle line:84% And each isotope, by force of nature 00:19:31.060 --> 00:19:34.500 align:middle line:84% has some time scale called the half life, 00:19:34.500 --> 00:19:36.720 align:middle line:84% which is going to tell you how fast this happens. 00:19:36.720 --> 00:19:41.100 align:middle line:84% And that's what you've got to work with, and that's it. 00:19:41.100 --> 00:19:43.320 align:middle line:84% So if you want to do something useful, 00:19:43.320 --> 00:19:45.160 align:middle line:84% we would like to have this happen, 00:19:45.160 --> 00:19:48.420 align:middle line:84% but we would like to stimulate it, do something to the atoms 00:19:48.420 --> 00:19:50.780 align:middle line:90% to control their release. 00:19:50.780 --> 00:19:53.920 align:middle line:84% And that gives us into these stimulated reactions, 00:19:53.920 --> 00:19:56.440 align:middle line:84% which is what nuclear engineering is about, 00:19:56.440 --> 00:19:58.460 align:middle line:84% figuring out how to make the nucleus do 00:19:58.460 --> 00:20:02.820 align:middle line:84% the things we want to do to extract this energy 00:20:02.820 --> 00:20:05.100 align:middle line:90% from the strong force. 00:20:05.100 --> 00:20:07.840 align:middle line:84% And that's what we're really going to look at. 00:20:07.840 --> 00:20:10.780 align:middle line:90% So how do we do this? 00:20:10.780 --> 00:20:11.320 align:middle line:90% All right. 00:20:11.320 --> 00:20:16.620 align:middle line:84% So we talked in the last class about the discovery 00:20:16.620 --> 00:20:19.260 align:middle line:84% of nuclear energy being tied to the question, what 00:20:19.260 --> 00:20:20.640 align:middle line:90% makes the stars burn? 00:20:20.640 --> 00:20:24.970 align:middle line:84% And indeed, that is one of the most useful reactions 00:20:24.970 --> 00:20:25.950 align:middle line:90% that we can have. 00:20:25.950 --> 00:20:30.410 align:middle line:84% So here's a very common fusion reaction 00:20:30.410 --> 00:20:32.810 align:middle line:84% where we're going to join together two 00:20:32.810 --> 00:20:36.290 align:middle line:90% nuclei to make a new nuclei. 00:20:36.290 --> 00:20:38.770 align:middle line:90% Therefore, it's fusion. 00:20:38.770 --> 00:20:41.770 align:middle line:84% And the ones we're going to choose for this example 00:20:41.770 --> 00:20:47.330 align:middle line:90% are hydrogen 2 and hydrogen 3. 00:20:47.330 --> 00:20:50.730 align:middle line:84% So hydrogen 2 means that it has one proton and one neutron, 00:20:50.730 --> 00:20:52.850 align:middle line:90% two nucleons, hydrogen 2. 00:20:52.850 --> 00:20:56.170 align:middle line:84% Hydrogen 3 is one proton and two neutrons. 00:20:56.170 --> 00:21:00.270 align:middle line:84% And these are also known as deuterium and tritium. 00:21:00.270 --> 00:21:02.630 align:middle line:84% Does anyone know what hydrogen 1 is called? 00:21:02.630 --> 00:21:05.330 align:middle line:90% 00:21:05.330 --> 00:21:06.530 align:middle line:90% STUDENT: Proton? 00:21:06.530 --> 00:21:07.030 align:middle line:90% No. 00:21:07.030 --> 00:21:08.238 align:middle line:90% PROFESSOR: You're very close. 00:21:08.238 --> 00:21:08.910 align:middle line:90% It is a proton. 00:21:08.910 --> 00:21:13.050 align:middle line:84% If it had no electron, it would just be a proton. 00:21:13.050 --> 00:21:15.030 align:middle line:84% STUDENT: That's why I tried to guess. 00:21:15.030 --> 00:21:17.730 align:middle line:90% PROFESSOR: You were very close. 00:21:17.730 --> 00:21:19.690 align:middle line:84% Actually, for some reason, we never 00:21:19.690 --> 00:21:22.330 align:middle line:84% tell students what hydrogen 1 is called. 00:21:22.330 --> 00:21:23.370 align:middle line:90% It's called protium. 00:21:23.370 --> 00:21:26.370 align:middle line:90% Useful to know. 00:21:26.370 --> 00:21:27.690 align:middle line:90% You'll run across it. 00:21:27.690 --> 00:21:32.650 align:middle line:84% Protium, deuterium and tritium are hydrogen 1, 2 and 3. 00:21:32.650 --> 00:21:34.510 align:middle line:90% So yes. 00:21:34.510 --> 00:21:37.950 align:middle line:84% So we won't use protium, but we'll use deuterium and tritium. 00:21:37.950 --> 00:21:39.750 align:middle line:84% Deuterium can be found in ocean water. 00:21:39.750 --> 00:21:42.450 align:middle line:90% Tritium is exotic. 00:21:42.450 --> 00:21:45.890 align:middle line:84% You have to make it in a nuclear reactor. 00:21:45.890 --> 00:21:51.850 align:middle line:84% But this is the reaction that powers 00:21:51.850 --> 00:21:56.570 align:middle line:84% virtually all practical modern fusion concepts. 00:21:56.570 --> 00:21:58.730 align:middle line:84% So when you put these together, you 00:21:58.730 --> 00:22:08.290 align:middle line:84% get two protons and three neutrons out. 00:22:08.290 --> 00:22:12.210 align:middle line:84% And so two protons means you get helium. 00:22:12.210 --> 00:22:15.350 align:middle line:84% And one of those neutrons will not bind. 00:22:15.350 --> 00:22:17.290 align:middle line:90% It comes out as a free neutron. 00:22:17.290 --> 00:22:21.920 align:middle line:84% And so you get helium 4 and free neutron. 00:22:21.920 --> 00:22:27.040 align:middle line:84% So let's just do a little quick calculation. 00:22:27.040 --> 00:22:32.180 align:middle line:90% So beforehand, we had-- 00:22:32.180 --> 00:22:32.680 align:middle line:90% let's see. 00:22:32.680 --> 00:22:34.480 align:middle line:90% We had one. 00:22:34.480 --> 00:22:36.900 align:middle line:84% Well, let's figure out how much energy out. 00:22:36.900 --> 00:22:40.160 align:middle line:84% So the binding energy of the final state 00:22:40.160 --> 00:22:42.740 align:middle line:84% minus the binding energy of the previous state. 00:22:42.740 --> 00:22:47.880 align:middle line:84% So the binding energy of the final state is helium 4. 00:22:47.880 --> 00:22:52.480 align:middle line:84% And if you look at the plot, it's 7 MeV per nucleon. 00:22:52.480 --> 00:22:56.960 align:middle line:84% So we have 7 times 4 because helium 4 has four nucleons. 00:22:56.960 --> 00:22:59.300 align:middle line:84% Plus what is the binding energy of a free neutron? 00:22:59.300 --> 00:23:02.040 align:middle line:90% 00:23:02.040 --> 00:23:07.640 align:middle line:90% Zero minus the input. 00:23:07.640 --> 00:23:08.700 align:middle line:90% That's not me moving. 00:23:08.700 --> 00:23:09.960 align:middle line:90% That is the projector moving. 00:23:09.960 --> 00:23:12.432 align:middle line:90% [LAUGHS] 00:23:12.432 --> 00:23:14.320 align:middle line:90% Minus the inputs. 00:23:14.320 --> 00:23:16.200 align:middle line:90% Whoa. 00:23:16.200 --> 00:23:17.580 align:middle line:90% It only happens when I talk. 00:23:17.580 --> 00:23:20.840 align:middle line:90% 00:23:20.840 --> 00:23:25.200 align:middle line:84% So we have one deuterium, which has a binding energy 00:23:25.200 --> 00:23:28.920 align:middle line:90% of about 1 MeV per nucleon. 00:23:28.920 --> 00:23:35.600 align:middle line:84% So 1 times 2 nucleons minus helium 3, 00:23:35.600 --> 00:23:39.960 align:middle line:84% which has a binding energy of about 2.8. 00:23:39.960 --> 00:23:43.620 align:middle line:90% 2.8 times 3 nucleons. 00:23:43.620 --> 00:23:45.620 align:middle line:84% Does anyone want to run that on a calculator? 00:23:45.620 --> 00:23:54.160 align:middle line:90% 00:23:54.160 --> 00:23:55.970 align:middle line:90% 7.4. 00:23:55.970 --> 00:24:00.960 align:middle line:84% 7 times 4, minus 2, minus 3, times 2.8. 00:24:00.960 --> 00:24:01.920 align:middle line:90% STUDENT: 17.6. 00:24:01.920 --> 00:24:03.200 align:middle line:90% PROFESSOR: 17.6. 00:24:03.200 --> 00:24:06.932 align:middle line:84% That is indeed the amount of energy you get from DT fusion. 00:24:06.932 --> 00:24:08.140 align:middle line:90% So that's where it come from. 00:24:08.140 --> 00:24:11.640 align:middle line:84% It's just this rearranging of the nucleons along this curve 00:24:11.640 --> 00:24:14.320 align:middle line:90% of binding energy. 00:24:14.320 --> 00:24:22.630 align:middle line:84% So 17.6 MeV, compared to approximately 1/10 to 1 MeV from 00:24:22.630 --> 00:24:24.110 align:middle line:90% radioactive decay. 00:24:24.110 --> 00:24:28.510 align:middle line:84% So we get a lot more energy from this stimulated reaction 00:24:28.510 --> 00:24:32.110 align:middle line:84% than we get if we just rely on radioactive decay. 00:24:32.110 --> 00:24:35.830 align:middle line:84% And we can control the rate, whereas radioactive decay, we're 00:24:35.830 --> 00:24:38.350 align:middle line:84% sitting around and saying half of them 00:24:38.350 --> 00:24:40.010 align:middle line:84% will decay in a certain period of time. 00:24:40.010 --> 00:24:42.270 align:middle line:90% Now we can just do our thing. 00:24:42.270 --> 00:24:47.230 align:middle line:84% So that is the magic of DT fusion. 00:24:47.230 --> 00:24:48.930 align:middle line:90% This is a reaction written out. 00:24:48.930 --> 00:24:49.930 align:middle line:90% There it is, the answer. 00:24:49.930 --> 00:24:53.070 align:middle line:90% Very good. 00:24:53.070 --> 00:24:55.230 align:middle line:90% Why is this hard? 00:24:55.230 --> 00:24:58.510 align:middle line:84% Well, remember the curve of-- remember that strong force fell 00:24:58.510 --> 00:25:02.750 align:middle line:90% to zero at around 3 Fermi? 00:25:02.750 --> 00:25:05.630 align:middle line:84% That's what has to stick these things together. 00:25:05.630 --> 00:25:08.190 align:middle line:84% In the meantime, we have the repulsive force 00:25:08.190 --> 00:25:09.550 align:middle line:90% from the protons. 00:25:09.550 --> 00:25:11.190 align:middle line:90% Like charges repel. 00:25:11.190 --> 00:25:13.850 align:middle line:84% And that is preventing this from happening. 00:25:13.850 --> 00:25:16.430 align:middle line:84% And that extends much further in space because it goes as 1 00:25:16.430 --> 00:25:18.150 align:middle line:90% over r squared. 00:25:18.150 --> 00:25:21.510 align:middle line:84% And so to overcome that, we have to speed these things up 00:25:21.510 --> 00:25:26.070 align:middle line:84% so fast that as the coulombic repulsion is slowing them down, 00:25:26.070 --> 00:25:29.510 align:middle line:84% it still keeps going to the point where it snaps together. 00:25:29.510 --> 00:25:34.030 align:middle line:84% And that requires temperatures of 10 million degrees. 00:25:34.030 --> 00:25:35.970 align:middle line:90% And that is why fusion is hard. 00:25:35.970 --> 00:25:39.350 align:middle line:84% We have to stably confine these molecules, 00:25:39.350 --> 00:25:42.350 align:middle line:84% get them close together, and heat them up 00:25:42.350 --> 00:25:44.190 align:middle line:90% to 10 million degrees. 00:25:44.190 --> 00:25:46.230 align:middle line:90% This is really annoying. 00:25:46.230 --> 00:25:51.270 align:middle line:90% And that is not easy to do. 00:25:51.270 --> 00:25:52.690 align:middle line:90% All right, so that's fusion. 00:25:52.690 --> 00:25:53.650 align:middle line:90% What about fission? 00:25:53.650 --> 00:25:56.310 align:middle line:84% What can you do with heavy nuclei? 00:25:56.310 --> 00:25:58.430 align:middle line:90% Story is a little bit different. 00:25:58.430 --> 00:26:01.350 align:middle line:84% Stimulated reactions to heavy nuclei, 00:26:01.350 --> 00:26:02.850 align:middle line:84% there are lots of them that exist. 00:26:02.850 --> 00:26:06.210 align:middle line:84% This is one example of what's called the n, 2n reaction. 00:26:06.210 --> 00:26:12.100 align:middle line:84% You shoot a neutron at a nucleus and it kicks off two neutrons. 00:26:12.100 --> 00:26:15.420 align:middle line:84% And so you unbind one neutron, and you get a change 00:26:15.420 --> 00:26:17.780 align:middle line:90% and you get some energy out. 00:26:17.780 --> 00:26:23.060 align:middle line:84% The problem with this is, when you do the math, what happens 00:26:23.060 --> 00:26:27.740 align:middle line:84% is you get basically the same as radioactive decay, fractions 00:26:27.740 --> 00:26:29.780 align:middle line:90% of an MeV, typically. 00:26:29.780 --> 00:26:34.380 align:middle line:84% So you can do this, but it's not buying you very much. 00:26:34.380 --> 00:26:36.940 align:middle line:84% And in order to make these high energy neutrons, to do this 00:26:36.940 --> 00:26:38.260 align:middle line:90% is not trivial. 00:26:38.260 --> 00:26:42.660 align:middle line:84% So it's not a net energy producer generally. 00:26:42.660 --> 00:26:46.300 align:middle line:84% However, for a few special nuclei, 00:26:46.300 --> 00:26:52.220 align:middle line:84% and indeed, only one nucleus found in nature, uranium 235, 00:26:52.220 --> 00:26:54.580 align:middle line:84% you can stimulate this weird reaction called 00:26:54.580 --> 00:26:57.340 align:middle line:84% a fission reaction, which is what is showing here. 00:26:57.340 --> 00:27:00.060 align:middle line:84% For some reason with this nucleus, 00:27:00.060 --> 00:27:03.300 align:middle line:84% when you shoot one neutron, the whole thing splits in half. 00:27:03.300 --> 00:27:07.660 align:middle line:84% And now when you do your calculation per nucleon, 00:27:07.660 --> 00:27:11.180 align:middle line:84% it turns out you get about 200 MeV out, 00:27:11.180 --> 00:27:13.700 align:middle line:90% which is a lot more energy. 00:27:13.700 --> 00:27:16.900 align:middle line:84% And so this is the reaction on which 00:27:16.900 --> 00:27:21.140 align:middle line:84% all fission, nuclear energy, all current working nuclear energy 00:27:21.140 --> 00:27:22.260 align:middle line:90% is based. 00:27:22.260 --> 00:27:26.420 align:middle line:84% And it exists strictly because this one isotope 00:27:26.420 --> 00:27:28.100 align:middle line:90% exists in nature. 00:27:28.100 --> 00:27:31.900 align:middle line:84% You could not do it if this isotope did not 00:27:31.900 --> 00:27:34.820 align:middle line:90% exist, uranium 235. 00:27:34.820 --> 00:27:37.680 align:middle line:84% So let's just briefly look at what's happening here. 00:27:37.680 --> 00:27:40.620 align:middle line:84% We're going from over there on the right. 00:27:40.620 --> 00:27:44.840 align:middle line:84% And the fission is random in where it lands. 00:27:44.840 --> 00:27:47.020 align:middle line:84% It breaks into different sized fragments and lands 00:27:47.020 --> 00:27:51.160 align:middle line:84% in different places, but roughly over there. 00:27:51.160 --> 00:27:54.620 align:middle line:84% And you see that the change in binding energy per nucleon 00:27:54.620 --> 00:27:59.720 align:middle line:84% is small, let's say, order 1 MeV per nucleon. 00:27:59.720 --> 00:28:05.190 align:middle line:84% But we have 200 nucleons, and so we get 200 MeV. 00:28:05.190 --> 00:28:06.940 align:middle line:84% And that's how you get a meaningful amount 00:28:06.940 --> 00:28:11.790 align:middle line:90% of energy out of fission. 00:28:11.790 --> 00:28:16.310 align:middle line:84% So a meaningful amount of energy still has quotes around it, 00:28:16.310 --> 00:28:23.550 align:middle line:84% because just to remind you, that 200 MeV is still 1/300 00:28:23.550 --> 00:28:27.210 align:middle line:84% trillionth of what you get from burning a matchstick. 00:28:27.210 --> 00:28:29.190 align:middle line:90% So it's tiny. 00:28:29.190 --> 00:28:34.850 align:middle line:84% So how do we get this up to macroscopic levels? 00:28:34.850 --> 00:28:38.970 align:middle line:84% Well, with a fusion reactor, we can figure out 00:28:38.970 --> 00:28:41.440 align:middle line:84% how much energy we want from our reaction, 00:28:41.440 --> 00:28:43.690 align:middle line:84% figure out how many moles of deuterium and tritium gas 00:28:43.690 --> 00:28:45.810 align:middle line:84% we need to pump into the reactor. 00:28:45.810 --> 00:28:48.890 align:middle line:84% You pump it in there, you heat it up, it goes poof, 00:28:48.890 --> 00:28:51.730 align:middle line:84% and it blows up, and it all reacts, you hope. 00:28:51.730 --> 00:28:55.690 align:middle line:84% And you get some energy out and everything is over. 00:28:55.690 --> 00:29:01.010 align:middle line:84% But you can't really do that with uranium because it's heavy, 00:29:01.010 --> 00:29:06.130 align:middle line:84% and not a gas and not easily moved around. 00:29:06.130 --> 00:29:10.890 align:middle line:84% And so you need to build a much more complicated device 00:29:10.890 --> 00:29:14.730 align:middle line:84% to control the rate of fission, if that's what you want to do. 00:29:14.730 --> 00:29:16.770 align:middle line:84% So in the next 10 minutes, we'll talk 00:29:16.770 --> 00:29:20.490 align:middle line:84% about how we control the rate of fission in reactors. 00:29:20.490 --> 00:29:23.330 align:middle line:84% Any questions up to this point regarding 00:29:23.330 --> 00:29:25.890 align:middle line:90% where the energy comes from? 00:29:25.890 --> 00:29:28.810 align:middle line:84% It's all related to the strong force. 00:29:28.810 --> 00:29:32.210 align:middle line:84% That's what you need to take away. 00:29:32.210 --> 00:29:35.250 align:middle line:84% So we use this concept called criticality, 00:29:35.250 --> 00:29:38.090 align:middle line:84% which I'm going to illustrate for you in a second. 00:29:38.090 --> 00:29:40.010 align:middle line:84% The key thing I want you to take away 00:29:40.010 --> 00:29:46.010 align:middle line:84% is, criticality is fundamentally a geometric consideration. 00:29:46.010 --> 00:29:49.850 align:middle line:84% We sometimes talk about a critical mass, 00:29:49.850 --> 00:29:54.110 align:middle line:84% but actually, the critical mass presumes a certain geometry. 00:29:54.110 --> 00:29:57.690 align:middle line:84% It presumes, usually, a spherical geometry. 00:29:57.690 --> 00:29:59.530 align:middle line:84% If you change the geometry or you 00:29:59.530 --> 00:30:01.590 align:middle line:84% put a neutron reflector or something else, 00:30:01.590 --> 00:30:04.040 align:middle line:90% the critical mass changes. 00:30:04.040 --> 00:30:08.200 align:middle line:84% The foundations of criticality are geometric, 00:30:08.200 --> 00:30:10.400 align:middle line:90% and you'll see why in a second. 00:30:10.400 --> 00:30:14.237 align:middle line:90% So Szilard had this. 00:30:14.237 --> 00:30:15.820 align:middle line:84% We saw in the last class Szilard said, 00:30:15.820 --> 00:30:17.920 align:middle line:84% I imagined if I was crossing the street 00:30:17.920 --> 00:30:21.300 align:middle line:84% and somehow one neutron could be released, 00:30:21.300 --> 00:30:23.760 align:middle line:84% it would do something that would generate 00:30:23.760 --> 00:30:28.100 align:middle line:84% two neutrons in the reaction, which would be a chain reaction. 00:30:28.100 --> 00:30:29.960 align:middle line:84% And he doesn't yet identify that it 00:30:29.960 --> 00:30:32.920 align:middle line:84% was uranium that is doing that, but that's 00:30:32.920 --> 00:30:34.520 align:middle line:90% the concept of a chain reaction. 00:30:34.520 --> 00:30:37.160 align:middle line:84% So here's how a chain reaction works. 00:30:37.160 --> 00:30:41.240 align:middle line:84% In a subcritical material, in fission-- 00:30:41.240 --> 00:30:42.780 align:middle line:90% I should probably go back here. 00:30:42.780 --> 00:30:45.840 align:middle line:90% 00:30:45.840 --> 00:30:48.520 align:middle line:84% The key feature of fission is-- we 00:30:48.520 --> 00:30:51.560 align:middle line:84% talked about this was Frederick Galileo's discovery. 00:30:51.560 --> 00:30:55.600 align:middle line:84% One neutron comes in, fissions, and more than one neutron 00:30:55.600 --> 00:30:56.800 align:middle line:90% comes out. 00:30:56.800 --> 00:30:59.320 align:middle line:84% That's what Frederick Julio published 00:30:59.320 --> 00:31:02.400 align:middle line:84% on the dawn of World War II that showed that the chain 00:31:02.400 --> 00:31:04.620 align:middle line:90% reaction was possible. 00:31:04.620 --> 00:31:09.440 align:middle line:84% So we will take-- we will watch as more than one fission-- 00:31:09.440 --> 00:31:11.920 align:middle line:84% more than one neutron comes out of fission. 00:31:11.920 --> 00:31:13.140 align:middle line:90% So let's run the simulation. 00:31:13.140 --> 00:31:13.920 align:middle line:90% Here we go. 00:31:13.920 --> 00:31:17.060 align:middle line:84% You see a neutron flies in, it fissions, 00:31:17.060 --> 00:31:20.040 align:middle line:84% it gives three neutrons, it causes more fissions, 00:31:20.040 --> 00:31:22.520 align:middle line:90% but it stops. 00:31:22.520 --> 00:31:26.680 align:middle line:84% Because eventually, the neutrons leak out. 00:31:26.680 --> 00:31:29.280 align:middle line:90% This is the subcritical state. 00:31:29.280 --> 00:31:34.000 align:middle line:84% It means that on average, fewer than one neutrons stays within 00:31:34.000 --> 00:31:37.280 align:middle line:84% the volume, and therefore, the chain reaction cannot be 00:31:37.280 --> 00:31:37.860 align:middle line:90% sustained. 00:31:37.860 --> 00:31:40.160 align:middle line:90% So it is subcritical. 00:31:40.160 --> 00:31:42.120 align:middle line:84% So we'll do the same simulation, but we'll just 00:31:42.120 --> 00:31:43.240 align:middle line:90% choose a larger mass. 00:31:43.240 --> 00:31:44.660 align:middle line:90% And you'll see what happens. 00:31:44.660 --> 00:31:48.160 align:middle line:90% 00:31:48.160 --> 00:31:50.760 align:middle line:90% It just takes off. 00:31:50.760 --> 00:31:54.000 align:middle line:84% And so that is the supercritical state. 00:31:54.000 --> 00:31:58.240 align:middle line:84% So in a reactor, we have to at least assemble 00:31:58.240 --> 00:32:02.990 align:middle line:84% a supercritical amount in order for the reactor to work at all. 00:32:02.990 --> 00:32:05.630 align:middle line:84% And eventually, what we want is for it 00:32:05.630 --> 00:32:10.950 align:middle line:84% to grow until we have a certain rate of fission reactions, 00:32:10.950 --> 00:32:13.645 align:middle line:84% and then we actually don't want it to keep growing. 00:32:13.645 --> 00:32:15.270 align:middle line:84% And so we're going to talk a little bit 00:32:15.270 --> 00:32:17.850 align:middle line:90% about that in a moment. 00:32:17.850 --> 00:32:20.630 align:middle line:90% 00:32:20.630 --> 00:32:23.550 align:middle line:84% You can see easily that if you don't do anything 00:32:23.550 --> 00:32:27.750 align:middle line:84% to prevent it from continuing this expansion, 00:32:27.750 --> 00:32:31.190 align:middle line:84% that the energy production just scales exponentially. 00:32:31.190 --> 00:32:34.110 align:middle line:84% And that's how you make a nuclear bomb. 00:32:34.110 --> 00:32:36.190 align:middle line:84% It's a simple version of a reactor. 00:32:36.190 --> 00:32:39.790 align:middle line:90% And so that's why it goes boom. 00:32:39.790 --> 00:32:40.370 align:middle line:90% All right. 00:32:40.370 --> 00:32:44.150 align:middle line:84% So how fast does this process grow? 00:32:44.150 --> 00:32:49.590 align:middle line:84% So in solid uranium metal, which is an important caveat here, 00:32:49.590 --> 00:32:55.270 align:middle line:84% the time between these fissions is about 10 nanoseconds. 00:32:55.270 --> 00:32:58.510 align:middle line:84% During the Manhattan Project, they 00:32:58.510 --> 00:33:02.990 align:middle line:84% discovered all of these fundamental measures of fission, 00:33:02.990 --> 00:33:05.350 align:middle line:84% and they had to give all these things names. 00:33:05.350 --> 00:33:09.030 align:middle line:90% And so this is called a shake. 00:33:09.030 --> 00:33:11.910 align:middle line:84% It's as long as the shake of a lamb's tail. 00:33:11.910 --> 00:33:13.550 align:middle line:90% That's where it comes from. 00:33:13.550 --> 00:33:18.270 align:middle line:84% And the probability of a certain reaction 00:33:18.270 --> 00:33:20.390 align:middle line:84% is measured by the cross section, 00:33:20.390 --> 00:33:24.270 align:middle line:84% which is a measure in units of square area. 00:33:24.270 --> 00:33:28.350 align:middle line:84% And the unit is called a barn because it's as big as a barn. 00:33:28.350 --> 00:33:30.630 align:middle line:90% It's the square area of a barn. 00:33:30.630 --> 00:33:36.510 align:middle line:84% And so all of these things are named after farm nomenclature. 00:33:36.510 --> 00:33:42.950 align:middle line:84% And the Los Alamos lab was called The Ranch. 00:33:42.950 --> 00:33:47.670 align:middle line:84% So this is all leftovers from the Manhattan Project. 00:33:47.670 --> 00:33:48.170 align:middle line:90% All right. 00:33:48.170 --> 00:33:53.150 align:middle line:84% So we have one shake, which is 10 to the minus 8 seconds, 00:33:53.150 --> 00:33:56.980 align:middle line:84% or 10 nanoseconds is the time in between. 00:33:56.980 --> 00:33:59.460 align:middle line:84% And you can say, well, how long does 00:33:59.460 --> 00:34:07.380 align:middle line:84% it take to get to, say, 10 to the 14 neutrons fissions, 00:34:07.380 --> 00:34:10.300 align:middle line:84% at a rate of 10 to the 14 neutrons happening 00:34:10.300 --> 00:34:14.380 align:middle line:84% simultaneously, which is what you would need if you take-- 00:34:14.380 --> 00:34:16.820 align:middle line:90% the energy you require is-- 00:34:16.820 --> 00:34:21.580 align:middle line:84% I think I've skipped over something in a previous slide. 00:34:21.580 --> 00:34:28.185 align:middle line:84% The energy you would require is something like 10 to the 20, 00:34:28.185 --> 00:34:31.840 align:middle line:84% 10 to the 22 fissions per second. 00:34:31.840 --> 00:34:36.880 align:middle line:84% But every fission takes 10 nanoseconds. 00:34:36.880 --> 00:34:39.560 align:middle line:84% So when you do the math, you want simultaneously, 00:34:39.560 --> 00:34:44.300 align:middle line:84% at any moment, there would be 10 to the approximately 14 00:34:44.300 --> 00:34:45.219 align:middle line:90% fissions. 00:34:45.219 --> 00:34:48.500 align:middle line:84% And then if you say, I need 10 to the 14 fissions, 00:34:48.500 --> 00:34:53.199 align:middle line:84% and I go up by, let's say, 2 and 1/2 per generation, 00:34:53.199 --> 00:34:57.020 align:middle line:84% because I get about 2 and 1/2 neutrons out of the re-fission, 00:34:57.020 --> 00:35:01.840 align:middle line:84% then I can say I have 2 and 1/2 to the n. 00:35:01.840 --> 00:35:10.500 align:middle line:90% 00:35:10.500 --> 00:35:12.420 align:middle line:90% And I can say let's solve for n. 00:35:12.420 --> 00:35:15.660 align:middle line:84% And if you solve for n, you get, what, log-- 00:35:15.660 --> 00:35:16.440 align:middle line:90% what is this? 00:35:16.440 --> 00:35:19.922 align:middle line:90% 00:35:19.922 --> 00:35:22.960 align:middle line:84% You go over log of 2.5 down here. 00:35:22.960 --> 00:35:27.500 align:middle line:90% And this is log of 10 to the 14. 00:35:27.500 --> 00:35:28.820 align:middle line:90% You can figure this out. 00:35:28.820 --> 00:35:36.380 align:middle line:90% And you find that n is about 35. 00:35:36.380 --> 00:35:42.660 align:middle line:84% So 35 generations gets you to the power of your reactor. 00:35:42.660 --> 00:35:46.820 align:middle line:84% And every generation is 10 nanoseconds. 00:35:46.820 --> 00:35:52.180 align:middle line:84% So 35 times 10 nanoseconds is about a third of a microsecond. 00:35:52.180 --> 00:35:55.050 align:middle line:84% So the reactor reaches full power 00:35:55.050 --> 00:35:58.470 align:middle line:90% in a third of a microsecond. 00:35:58.470 --> 00:36:00.590 align:middle line:84% Again, you can see why it's easy to make a bomb. 00:36:00.590 --> 00:36:02.690 align:middle line:84% And now we ask the question, how in the world 00:36:02.690 --> 00:36:07.370 align:middle line:84% do we control this thing in a practical nuclear reactor 00:36:07.370 --> 00:36:09.530 align:middle line:84% if you have a third of a microsecond 00:36:09.530 --> 00:36:12.310 align:middle line:84% to put the brakes on, to change the geometry, 00:36:12.310 --> 00:36:15.690 align:middle line:84% or do something so that this won't continue to grow? 00:36:15.690 --> 00:36:18.090 align:middle line:90% So it's hard. 00:36:18.090 --> 00:36:19.850 align:middle line:84% And the magic comes from something 00:36:19.850 --> 00:36:23.010 align:middle line:90% called delayed neutrons. 00:36:23.010 --> 00:36:28.850 align:middle line:84% So it is true that you get 2 and 1/2 neutrons on average coming 00:36:28.850 --> 00:36:30.210 align:middle line:90% out of fission. 00:36:30.210 --> 00:36:35.050 align:middle line:84% But a very small fraction, about 0.6% of those neutrons 00:36:35.050 --> 00:36:39.490 align:middle line:84% don't come directly from the fissioning nucleus, 00:36:39.490 --> 00:36:44.370 align:middle line:84% but show up later because of radioactive decay. 00:36:44.370 --> 00:36:48.250 align:middle line:84% And let me actually be a little bit more specific. 00:36:48.250 --> 00:36:50.810 align:middle line:84% So let's go back here to the table of isotopes. 00:36:50.810 --> 00:36:52.650 align:middle line:90% So we start here. 00:36:52.650 --> 00:36:55.050 align:middle line:84% I don't have a laser pointer, but I'll 00:36:55.050 --> 00:36:56.290 align:middle line:90% use the shadow of a shadow. 00:36:56.290 --> 00:37:02.770 align:middle line:84% So we start here around uranium is right around here, 00:37:02.770 --> 00:37:04.250 align:middle line:90% maybe that red one. 00:37:04.250 --> 00:37:05.830 align:middle line:90% And it's going to split in half. 00:37:05.830 --> 00:37:09.010 align:middle line:84% Conservation of mass requires the number of protons 00:37:09.010 --> 00:37:12.290 align:middle line:84% and neutrons to be conserved, so we wind up with some spots 00:37:12.290 --> 00:37:13.570 align:middle line:90% down here and here. 00:37:13.570 --> 00:37:16.130 align:middle line:84% Probabilistically, you're probably not going 00:37:16.130 --> 00:37:18.450 align:middle line:90% to land on these blacks. 00:37:18.450 --> 00:37:21.730 align:middle line:84% Probabilistically, you're going to land on a blue. 00:37:21.730 --> 00:37:25.810 align:middle line:84% And that means there will be some additional radioactive 00:37:25.810 --> 00:37:28.730 align:middle line:90% decays after fission. 00:37:28.730 --> 00:37:34.770 align:middle line:84% And some of those decays will just by chance issue neutrons. 00:37:34.770 --> 00:37:38.370 align:middle line:84% And those neutrons are called delayed neutrons. 00:37:38.370 --> 00:37:40.710 align:middle line:84% So what you do is you design your reactor 00:37:40.710 --> 00:37:43.970 align:middle line:90% so it's actually subcritical. 00:37:43.970 --> 00:37:46.570 align:middle line:90% We call it prompt subcritical. 00:37:46.570 --> 00:37:48.310 align:middle line:84% It will not sustain the reaction. 00:37:48.310 --> 00:37:51.660 align:middle line:84% It's like that smaller green ball that stops, 00:37:51.660 --> 00:37:55.480 align:middle line:84% but then this radioactive decay happens 00:37:55.480 --> 00:37:59.400 align:middle line:84% because one of these fragments is on a light blue square. 00:37:59.400 --> 00:38:01.995 align:middle line:84% And it gives another neutron, and it's been momentarily 00:38:01.995 --> 00:38:04.120 align:middle line:84% supercritical, and the reaction continues, and then 00:38:04.120 --> 00:38:06.137 align:middle line:90% it stops again. 00:38:06.137 --> 00:38:07.720 align:middle line:84% And that's actually what's controlling 00:38:07.720 --> 00:38:09.360 align:middle line:90% the rate of the reactor. 00:38:09.360 --> 00:38:15.920 align:middle line:84% It's the half life of all those little-- 00:38:15.920 --> 00:38:17.860 align:middle line:90% all these little blue squares. 00:38:17.860 --> 00:38:20.640 align:middle line:84% The half life of these things are 00:38:20.640 --> 00:38:24.520 align:middle line:84% what are controlling the speed, rather than the prompt growth 00:38:24.520 --> 00:38:25.580 align:middle line:90% of the reactor. 00:38:25.580 --> 00:38:27.320 align:middle line:84% Does that make sense to everyone? 00:38:27.320 --> 00:38:28.360 align:middle line:90% All right. 00:38:28.360 --> 00:38:30.680 align:middle line:84% So now you say, well, what is the half life? 00:38:30.680 --> 00:38:31.860 align:middle line:90% Actually, you have no idea. 00:38:31.860 --> 00:38:33.902 align:middle line:84% You have no idea what it's going to fission into. 00:38:33.902 --> 00:38:39.920 align:middle line:84% But on average, we find certain delayed neutrons fall 00:38:39.920 --> 00:38:41.360 align:middle line:90% into certain time bins. 00:38:41.360 --> 00:38:43.360 align:middle line:84% And that gives us a kind of average behavior 00:38:43.360 --> 00:38:45.000 align:middle line:90% that we can count on. 00:38:45.000 --> 00:38:47.327 align:middle line:84% Thanks to Avogadro's number being very large, 00:38:47.327 --> 00:38:49.660 align:middle line:84% we can average over a very large number of these things, 00:38:49.660 --> 00:38:51.520 align:middle line:90% and it's relatively predictable. 00:38:51.520 --> 00:38:56.760 align:middle line:84% But fundamentally, it's still a chaotic system 00:38:56.760 --> 00:39:00.480 align:middle line:84% because it just has to statistically work out 00:39:00.480 --> 00:39:04.560 align:middle line:84% in your favor, which is why you might have 00:39:04.560 --> 00:39:08.560 align:middle line:84% a reactor start to have an energy excursion 00:39:08.560 --> 00:39:10.480 align:middle line:90% and start to take off. 00:39:10.480 --> 00:39:13.080 align:middle line:84% And you have to do something to stop that, otherwise you 00:39:13.080 --> 00:39:14.640 align:middle line:90% have a meltdown. 00:39:14.640 --> 00:39:19.293 align:middle line:84% And so this is why reactors have to be dynamically controlled 00:39:19.293 --> 00:39:20.960 align:middle line:84% in order to prevent that from happening, 00:39:20.960 --> 00:39:22.668 align:middle line:84% because it's fundamentally still chaotic. 00:39:22.668 --> 00:39:25.960 align:middle line:90% 00:39:25.960 --> 00:39:28.300 align:middle line:90% So that's the art. 00:39:28.300 --> 00:39:30.520 align:middle line:84% You balance the neutron generation 00:39:30.520 --> 00:39:34.440 align:middle line:84% just right, so that it is, in fact, a subcritical device. 00:39:34.440 --> 00:39:38.920 align:middle line:84% But then these decayed delayed neutrons momentarily 00:39:38.920 --> 00:39:41.240 align:middle line:84% move it into supercriticality, and the reaction 00:39:41.240 --> 00:39:42.880 align:middle line:90% proceeds forward. 00:39:42.880 --> 00:39:45.160 align:middle line:90% So how is that-- 00:39:45.160 --> 00:39:47.030 align:middle line:90% how is that done? 00:39:47.030 --> 00:39:50.590 align:middle line:84% Well, the generation rate is kind of 00:39:50.590 --> 00:39:55.150 align:middle line:84% fixed by the fuel, which is a solid amount of uranium 00:39:55.150 --> 00:39:58.150 align:middle line:90% or other fuel in there. 00:39:58.150 --> 00:40:03.710 align:middle line:84% The number of neutrons that continued to run 00:40:03.710 --> 00:40:07.765 align:middle line:84% is controlled by, principally, the loss, 00:40:07.765 --> 00:40:08.890 align:middle line:90% which we talked about here. 00:40:08.890 --> 00:40:12.550 align:middle line:84% So the loss is higher in a smaller geometry 00:40:12.550 --> 00:40:16.070 align:middle line:84% because the surface-to-volume ratio is higher than 00:40:16.070 --> 00:40:17.250 align:middle line:90% in a larger geometry. 00:40:17.250 --> 00:40:19.070 align:middle line:84% So we build the reactor so that it's 00:40:19.070 --> 00:40:21.470 align:middle line:84% called the leakage term is such that we 00:40:21.470 --> 00:40:23.750 align:middle line:90% are very close to criticality. 00:40:23.750 --> 00:40:29.790 align:middle line:84% And then we slightly adjust, tweak 00:40:29.790 --> 00:40:34.610 align:middle line:84% the level using something that absorbs excess neutrons. 00:40:34.610 --> 00:40:36.910 align:middle line:84% So we make it so there's a little extra neutrons. 00:40:36.910 --> 00:40:39.710 align:middle line:84% And then we do some we add some additional stuff that 00:40:39.710 --> 00:40:40.930 align:middle line:90% absorbs neutrons. 00:40:40.930 --> 00:40:43.580 align:middle line:84% We call these things control rods. 00:40:43.580 --> 00:40:47.420 align:middle line:84% And a common material, for example, is boron. 00:40:47.420 --> 00:40:50.100 align:middle line:84% So there's other things you can put in control rods that 00:40:50.100 --> 00:40:52.980 align:middle line:84% have different features, but boron is the principal thing 00:40:52.980 --> 00:40:55.577 align:middle line:84% that we use, because boron has a large neutron capture 00:40:55.577 --> 00:40:56.160 align:middle line:90% cross section. 00:40:56.160 --> 00:40:57.740 align:middle line:90% It wants to suck up neutrons. 00:40:57.740 --> 00:41:00.700 align:middle line:84% So we put that in there, and we use that to just fine tune 00:41:00.700 --> 00:41:01.360 align:middle line:90% the rate. 00:41:01.360 --> 00:41:03.950 align:middle line:90% 00:41:03.950 --> 00:41:08.900 align:middle line:84% So that brings us to the engineering of fission reactors. 00:41:08.900 --> 00:41:09.480 align:middle line:90% All right. 00:41:09.480 --> 00:41:16.060 align:middle line:84% So one of the things that we will hear about 00:41:16.060 --> 00:41:20.420 align:middle line:84% are something called thermal reactors versus fast reactors. 00:41:20.420 --> 00:41:23.460 align:middle line:84% So let me just explain what that means. 00:41:23.460 --> 00:41:27.980 align:middle line:84% As it turns out, when you fission a nucleus of uranium 00:41:27.980 --> 00:41:31.300 align:middle line:84% and it gives off of two to three neutrons, 00:41:31.300 --> 00:41:35.580 align:middle line:84% these neutrons come zipping out with a lot of speed 00:41:35.580 --> 00:41:38.340 align:middle line:84% because they are carrying the way the binding 00:41:38.340 --> 00:41:39.900 align:middle line:90% energy of the nucleus. 00:41:39.900 --> 00:41:42.740 align:middle line:84% That is part of where the energy goes. 00:41:42.740 --> 00:41:46.620 align:middle line:84% And on average, they're born with approximately 2 MeV 00:41:46.620 --> 00:41:51.760 align:middle line:84% of kinetic energy per neutron at the time in which they are born. 00:41:51.760 --> 00:41:54.740 align:middle line:90% Oh my gosh, this projector. 00:41:54.740 --> 00:41:56.900 align:middle line:90% And so that's over here. 00:41:56.900 --> 00:42:02.980 align:middle line:84% This is a plot of the energy of the neutrons. 00:42:02.980 --> 00:42:06.220 align:middle line:84% And you can see it goes all the way up here to 10 MeV, 00:42:06.220 --> 00:42:10.580 align:middle line:84% and it goes all the way down here to 10 00:42:10.580 --> 00:42:13.500 align:middle line:90% to the minus 5 electron volts. 00:42:13.500 --> 00:42:17.380 align:middle line:84% Very, very tiny, so many decades. 00:42:17.380 --> 00:42:22.100 align:middle line:84% This is approximately what we call thermal neutrons. 00:42:22.100 --> 00:42:27.220 align:middle line:84% So a fast reactor operates over here, 00:42:27.220 --> 00:42:32.140 align:middle line:84% and a thermal reactor, like most of the reactors in the world, 00:42:32.140 --> 00:42:35.420 align:middle line:84% operate with neutrons in this region. 00:42:35.420 --> 00:42:38.740 align:middle line:84% Why would you build a thermal reactor? 00:42:38.740 --> 00:42:43.170 align:middle line:84% Well, the principal reason is that this is the cross section 00:42:43.170 --> 00:42:46.170 align:middle line:90% in units of barns. 00:42:46.170 --> 00:42:50.250 align:middle line:84% And you can see that it is basically 00:42:50.250 --> 00:42:53.670 align:middle line:84% a measure of the probability that a reaction will happen. 00:42:53.670 --> 00:42:56.050 align:middle line:84% And you see that the probability of-- this 00:42:56.050 --> 00:42:57.750 align:middle line:90% is the probability of fission. 00:42:57.750 --> 00:43:00.210 align:middle line:84% This is the cross section for fission of uranium. 00:43:00.210 --> 00:43:03.130 align:middle line:84% So the probability of fission goes up 00:43:03.130 --> 00:43:07.210 align:middle line:84% by many orders of magnitude when you slow the neutron down. 00:43:07.210 --> 00:43:11.290 align:middle line:84% So that makes the reactor easier to design. 00:43:11.290 --> 00:43:13.530 align:middle line:84% But it also makes the reactor safer 00:43:13.530 --> 00:43:18.710 align:middle line:84% to design because these neutrons are born fast. 00:43:18.710 --> 00:43:22.630 align:middle line:84% But if they start colliding with things, they slow down. 00:43:22.630 --> 00:43:27.450 align:middle line:84% They transfer some kinetic energy to other atoms, 00:43:27.450 --> 00:43:28.990 align:middle line:90% and then they will slow down. 00:43:28.990 --> 00:43:31.690 align:middle line:84% And then the probability that the fission reaction happens 00:43:31.690 --> 00:43:32.770 align:middle line:90% goes up. 00:43:32.770 --> 00:43:35.550 align:middle line:84% So in order to prevent accidental fission, 00:43:35.550 --> 00:43:39.610 align:middle line:84% we actually require that they slow down. 00:43:39.610 --> 00:43:42.410 align:middle line:84% This process is called thermalization. 00:43:42.410 --> 00:43:46.810 align:middle line:84% They slow down so much that they have the same kinetic energy 00:43:46.810 --> 00:43:50.510 align:middle line:84% on average as the average of the molecules in this table? 00:43:50.510 --> 00:43:54.010 align:middle line:90% That is 0.02 eV. 00:43:54.010 --> 00:43:58.330 align:middle line:84% That is the room temperature energy per nucleon. 00:43:58.330 --> 00:44:01.130 align:middle line:84% And so they don't get any cooler. 00:44:01.130 --> 00:44:02.510 align:middle line:90% That's just where they land. 00:44:02.510 --> 00:44:04.052 align:middle line:84% And they kind of stay in that region, 00:44:04.052 --> 00:44:06.590 align:middle line:84% and we design the reactor to operate in that region. 00:44:06.590 --> 00:44:09.690 align:middle line:90% 00:44:09.690 --> 00:44:13.830 align:middle line:84% This is the cross section for uranium. 00:44:13.830 --> 00:44:16.330 align:middle line:84% And we see it has this feature where 00:44:16.330 --> 00:44:18.850 align:middle line:90% thermal reactivity goes up. 00:44:18.850 --> 00:44:21.310 align:middle line:84% But that's actually kind of a rare thing in nature. 00:44:21.310 --> 00:44:23.850 align:middle line:84% So just to show you how rare, here 00:44:23.850 --> 00:44:27.290 align:middle line:84% is the cross section for thorium. 00:44:27.290 --> 00:44:30.590 align:middle line:84% And you can see that it goes down as they thermalize. 00:44:30.590 --> 00:44:35.090 align:middle line:84% So you can't make a thorium reactor easily. 00:44:35.090 --> 00:44:38.360 align:middle line:84% You say, well, I've heard of thorium nuclear power. 00:44:38.360 --> 00:44:42.680 align:middle line:84% Does anyone know how thorium nuclear power works? 00:44:42.680 --> 00:44:45.640 align:middle line:84% You would change it into uranium. 00:44:45.640 --> 00:44:48.200 align:middle line:90% That's the magic of thorium. 00:44:48.200 --> 00:44:51.480 align:middle line:84% Thorium will, by itself, not actually produce a good reactor. 00:44:51.480 --> 00:44:53.592 align:middle line:84% You have to change it into uranium. 00:44:53.592 --> 00:44:55.800 align:middle line:84% You change it into a different kind of uranium called 00:44:55.800 --> 00:44:58.340 align:middle line:84% uranium 233 instead of uranium 235, 00:44:58.340 --> 00:45:01.852 align:middle line:84% but you still have to change it into uranium first. 00:45:01.852 --> 00:45:04.960 align:middle line:84% So thorium by itself does not make a reactor. 00:45:04.960 --> 00:45:07.940 align:middle line:84% And if we did not have uranium from nature, 00:45:07.940 --> 00:45:10.268 align:middle line:84% we would not be able to ever have built these. 00:45:10.268 --> 00:45:11.060 align:middle line:90% Here's another one. 00:45:11.060 --> 00:45:14.800 align:middle line:90% This is uranium 238. 00:45:14.800 --> 00:45:17.320 align:middle line:90% This is another kind of uranium. 00:45:17.320 --> 00:45:22.320 align:middle line:84% This is in fact, 99.3% of the uranium in the Earth 00:45:22.320 --> 00:45:24.840 align:middle line:90% is uranium 238. 00:45:24.840 --> 00:45:29.640 align:middle line:84% This is only 0.7% of uranium is uranium 235. 00:45:29.640 --> 00:45:33.680 align:middle line:84% So most of the stuff out there has this weird feature 00:45:33.680 --> 00:45:36.400 align:middle line:84% that the probability of fission is very low. 00:45:36.400 --> 00:45:39.120 align:middle line:84% It requires very high energy neutrons to do it. 00:45:39.120 --> 00:45:41.840 align:middle line:84% If the neutron is thermalized, you're out of luck. 00:45:41.840 --> 00:45:44.400 align:middle line:84% The reactor is very, very hard to build. 00:45:44.400 --> 00:45:48.720 align:middle line:84% But for 238 in particular, it has this feature where 00:45:48.720 --> 00:45:50.920 align:middle line:90% it goes in the other direction. 00:45:50.920 --> 00:45:53.540 align:middle line:84% So we can ask, well, that's great. 00:45:53.540 --> 00:45:56.600 align:middle line:90% How do we get uranium 238? 00:45:56.600 --> 00:45:59.240 align:middle line:84% Well, as I mentioned, natural uranium on Earth 00:45:59.240 --> 00:46:03.200 align:middle line:90% is only about 0.7% uranium 235. 00:46:03.200 --> 00:46:05.960 align:middle line:84% Sorry, a moment ago, I said, how do we get uranium 238? 00:46:05.960 --> 00:46:08.880 align:middle line:84% But I meant to say, how do you get uranium 235, the stuff that 00:46:08.880 --> 00:46:11.280 align:middle line:90% has this nice feature? 00:46:11.280 --> 00:46:15.000 align:middle line:84% So we have to get rid of the extra uranium 00:46:15.000 --> 00:46:19.480 align:middle line:84% 238, which we don't want, because it's a mix of these two 00:46:19.480 --> 00:46:21.800 align:middle line:90% isotopes in nature. 00:46:21.800 --> 00:46:26.000 align:middle line:84% And the only distinguishing feature between the two 00:46:26.000 --> 00:46:28.840 align:middle line:90% are their mass. 00:46:28.840 --> 00:46:31.420 align:middle line:84% And so chemically, they're the same. 00:46:31.420 --> 00:46:34.030 align:middle line:84% They're both uranium, the same number of protons, therefore 00:46:34.030 --> 00:46:37.150 align:middle line:84% the same number of electrons, therefore the same chemistry. 00:46:37.150 --> 00:46:40.090 align:middle line:84% So we have to find a way to, atom by atom, 00:46:40.090 --> 00:46:43.830 align:middle line:90% sort them out by their mass. 00:46:43.830 --> 00:46:45.807 align:middle line:84% And this is the process of uranium enrichment. 00:46:45.807 --> 00:46:47.390 align:middle line:84% We'll talk a little bit more about how 00:46:47.390 --> 00:46:51.790 align:middle line:84% this works in future class, and the process 00:46:51.790 --> 00:46:56.070 align:middle line:84% by which people like Iran is trying to make a nuclear bomb. 00:46:56.070 --> 00:47:00.950 align:middle line:84% They're doing something to get purified uranium 235. 00:47:00.950 --> 00:47:03.790 align:middle line:84% But the short of it is that you accelerate them. 00:47:03.790 --> 00:47:07.110 align:middle line:84% You spin them around, and the heavy ones go to the outside, 00:47:07.110 --> 00:47:09.890 align:middle line:84% and you throw that away because that's uranium 238. 00:47:09.890 --> 00:47:13.070 align:middle line:84% It's heavier, and you keep the others towards the center. 00:47:13.070 --> 00:47:16.890 align:middle line:90% Basically, that's the concept. 00:47:16.890 --> 00:47:19.190 align:middle line:84% But this gives us some categories. 00:47:19.190 --> 00:47:22.990 align:middle line:84% If you get just rid of some of the 238 00:47:22.990 --> 00:47:29.630 align:middle line:84% so that we can say increase the amount of 235 by a factor of 5, 00:47:29.630 --> 00:47:32.910 align:middle line:84% so we've actually gotten rid of quite a lot. 00:47:32.910 --> 00:47:37.438 align:middle line:84% The material is called low enriched uranium. 00:47:37.438 --> 00:47:39.730 align:middle line:84% This is actually most of the uranium used for reactors. 00:47:39.730 --> 00:47:41.550 align:middle line:90% This is good enough. 00:47:41.550 --> 00:47:46.510 align:middle line:84% Even though there's still a lot of 230-- 00:47:46.510 --> 00:47:53.270 align:middle line:84% even though the fuel is still 95% uranium 238 and not very 00:47:53.270 --> 00:47:57.190 align:middle line:84% useful, it doesn't matter, because this cross section goes 00:47:57.190 --> 00:47:59.790 align:middle line:84% up by so many orders of magnitude, 00:47:59.790 --> 00:48:03.710 align:middle line:84% and we have enough of it that we can get by. 00:48:03.710 --> 00:48:04.590 align:middle line:90% Yep. 00:48:04.590 --> 00:48:07.038 align:middle line:84% STUDENT: Why is it so jagged in the middle? 00:48:07.038 --> 00:48:08.830 align:middle line:84% PROFESSOR: Those are called the-- so that's 00:48:08.830 --> 00:48:10.990 align:middle line:90% called the resonance region. 00:48:10.990 --> 00:48:16.710 align:middle line:84% And it has to do with the fact that the nucleus has, 00:48:16.710 --> 00:48:20.430 align:middle line:84% just like the atomic states, it has these resonance energy 00:48:20.430 --> 00:48:22.030 align:middle line:90% structures in it. 00:48:22.030 --> 00:48:26.710 align:middle line:84% And if you happen to have a neutron coming 00:48:26.710 --> 00:48:30.040 align:middle line:84% in that is exactly the right energy, 00:48:30.040 --> 00:48:35.740 align:middle line:84% it can resonantly excite the process that 00:48:35.740 --> 00:48:39.132 align:middle line:90% allows for the fusion to occur. 00:48:39.132 --> 00:48:41.340 align:middle line:84% So it goes up, and then it goes down, and it goes up, 00:48:41.340 --> 00:48:42.140 align:middle line:90% and it goes down. 00:48:42.140 --> 00:48:46.620 align:middle line:84% And then actually, really, these things continue forever. 00:48:46.620 --> 00:48:49.260 align:middle line:84% But we don't plot them because they just 00:48:49.260 --> 00:48:51.020 align:middle line:90% get to be stupidly dense. 00:48:51.020 --> 00:48:53.080 align:middle line:90% So then we stop. 00:48:53.080 --> 00:48:55.780 align:middle line:90% We just plot the average. 00:48:55.780 --> 00:48:57.440 align:middle line:84% But yeah, that is what's going on. 00:48:57.440 --> 00:49:00.180 align:middle line:84% You're seeing these resonance harmonics of the structure 00:49:00.180 --> 00:49:02.460 align:middle line:90% of the nucleus. 00:49:02.460 --> 00:49:04.860 align:middle line:84% And yeah, this really just keeps going. 00:49:04.860 --> 00:49:06.400 align:middle line:90% It's like that, then keep going. 00:49:06.400 --> 00:49:09.620 align:middle line:90% 00:49:09.620 --> 00:49:12.020 align:middle line:90% So where was I? 00:49:12.020 --> 00:49:17.200 align:middle line:84% So 5% is enough to get by for most reactors, 00:49:17.200 --> 00:49:21.340 align:middle line:84% as long as the reactor is a thermal reactor, not 00:49:21.340 --> 00:49:22.800 align:middle line:90% a fast reactor. 00:49:22.800 --> 00:49:25.300 align:middle line:84% So right now, there's a lot of people interested in building 00:49:25.300 --> 00:49:26.000 align:middle line:90% fast reactors. 00:49:26.000 --> 00:49:28.400 align:middle line:84% Again, they want to keep the neutrons over here. 00:49:28.400 --> 00:49:30.900 align:middle line:84% They don't want to thermalize them. 00:49:30.900 --> 00:49:34.700 align:middle line:84% That means you can't make it out of 5% uranium. 00:49:34.700 --> 00:49:41.020 align:middle line:84% You need something that has either a lot more uranium 00:49:41.020 --> 00:49:46.380 align:middle line:84% 235 than 238 because this just drops to nothing. 00:49:46.380 --> 00:49:50.140 align:middle line:84% Or you need to use something else like plutonium to make it. 00:49:50.140 --> 00:49:52.900 align:middle line:84% And so fast reactors always use fuels 00:49:52.900 --> 00:49:57.540 align:middle line:84% that are also, because they're fast, useful for making bombs. 00:49:57.540 --> 00:50:00.660 align:middle line:90% Because bombs are fast reactors. 00:50:00.660 --> 00:50:02.520 align:middle line:84% They're just run-away fast reactors. 00:50:02.520 --> 00:50:04.540 align:middle line:90% That's what nuclear bombs are. 00:50:04.540 --> 00:50:06.120 align:middle line:90% So you build a fast reactor. 00:50:06.120 --> 00:50:09.620 align:middle line:84% You have to use, basically, bomb grade material. 00:50:09.620 --> 00:50:12.860 align:middle line:84% And on top of that, if the neutrons slow down 00:50:12.860 --> 00:50:14.740 align:middle line:84% for whatever reason, the reactor becomes 00:50:14.740 --> 00:50:18.280 align:middle line:84% uncontrolled because the probability of fission goes up. 00:50:18.280 --> 00:50:22.860 align:middle line:84% So fast reactors are very hard to build and control. 00:50:22.860 --> 00:50:26.410 align:middle line:84% Thermal reactors are straightforward. 00:50:26.410 --> 00:50:30.010 align:middle line:90% So where was I? 00:50:30.010 --> 00:50:37.770 align:middle line:90% It goes up by a factor of 1,000. 00:50:37.770 --> 00:50:41.490 align:middle line:84% So for thermal reactors, this will do just fine. 00:50:41.490 --> 00:50:44.690 align:middle line:84% As the reactor becomes smaller and smaller, 00:50:44.690 --> 00:50:48.930 align:middle line:84% leakage goes up and up, so we have to compensate for that. 00:50:48.930 --> 00:50:50.670 align:middle line:90% So we typically keep enriching. 00:50:50.670 --> 00:50:54.570 align:middle line:84% We call this stuff right here high assay low enriched uranium, 00:50:54.570 --> 00:50:59.090 align:middle line:84% right at the cutoff where it's 20% uranium 235. 00:50:59.090 --> 00:51:03.890 align:middle line:84% Most new reactor concepts are looking at this material 00:51:03.890 --> 00:51:06.570 align:middle line:84% because they want to make the cores very small. 00:51:06.570 --> 00:51:08.590 align:middle line:84% Therefore, the leakage is very high, 00:51:08.590 --> 00:51:13.810 align:middle line:84% so they compensate by upping the amount of 235. 00:51:13.810 --> 00:51:16.270 align:middle line:84% But you can in principle go all the way to weapons grade. 00:51:16.270 --> 00:51:19.250 align:middle line:84% And in fact, our reactors that would 00:51:19.250 --> 00:51:22.330 align:middle line:84% put on submarines, which are very small, 00:51:22.330 --> 00:51:24.890 align:middle line:84% use weapons grade uranium, which is to say 00:51:24.890 --> 00:51:28.890 align:middle line:90% greater than 90% uranium 235. 00:51:28.890 --> 00:51:32.090 align:middle line:84% We'll talk about it in class, but in fact, anything above 20% 00:51:32.090 --> 00:51:34.810 align:middle line:84% can-- actually, anything above 12% 00:51:34.810 --> 00:51:38.370 align:middle line:84% can really be used to make a bomb. 00:51:38.370 --> 00:51:48.053 align:middle line:84% So the MIT reactor runs on weapons grade 93% uranium. 00:51:48.053 --> 00:51:48.970 align:middle line:90% Just letting you know. 00:51:48.970 --> 00:51:52.530 align:middle line:90% It's over there. 00:51:52.530 --> 00:51:54.130 align:middle line:84% Yeah, get a couple of MIT reactors 00:51:54.130 --> 00:51:57.470 align:middle line:84% together and you can build a bomb pretty easy. 00:51:57.470 --> 00:52:00.130 align:middle line:90% 00:52:00.130 --> 00:52:05.010 align:middle line:84% So just briefly, for those of you who are not physicists, 00:52:05.010 --> 00:52:07.012 align:middle line:84% and I talk about thermalizing neutrons 00:52:07.012 --> 00:52:08.470 align:middle line:84% and you're like, slowing them down. 00:52:08.470 --> 00:52:09.803 align:middle line:90% I have no idea how this happens. 00:52:09.803 --> 00:52:10.570 align:middle line:90% What do you mean? 00:52:10.570 --> 00:52:13.610 align:middle line:84% This is just a little illustration. 00:52:13.610 --> 00:52:15.930 align:middle line:90% Here's a heavy atom. 00:52:15.930 --> 00:52:17.150 align:middle line:90% Here's a neutron coming in. 00:52:17.150 --> 00:52:18.790 align:middle line:84% You can see the neutron bounced off, 00:52:18.790 --> 00:52:20.962 align:middle line:84% and this thing is like it's moving a little bit. 00:52:20.962 --> 00:52:23.420 align:middle line:84% But mostly the neutrons just bounced off at the same speed. 00:52:23.420 --> 00:52:27.360 align:middle line:84% It's like throwing a tennis ball against a wall. 00:52:27.360 --> 00:52:28.500 align:middle line:90% The wall is a lot heavier. 00:52:28.500 --> 00:52:30.417 align:middle line:84% Conservation of momentum means the tennis ball 00:52:30.417 --> 00:52:33.320 align:middle line:84% is going to bounce at roughly the same speed. 00:52:33.320 --> 00:52:40.040 align:middle line:84% If you have a light thing instead, like this molecule of-- 00:52:40.040 --> 00:52:43.000 align:middle line:84% I guess it's a molecule of helium, 00:52:43.000 --> 00:52:49.320 align:middle line:84% then you see that conservation of mass 00:52:49.320 --> 00:52:52.180 align:middle line:84% causes that thing to move off-- the neutron 00:52:52.180 --> 00:52:53.640 align:middle line:90% to move off more slowly. 00:52:53.640 --> 00:52:56.440 align:middle line:84% And so if we want to slow things down, 00:52:56.440 --> 00:52:59.720 align:middle line:84% we want to put a lot of light atoms in there. 00:52:59.720 --> 00:53:01.380 align:middle line:90% What is the lightest atom? 00:53:01.380 --> 00:53:04.160 align:middle line:90% 00:53:04.160 --> 00:53:07.440 align:middle line:84% Hydrogen. What has a lot of hydrogen? 00:53:07.440 --> 00:53:08.160 align:middle line:90% Water. 00:53:08.160 --> 00:53:12.800 align:middle line:84% And so now you know why water is the principal so-called 00:53:12.800 --> 00:53:13.500 align:middle line:90% moderator. 00:53:13.500 --> 00:53:17.760 align:middle line:84% The moderator is the thing that is slowing down the neutrons. 00:53:17.760 --> 00:53:21.120 align:middle line:84% And that's why we built reactors out of water. 00:53:21.120 --> 00:53:23.280 align:middle line:84% You can build it out of other light stuff. 00:53:23.280 --> 00:53:25.500 align:middle line:84% You can use graphite, which is carbon, 00:53:25.500 --> 00:53:29.200 align:middle line:84% which is heavier than hydrogen, but it's still pretty light. 00:53:29.200 --> 00:53:31.080 align:middle line:84% And a variety of other things people 00:53:31.080 --> 00:53:35.000 align:middle line:84% have-- there are other sources of hydrogen, like oils. 00:53:35.000 --> 00:53:38.160 align:middle line:90% Oils are also hydrogen-rich. 00:53:38.160 --> 00:53:42.500 align:middle line:84% And so people talk about organic reactors and things like this, 00:53:42.500 --> 00:53:49.100 align:middle line:84% but basically, kerosene in there to slow down the neutrons. 00:53:49.100 --> 00:53:51.720 align:middle line:90% 00:53:51.720 --> 00:53:54.920 align:middle line:84% Early reactors built during World War II to make plutonium 00:53:54.920 --> 00:53:58.320 align:middle line:84% used graphite, which was nice because you could just 00:53:58.320 --> 00:53:59.620 align:middle line:90% get it in these big blocks. 00:53:59.620 --> 00:54:00.940 align:middle line:90% You drill these holes. 00:54:00.940 --> 00:54:03.280 align:middle line:90% You put the fuel into the holes. 00:54:03.280 --> 00:54:05.820 align:middle line:84% The neutrons shoot out of the fuel as they're fissioning. 00:54:05.820 --> 00:54:09.060 align:middle line:84% They travel, bounce around in the carbonaceous graphite. 00:54:09.060 --> 00:54:11.240 align:middle line:84% They slow down to the point where they can then 00:54:11.240 --> 00:54:12.800 align:middle line:90% trigger the next fission. 00:54:12.800 --> 00:54:16.960 align:middle line:90% And so the reactor proceeds. 00:54:16.960 --> 00:54:22.710 align:middle line:84% So that is how the first, what we call, production reactors 00:54:22.710 --> 00:54:25.490 align:middle line:84% were built. They would, of course, heat up. 00:54:25.490 --> 00:54:27.310 align:middle line:84% So we would flow air through them, 00:54:27.310 --> 00:54:30.110 align:middle line:84% trying to keep all this stuff cool. 00:54:30.110 --> 00:54:32.330 align:middle line:84% And I don't have a photo, unfortunately, 00:54:32.330 --> 00:54:35.150 align:middle line:84% but they would put some shielding out in front. 00:54:35.150 --> 00:54:37.910 align:middle line:84% And literally, there'd be people standing 00:54:37.910 --> 00:54:42.510 align:middle line:84% in front of the open face reactor shoveling uranium 00:54:42.510 --> 00:54:46.230 align:middle line:90% into these holes manually. 00:54:46.230 --> 00:54:48.570 align:middle line:84% And then inside, deep inside the graphite block, 00:54:48.570 --> 00:54:50.550 align:middle line:84% there would be this reaction happening. 00:54:50.550 --> 00:54:54.570 align:middle line:84% And they ran these reactors and made a whole bunch of plutonium. 00:54:54.570 --> 00:54:56.870 align:middle line:84% We'll talk more about how plutonium was made. 00:54:56.870 --> 00:55:01.990 align:middle line:84% But this is how the early reactor is built. 00:55:01.990 --> 00:55:06.430 align:middle line:84% The water reactors, we have a couple of different concepts. 00:55:06.430 --> 00:55:09.802 align:middle line:84% So one is called the-- this is actually a later concept. 00:55:09.802 --> 00:55:12.010 align:middle line:84% The original concept was a pressurized water reactor, 00:55:12.010 --> 00:55:14.870 align:middle line:84% which we talked about briefly as being used on submarines. 00:55:14.870 --> 00:55:16.410 align:middle line:90% I'll get to that in a moment. 00:55:16.410 --> 00:55:20.670 align:middle line:84% This is a newer concept called the boiling water reactor. 00:55:20.670 --> 00:55:24.990 align:middle line:84% So here is your uranium fuel in yellow. 00:55:24.990 --> 00:55:28.390 align:middle line:84% The neutrons come out and they hit the water. 00:55:28.390 --> 00:55:30.610 align:middle line:84% They then trigger more fissions in the rods. 00:55:30.610 --> 00:55:31.410 align:middle line:90% The rods heat up. 00:55:31.410 --> 00:55:33.050 align:middle line:84% The water happens to do two things. 00:55:33.050 --> 00:55:35.310 align:middle line:84% It's not only slowing the neutrons down. 00:55:35.310 --> 00:55:38.430 align:middle line:90% It's also cooling the rods. 00:55:38.430 --> 00:55:44.230 align:middle line:84% It absorbs so much heat that the water boils. 00:55:44.230 --> 00:55:45.990 align:middle line:90% It makes steam. 00:55:45.990 --> 00:55:49.470 align:middle line:84% You pump the steam out and you turn a generator. 00:55:49.470 --> 00:55:51.990 align:middle line:84% It condenses on the generator blades, and then 00:55:51.990 --> 00:55:56.590 align:middle line:84% the turbine generator, and then you pump the water back in. 00:55:56.590 --> 00:55:59.750 align:middle line:84% And this is how you run your reactor. 00:55:59.750 --> 00:56:02.770 align:middle line:84% So reactors are just water boilers, basically. 00:56:02.770 --> 00:56:04.550 align:middle line:90% That's what they are. 00:56:04.550 --> 00:56:06.807 align:middle line:84% This idea was promoted by General Electric. 00:56:06.807 --> 00:56:08.390 align:middle line:84% We'll talk more about General Electric 00:56:08.390 --> 00:56:12.110 align:middle line:84% in the next class as a principal competitor 00:56:12.110 --> 00:56:15.060 align:middle line:84% to Westinghouse, who came up with the pressurized water 00:56:15.060 --> 00:56:15.860 align:middle line:90% reactor. 00:56:15.860 --> 00:56:19.980 align:middle line:84% Approximately 20% of the world's fleet is based on this concept. 00:56:19.980 --> 00:56:25.980 align:middle line:84% A lot of the reactors in Japan, the Fukushima reactor, 00:56:25.980 --> 00:56:29.740 align:middle line:90% was of this design. 00:56:29.740 --> 00:56:36.340 align:middle line:84% Because the water is directly in contact with the fuel, 00:56:36.340 --> 00:56:40.780 align:middle line:84% one of the things that happens is the fuel corrodes 00:56:40.780 --> 00:56:45.300 align:middle line:84% and some radioactive material leaks into the water. 00:56:45.300 --> 00:56:49.900 align:middle line:84% And so this loop of water becomes contaminated 00:56:49.900 --> 00:56:53.100 align:middle line:84% with radiation, including the turbine that 00:56:53.100 --> 00:56:54.780 align:middle line:90% is producing your electricity. 00:56:54.780 --> 00:56:57.180 align:middle line:84% So you have a whole lot more radiation 00:56:57.180 --> 00:56:59.240 align:middle line:84% to deal with all throughout the plant. 00:56:59.240 --> 00:57:02.400 align:middle line:84% If you spring a leak, the water is already radioactive. 00:57:02.400 --> 00:57:04.000 align:middle line:90% You have to deal with it. 00:57:04.000 --> 00:57:09.627 align:middle line:84% So this is a complicated issue if there's a problem. 00:57:09.627 --> 00:57:11.460 align:middle line:84% You have to worry about corrosion and things 00:57:11.460 --> 00:57:14.940 align:middle line:84% like that more because of the radiation. 00:57:14.940 --> 00:57:17.940 align:middle line:84% Here is the pressurized water reactor concept. 00:57:17.940 --> 00:57:19.660 align:middle line:84% The idea of pressurized water reactor 00:57:19.660 --> 00:57:23.860 align:middle line:84% is that you put this water in a pressure vessel, 00:57:23.860 --> 00:57:26.860 align:middle line:84% and you increase the pressure so high, the water doesn't boil, 00:57:26.860 --> 00:57:30.940 align:middle line:84% because the boiling point is a function of the pressure. 00:57:30.940 --> 00:57:33.602 align:middle line:84% So you raise it above the vapor point. 00:57:33.602 --> 00:57:34.560 align:middle line:90% The water doesn't boil. 00:57:34.560 --> 00:57:35.600 align:middle line:90% It's just superheated water. 00:57:35.600 --> 00:57:37.225 align:middle line:84% It's like your pressure cooker at home. 00:57:37.225 --> 00:57:39.500 align:middle line:90% You're canning stuff. 00:57:39.500 --> 00:57:42.220 align:middle line:84% And instead, it sends super hot-- 00:57:42.220 --> 00:57:43.320 align:middle line:90% I'm sorry about this. 00:57:43.320 --> 00:57:45.840 align:middle line:84% I'm going to ask them about this jiggly projector. 00:57:45.840 --> 00:57:48.780 align:middle line:84% It sends super hot water into this thing 00:57:48.780 --> 00:57:55.022 align:middle line:84% called a steam generator, which is just a heat exchanger. 00:57:55.022 --> 00:57:56.480 align:middle line:84% If you look at a real one of these, 00:57:56.480 --> 00:58:00.940 align:middle line:84% it's just hundreds and hundreds of little tubes going through 00:58:00.940 --> 00:58:02.540 align:middle line:90% in a big tank. 00:58:02.540 --> 00:58:04.380 align:middle line:84% And the super hot water that contains 00:58:04.380 --> 00:58:08.260 align:middle line:84% some of those corrosion products are confined to the tubes. 00:58:08.260 --> 00:58:09.820 align:middle line:84% And then we pump other water, which 00:58:09.820 --> 00:58:13.590 align:middle line:84% is nice and clean, around the outside of the tubes, 00:58:13.590 --> 00:58:16.390 align:middle line:84% and it exchanges the heat to the other water. 00:58:16.390 --> 00:58:18.890 align:middle line:84% And it boils the water on the outside of the tubes 00:58:18.890 --> 00:58:20.170 align:middle line:90% and makes steam. 00:58:20.170 --> 00:58:22.570 align:middle line:84% And then we use that steam, which is nice and clean, 00:58:22.570 --> 00:58:26.330 align:middle line:84% take it out of the containment, and run the turbine 00:58:26.330 --> 00:58:28.330 align:middle line:90% to make electricity. 00:58:28.330 --> 00:58:31.050 align:middle line:84% That way, everything that contains radiation 00:58:31.050 --> 00:58:34.330 align:middle line:90% is inside this containment. 00:58:34.330 --> 00:58:37.290 align:middle line:84% And if there's a problem, everything will be contained. 00:58:37.290 --> 00:58:40.250 align:middle line:90% And that is the idea behind PWR. 00:58:40.250 --> 00:58:43.830 align:middle line:84% So in that sense, they are supposed to be, if you will, 00:58:43.830 --> 00:58:44.810 align:middle line:90% safer. 00:58:44.810 --> 00:58:47.230 align:middle line:84% So these reactors typically operate around 300 degrees 00:58:47.230 --> 00:58:51.457 align:middle line:84% C, 3 times the normal boiling point of water. 00:58:51.457 --> 00:58:53.290 align:middle line:84% But they do that because they keep the water 00:58:53.290 --> 00:58:54.650 align:middle line:90% under very high pressure. 00:58:54.650 --> 00:58:59.010 align:middle line:84% About 63% of the commercial fleet uses these designs. 00:58:59.010 --> 00:59:01.910 align:middle line:90% Of course, there's consequences. 00:59:01.910 --> 00:59:04.290 align:middle line:84% You have very high pressures, which 00:59:04.290 --> 00:59:10.010 align:middle line:84% means that things may stress from cooling and heating, 00:59:10.010 --> 00:59:12.370 align:middle line:90% and weaken, and blow out. 00:59:12.370 --> 00:59:14.450 align:middle line:84% And so you have a different kind of safety issue 00:59:14.450 --> 00:59:19.090 align:middle line:84% to think about related to material integrity and so on. 00:59:19.090 --> 00:59:22.070 align:middle line:84% And so you overbuild these things to try to prevent that, 00:59:22.070 --> 00:59:23.670 align:middle line:90% but things can still happen. 00:59:23.670 --> 00:59:25.770 align:middle line:84% So some of the pressurized water reactors 00:59:25.770 --> 00:59:30.210 align:middle line:84% that are currently being stood up in France, 00:59:30.210 --> 00:59:34.713 align:middle line:84% what's called the EPR, and the top cap of-- 00:59:34.713 --> 00:59:36.130 align:middle line:84% Actually, I can't remember if it's 00:59:36.130 --> 00:59:37.380 align:middle line:90% the top cap or the bottom cap. 00:59:37.380 --> 00:59:41.170 align:middle line:84% One of the caps, really thick steel casting 00:59:41.170 --> 00:59:42.650 align:middle line:90% to hold all the pressure in. 00:59:42.650 --> 00:59:45.170 align:middle line:84% But it turned out when they cast it, 00:59:45.170 --> 00:59:49.210 align:middle line:84% there were some precipitates of carbon 00:59:49.210 --> 00:59:52.210 align:middle line:84% in the steel, which weakens the steel, which 00:59:52.210 --> 00:59:54.690 align:middle line:84% means that now they're not sure if that thing will 00:59:54.690 --> 00:59:56.090 align:middle line:90% last very long. 00:59:56.090 --> 00:59:58.790 align:middle line:84% So they've given it a provisional operating license, 00:59:58.790 --> 01:00:03.010 align:middle line:84% but they may have to shut the reactor down early. 01:00:03.010 --> 01:00:05.410 align:middle line:84% And unfortunately, that cap is so ginormous 01:00:05.410 --> 01:00:08.680 align:middle line:84% that you can't actually replace it without disassembling 01:00:08.680 --> 01:00:10.520 align:middle line:90% the whole reactor building. 01:00:10.520 --> 01:00:12.860 align:middle line:84% This thing has to be built and put in place, 01:00:12.860 --> 01:00:14.840 align:middle line:84% and then you build the building around it. 01:00:14.840 --> 01:00:21.000 align:middle line:84% And so yeah, getting the material science right 01:00:21.000 --> 01:00:22.840 align:middle line:90% is really important. 01:00:22.840 --> 01:00:27.680 align:middle line:84% And this is what makes these things hard to build. 01:00:27.680 --> 01:00:28.180 align:middle line:90% All right. 01:00:28.180 --> 01:00:31.240 align:middle line:84% So I've mentioned I keep talking about the generator. 01:00:31.240 --> 01:00:31.940 align:middle line:90% What do I mean? 01:00:31.940 --> 01:00:35.112 align:middle line:84% So again, this is a pressurized water reactor. 01:00:35.112 --> 01:00:35.820 align:middle line:90% Here's a reactor. 01:00:35.820 --> 01:00:39.040 align:middle line:84% Here's the steam generator or the heat exchanger. 01:00:39.040 --> 01:00:40.600 align:middle line:84% This is the pressurizer which keeps 01:00:40.600 --> 01:00:42.640 align:middle line:90% everything in high pressure. 01:00:42.640 --> 01:00:44.260 align:middle line:90% Here comes the steam. 01:00:44.260 --> 01:00:45.920 align:middle line:84% There's a turbine that is controlling 01:00:45.920 --> 01:00:48.140 align:middle line:84% your electric generator that is making electricity. 01:00:48.140 --> 01:00:50.160 align:middle line:90% It's making us happy. 01:00:50.160 --> 01:00:52.700 align:middle line:84% And then the cool steam comes out here, 01:00:52.700 --> 01:00:56.320 align:middle line:84% and we want to cool it down as much as possible. 01:00:56.320 --> 01:01:00.080 align:middle line:84% So we send it to these big towers 01:01:00.080 --> 01:01:02.680 align:middle line:84% or to some other source of ultimate heat sink, where 01:01:02.680 --> 01:01:06.000 align:middle line:84% we cool the water down so we can start over and get 01:01:06.000 --> 01:01:07.620 align:middle line:90% good thermodynamic efficiency. 01:01:07.620 --> 01:01:09.300 align:middle line:84% If the water comes in too hot here, 01:01:09.300 --> 01:01:12.200 align:middle line:84% we can't run the reactor at full power. 01:01:12.200 --> 01:01:16.040 align:middle line:84% So you have to de-rate the reactor. 01:01:16.040 --> 01:01:19.380 align:middle line:84% So we have these big towers, and that's, of course, 01:01:19.380 --> 01:01:21.880 align:middle line:84% what we associate with pictures of nuclear power 01:01:21.880 --> 01:01:24.280 align:middle line:90% plants or these towers. 01:01:24.280 --> 01:01:26.360 align:middle line:84% But they are just there to cool off the water 01:01:26.360 --> 01:01:28.680 align:middle line:84% after it goes through the turbine, 01:01:28.680 --> 01:01:32.080 align:middle line:90% before it goes pumped back in. 01:01:32.080 --> 01:01:34.743 align:middle line:84% Ideally, all this stuff that handles the radiation, 01:01:34.743 --> 01:01:36.160 align:middle line:84% the radioactive water, all of this 01:01:36.160 --> 01:01:39.400 align:middle line:84% goes in a big concrete building that is steel-reinforced. 01:01:39.400 --> 01:01:41.240 align:middle line:90% We called that the containment. 01:01:41.240 --> 01:01:44.200 align:middle line:84% And it's designed so that if this thing explodes and all 01:01:44.200 --> 01:01:47.400 align:middle line:84% this water flashes to steam, which turns it 01:01:47.400 --> 01:01:53.080 align:middle line:84% into a big hot balloon, that this thing can 01:01:53.080 --> 01:01:56.740 align:middle line:84% handle all that pressure, at least temporarily. 01:01:56.740 --> 01:02:00.160 align:middle line:84% It turns out it can't handle it forever. 01:02:00.160 --> 01:02:02.780 align:middle line:84% It can only handle it for a couple of days, 01:02:02.780 --> 01:02:04.090 align:middle line:90% and then you have to vent it. 01:02:04.090 --> 01:02:05.750 align:middle line:90% Otherwise, it will explode. 01:02:05.750 --> 01:02:09.270 align:middle line:84% And so that in Three Mile Island accident, 01:02:09.270 --> 01:02:12.050 align:middle line:84% those vents, to keep this thing from exploding, 01:02:12.050 --> 01:02:15.590 align:middle line:84% that venting process is what led to the release of radioactivity 01:02:15.590 --> 01:02:17.630 align:middle line:84% in the Three Mile Island accident. 01:02:17.630 --> 01:02:20.170 align:middle line:84% And in fact, the same thing also occurred at Fukushima. 01:02:20.170 --> 01:02:22.030 align:middle line:90% They had to vent the-- 01:02:22.030 --> 01:02:24.470 align:middle line:84% it wasn't a PWR, but they also had to vent. 01:02:24.470 --> 01:02:30.550 align:middle line:84% And you see this little burst of radiation occurring. 01:02:30.550 --> 01:02:35.310 align:middle line:84% There are, of course, concepts that don't rely on water. 01:02:35.310 --> 01:02:38.170 align:middle line:84% So this is called high temperature graphite reactor, 01:02:38.170 --> 01:02:40.350 align:middle line:90% HTGR. 01:02:40.350 --> 01:02:48.230 align:middle line:84% This concept, you have here these fuel pebbles, 01:02:48.230 --> 01:02:52.590 align:middle line:84% which have little grains of uranium sand in them coated 01:02:52.590 --> 01:02:54.350 align:middle line:90% with graphite. 01:02:54.350 --> 01:02:58.010 align:middle line:84% The graphite is a moderator, slows things down. 01:02:58.010 --> 01:03:02.590 align:middle line:84% And instead, you cool it with gas, usually, 01:03:02.590 --> 01:03:06.910 align:middle line:84% an inert gas like helium that doesn't chemically react. 01:03:06.910 --> 01:03:13.450 align:middle line:84% And that is nice because gas is not-- it's already gas form, 01:03:13.450 --> 01:03:15.130 align:middle line:84% so it won't undergo a phase change. 01:03:15.130 --> 01:03:17.230 align:middle line:84% If it gets too hot, it stays as gas. 01:03:17.230 --> 01:03:19.470 align:middle line:84% So water, when it gets too hot, it boils, 01:03:19.470 --> 01:03:21.750 align:middle line:84% and the volume expands dramatically 01:03:21.750 --> 01:03:23.870 align:middle line:90% and explodes things. 01:03:23.870 --> 01:03:26.592 align:middle line:84% Here the gas just stays as gas and the pressure does go up, 01:03:26.592 --> 01:03:28.050 align:middle line:84% but you don't get the phase change, 01:03:28.050 --> 01:03:32.550 align:middle line:84% so it's a much safer device from that perspective. 01:03:32.550 --> 01:03:34.950 align:middle line:84% And so China is currently marketing 01:03:34.950 --> 01:03:41.230 align:middle line:84% a reactor of this concept called the H-- it's called the PM 10. 01:03:41.230 --> 01:03:42.130 align:middle line:90% Is that right? 01:03:42.130 --> 01:03:42.710 align:middle line:90% HTR PM? 01:03:42.710 --> 01:03:45.830 align:middle line:84% No HTR PM 10 was the prototype HTR PR I think 01:03:45.830 --> 01:03:48.450 align:middle line:90% is the current one. 01:03:48.450 --> 01:03:51.318 align:middle line:84% This is called pebble bed reactor. 01:03:51.318 --> 01:03:53.110 align:middle line:84% It's called pebble bed because the reactors 01:03:53.110 --> 01:03:55.390 align:middle line:90% are in little pebbles. 01:03:55.390 --> 01:03:57.720 align:middle line:84% You can also build this without the fuel being-- sorry. 01:03:57.720 --> 01:03:58.970 align:middle line:90% The fuel is in little pebbles. 01:03:58.970 --> 01:04:01.440 align:middle line:84% You can also build this without the fuel being in pebbles. 01:04:01.440 --> 01:04:04.480 align:middle line:84% You can just have them as sticks instead of reactor turnout. 01:04:04.480 --> 01:04:05.620 align:middle line:90% That's a better design. 01:04:05.620 --> 01:04:07.260 align:middle line:84% But people like the pebble bed reactor 01:04:07.260 --> 01:04:09.552 align:middle line:84% because the idea is that you put the fuel in one pebble 01:04:09.552 --> 01:04:12.100 align:middle line:84% at a time, and the old fuel comes out one pebble at a time, 01:04:12.100 --> 01:04:14.540 align:middle line:84% and you can just keep the reactor running. 01:04:14.540 --> 01:04:17.980 align:middle line:84% The problem with pebble beds is, remember, criticality 01:04:17.980 --> 01:04:22.060 align:middle line:84% is fundamentally an issue of geometry. 01:04:22.060 --> 01:04:24.260 align:middle line:84% And the geometry is always changing every time 01:04:24.260 --> 01:04:25.980 align:middle line:90% you add and remove pebble. 01:04:25.980 --> 01:04:28.860 align:middle line:84% So the criticality goes [VOCALIZING] and moves around, 01:04:28.860 --> 01:04:31.780 align:middle line:84% and you have to control it and deal with it. 01:04:31.780 --> 01:04:35.780 align:middle line:84% So there was recently a proposal to put one of these things 01:04:35.780 --> 01:04:37.153 align:middle line:90% on a boat. 01:04:37.153 --> 01:04:38.820 align:middle line:84% It's a really dumb idea because the boat 01:04:38.820 --> 01:04:42.320 align:middle line:84% goes like this and the geometry is changing, and right. 01:04:42.320 --> 01:04:44.500 align:middle line:84% So nonetheless, there's a startup company 01:04:44.500 --> 01:04:47.380 align:middle line:90% trying to do this. 01:04:47.380 --> 01:04:50.700 align:middle line:84% Yeah, so there's some nice features. 01:04:50.700 --> 01:04:53.900 align:middle line:84% These reactors, because they don't use water, 01:04:53.900 --> 01:04:56.460 align:middle line:84% and because they don't use metal cladding 01:04:56.460 --> 01:04:59.900 align:middle line:84% but use graphite around the fuel, 01:04:59.900 --> 01:05:01.760 align:middle line:84% they can go to very, very high temperatures. 01:05:01.760 --> 01:05:06.180 align:middle line:84% So in principle, as long as the temperature stays below 1,600 01:05:06.180 --> 01:05:09.180 align:middle line:84% degrees C, which is the temperature at which graphite 01:05:09.180 --> 01:05:15.220 align:middle line:84% begins to autocatalytically burn, this reactor is fine. 01:05:15.220 --> 01:05:17.480 align:middle line:84% So because they go to much higher temperatures, 01:05:17.480 --> 01:05:19.600 align:middle line:84% you can get better thermal efficiencies, 01:05:19.600 --> 01:05:21.260 align:middle line:84% which are a function of this delta t. 01:05:21.260 --> 01:05:26.300 align:middle line:84% And you can do things that need a really high temperature 01:05:26.300 --> 01:05:28.740 align:middle line:90% heat, like melt steel. 01:05:28.740 --> 01:05:30.660 align:middle line:84% And so you could use these for all kinds 01:05:30.660 --> 01:05:33.460 align:middle line:84% of industrial processes in principle. 01:05:33.460 --> 01:05:35.620 align:middle line:84% We'll have a whole lecture about that. 01:05:35.620 --> 01:05:38.500 align:middle line:84% But in principle, you can use these kinds of reactors 01:05:38.500 --> 01:05:39.680 align:middle line:90% for different concepts. 01:05:39.680 --> 01:05:42.660 align:middle line:90% 01:05:42.660 --> 01:05:47.740 align:middle line:84% STUDENT: Is it easy to change out the spent fuel container? 01:05:47.740 --> 01:05:48.780 align:middle line:90% How big is it? 01:05:48.780 --> 01:05:51.020 align:middle line:84% PROFESSOR: It doesn't really look like this. 01:05:51.020 --> 01:05:54.513 align:middle line:84% Yeah, there's all kinds of-- it's much more complicated. 01:05:54.513 --> 01:05:55.930 align:middle line:84% Every pebble comes out and there's 01:05:55.930 --> 01:05:59.410 align:middle line:84% a radiation sensor that measures how much of the uranium 01:05:59.410 --> 01:06:01.770 align:middle line:84% is burned, and they decide either to recycle it 01:06:01.770 --> 01:06:03.210 align:middle line:90% or to dispose of it. 01:06:03.210 --> 01:06:05.097 align:middle line:84% And it's all controlled by robotics. 01:06:05.097 --> 01:06:06.930 align:middle line:84% And yeah, it is not just like this container 01:06:06.930 --> 01:06:08.250 align:middle line:90% that you take away. 01:06:08.250 --> 01:06:10.450 align:middle line:90% So this is figurative. 01:06:10.450 --> 01:06:13.210 align:middle line:90% 01:06:13.210 --> 01:06:15.790 align:middle line:84% Although these reactors are back in fashion, 01:06:15.790 --> 01:06:17.970 align:middle line:84% I just want to point out that this concept actually 01:06:17.970 --> 01:06:21.690 align:middle line:90% dates back to 1960. 01:06:21.690 --> 01:06:23.850 align:middle line:90% We built a few of these things. 01:06:23.850 --> 01:06:25.230 align:middle line:90% They were not cost effective. 01:06:25.230 --> 01:06:28.930 align:middle line:84% They had lots and lots of problems with the fuel. 01:06:28.930 --> 01:06:31.390 align:middle line:84% The fuel, because of rubbing against each other, 01:06:31.390 --> 01:06:33.810 align:middle line:84% would abrade and create all this dust. 01:06:33.810 --> 01:06:39.170 align:middle line:84% Then it turned out that fission products would diffuse out 01:06:39.170 --> 01:06:41.850 align:middle line:84% of the fuel because the temperatures were so high, 01:06:41.850 --> 01:06:45.330 align:middle line:84% so just kinetically move around and get out of the fuel. 01:06:45.330 --> 01:06:47.730 align:middle line:84% And then you had all this dust, and so it adsorbed 01:06:47.730 --> 01:06:50.050 align:middle line:90% this radiation onto this dust. 01:06:50.050 --> 01:06:54.250 align:middle line:84% And then you had so you had all this radioactive dust floating 01:06:54.250 --> 01:06:55.510 align:middle line:90% around inside. 01:06:55.510 --> 01:06:57.390 align:middle line:84% And if you tried to open it to service it, 01:06:57.390 --> 01:06:59.570 align:middle line:84% all this radioactive dust would come out. 01:06:59.570 --> 01:07:02.490 align:middle line:84% And so the facility in Germany, I 01:07:02.490 --> 01:07:05.030 align:middle line:84% think it was called the AVR, where they built this. 01:07:05.030 --> 01:07:08.410 align:middle line:84% It's to this day, the most strontium contaminated facility 01:07:08.410 --> 01:07:10.450 align:middle line:84% in the world, has never been disassembled, 01:07:10.450 --> 01:07:12.570 align:middle line:84% despite it being closed for decades and decades 01:07:12.570 --> 01:07:15.250 align:middle line:90% because it's just too reactive. 01:07:15.250 --> 01:07:17.930 align:middle line:84% Some of these problems have been maybe fixed. 01:07:17.930 --> 01:07:20.650 align:middle line:84% They're fixed in the lab, but whether they're 01:07:20.650 --> 01:07:23.770 align:middle line:84% fixed in real life is TBD, because it 01:07:23.770 --> 01:07:27.650 align:middle line:84% depends on engineering tolerances and getting 01:07:27.650 --> 01:07:29.290 align:middle line:90% everything just right. 01:07:29.290 --> 01:07:31.330 align:middle line:84% And as you scale things up, things 01:07:31.330 --> 01:07:33.090 align:middle line:90% tend not to be just right. 01:07:33.090 --> 01:07:36.130 align:middle line:84% So these reactor concepts are still 01:07:36.130 --> 01:07:39.210 align:middle line:84% kind of on the edge of whether or not 01:07:39.210 --> 01:07:42.050 align:middle line:84% they're really mature enough to be deployed. 01:07:42.050 --> 01:07:45.473 align:middle line:84% But they're also more complicated because of all 01:07:45.473 --> 01:07:47.890 align:middle line:84% these pebbles, and all these pebble sorters and everything 01:07:47.890 --> 01:07:48.930 align:middle line:90% is going on. 01:07:48.930 --> 01:07:50.810 align:middle line:84% So to keep the cost down, one of the things 01:07:50.810 --> 01:07:54.720 align:middle line:84% that has been done, for example, by the Chinese, who 01:07:54.720 --> 01:07:57.240 align:middle line:84% are the only ones operating these right now, 01:07:57.240 --> 01:08:00.560 align:middle line:84% is to just get rid of the containment building. 01:08:00.560 --> 01:08:04.520 align:middle line:84% And so they say these things, they don't have water 01:08:04.520 --> 01:08:07.400 align:middle line:90% and they don't melt down easily. 01:08:07.400 --> 01:08:12.560 align:middle line:84% So as long as nothing runs into the reactor and opens it up, 01:08:12.560 --> 01:08:16.359 align:middle line:84% or there's not some weird unexplained excursion, 01:08:16.359 --> 01:08:19.080 align:middle line:84% we deem that it is safe enough to operate these 01:08:19.080 --> 01:08:21.418 align:middle line:84% without containment, which is very questionable. 01:08:21.418 --> 01:08:23.460 align:middle line:84% That was also the case for the Chernobyl reactor. 01:08:23.460 --> 01:08:26.479 align:middle line:84% They deemed it was safe enough to operate without containment, 01:08:26.479 --> 01:08:28.760 align:middle line:84% and then there was a major accident 01:08:28.760 --> 01:08:31.540 align:middle line:84% without containment to contain new stuff. 01:08:31.540 --> 01:08:34.580 align:middle line:84% One of the ways these reactors can fail 01:08:34.580 --> 01:08:36.520 align:middle line:90% is if there's water ingress. 01:08:36.520 --> 01:08:40.040 align:middle line:84% So you could imagine a variety of situations 01:08:40.040 --> 01:08:44.439 align:middle line:84% where, for example, you build this reactor in an area that's 01:08:44.439 --> 01:08:47.640 align:middle line:84% a flood zone, and suddenly, this reactor becomes flooded, 01:08:47.640 --> 01:08:50.120 align:middle line:84% and water then over moderates the neutrons. 01:08:50.120 --> 01:08:53.720 align:middle line:84% And suddenly, this thing explodes. 01:08:53.720 --> 01:08:57.240 align:middle line:84% There's all kinds of things you need to worry about. 01:08:57.240 --> 01:08:59.915 align:middle line:84% So every time you come up with a reactor 01:08:59.915 --> 01:09:01.540 align:middle line:84% that is different than what we've seen, 01:09:01.540 --> 01:09:03.165 align:middle line:84% there are all these wonderful features, 01:09:03.165 --> 01:09:05.560 align:middle line:84% and then there are all these other problems that people 01:09:05.560 --> 01:09:06.620 align:middle line:90% tend not to talk about. 01:09:06.620 --> 01:09:10.640 align:middle line:84% And that's kind of an important message here. 01:09:10.640 --> 01:09:13.319 align:middle line:90% Here's another variation. 01:09:13.319 --> 01:09:14.160 align:middle line:90% What? 01:09:14.160 --> 01:09:15.340 align:middle line:90% Oh, it looks great. 01:09:15.340 --> 01:09:16.819 align:middle line:90% It looks virtually the same. 01:09:16.819 --> 01:09:18.560 align:middle line:90% Yeah. 01:09:18.560 --> 01:09:20.279 align:middle line:90% Here's what changes. 01:09:20.279 --> 01:09:24.920 align:middle line:84% The coolant goes from helium to liquid salt. Yeah, 01:09:24.920 --> 01:09:27.520 align:middle line:84% so a very high temperature salt. Usually, 01:09:27.520 --> 01:09:29.080 align:middle line:90% this is a salt called FLiBe. 01:09:29.080 --> 01:09:32.200 align:middle line:90% Fluorine, lithium, beryllium. 01:09:32.200 --> 01:09:34.720 align:middle line:90% It also has these nice features. 01:09:34.720 --> 01:09:39.979 align:middle line:84% FLiBe, I'll just write it out so you know what I'm saying. 01:09:39.979 --> 01:09:46.399 align:middle line:90% 01:09:46.399 --> 01:09:50.270 align:middle line:84% Fluorine, lithium, beryllium, FLiBe, 01:09:50.270 --> 01:09:52.010 align:middle line:84% it expands even less than helium. 01:09:52.010 --> 01:09:53.750 align:middle line:84% One of the problems with helium is 01:09:53.750 --> 01:09:56.790 align:middle line:84% that the conductivity, the thermal conductivity of helium 01:09:56.790 --> 01:09:58.750 align:middle line:90% goes down when it gets hotter. 01:09:58.750 --> 01:10:00.030 align:middle line:90% It's weird. 01:10:00.030 --> 01:10:03.530 align:middle line:84% So the result is that in a helium version, 01:10:03.530 --> 01:10:06.190 align:middle line:84% if I get a hotspot here, the helium cools it even less, 01:10:06.190 --> 01:10:07.950 align:middle line:84% and the hotspot becomes even hotter. 01:10:07.950 --> 01:10:10.550 align:middle line:84% And then you have the fuel exceeding its design 01:10:10.550 --> 01:10:12.710 align:middle line:90% temperatures and problems. 01:10:12.710 --> 01:10:15.630 align:middle line:84% So FLiBe doesn't have that feature, 01:10:15.630 --> 01:10:17.570 align:middle line:90% and so it also expands less. 01:10:17.570 --> 01:10:20.230 align:middle line:84% And you see it's in principle, a variant on this 01:10:20.230 --> 01:10:22.990 align:middle line:90% that is more robust. 01:10:22.990 --> 01:10:24.430 align:middle line:90% So that's nice. 01:10:24.430 --> 01:10:26.710 align:middle line:84% But of course, there are other problems with FLiBe. 01:10:26.710 --> 01:10:28.630 align:middle line:84% One is it's a salt. So if it gets cold, 01:10:28.630 --> 01:10:31.990 align:middle line:84% the whole thing congeals into an ice, a block of salt, 01:10:31.990 --> 01:10:33.190 align:middle line:90% and it's a problem. 01:10:33.190 --> 01:10:35.290 align:middle line:84% Another problem is that beryllium, 01:10:35.290 --> 01:10:41.030 align:middle line:84% which is a key component of the salt, is extremely toxic. 01:10:41.030 --> 01:10:43.750 align:middle line:84% So there's all kinds of issues again. 01:10:43.750 --> 01:10:45.790 align:middle line:84% And a third one is it turns out FLiBe, 01:10:45.790 --> 01:10:48.030 align:middle line:84% if it gets any contaminants in it, 01:10:48.030 --> 01:10:53.230 align:middle line:84% because it's an ionic material, it's really good at corrosion. 01:10:53.230 --> 01:10:57.310 align:middle line:84% It will just corrode anything very rapidly 01:10:57.310 --> 01:11:00.510 align:middle line:84% because you can just move electrons around and chemically 01:11:00.510 --> 01:11:04.030 align:middle line:90% facilitate chemical corrosion. 01:11:04.030 --> 01:11:07.690 align:middle line:84% So these reactors are also being designed. 01:11:07.690 --> 01:11:10.150 align:middle line:84% There's a group here in our department at MIT 01:11:10.150 --> 01:11:13.930 align:middle line:84% that is a big fan of these, but again, also has issues. 01:11:13.930 --> 01:11:17.670 align:middle line:84% FLiBe is also Incidentally the coolant inside the spark fusion 01:11:17.670 --> 01:11:18.630 align:middle line:90% reactor. 01:11:18.630 --> 01:11:21.350 align:middle line:84% And all these same problems exist with FLiBe 01:11:21.350 --> 01:11:24.150 align:middle line:90% in that application as well. 01:11:24.150 --> 01:11:26.910 align:middle line:90% Here's another concept. 01:11:26.910 --> 01:11:28.630 align:middle line:90% What is this concept? 01:11:28.630 --> 01:11:31.170 align:middle line:84% I guess this is-- oh, this is the fast reactor. 01:11:31.170 --> 01:11:37.870 align:middle line:84% So here, this reactor is a pool type reactor. 01:11:37.870 --> 01:11:41.630 align:middle line:84% I think EVR 2 was of this design. 01:11:41.630 --> 01:11:44.330 align:middle line:84% They basically have the fuel sitting in here. 01:11:44.330 --> 01:11:49.340 align:middle line:84% They have a hot liquid, non-boiling fluid 01:11:49.340 --> 01:11:52.260 align:middle line:84% circulating heat around this heat exchanger. 01:11:52.260 --> 01:11:54.260 align:middle line:84% And then they just take the heat out 01:11:54.260 --> 01:11:57.160 align:middle line:90% of this big caldron of liquid. 01:11:57.160 --> 01:11:58.300 align:middle line:90% What is the liquid? 01:11:58.300 --> 01:11:59.967 align:middle line:84% Well, it has to be something with really 01:11:59.967 --> 01:12:03.860 align:middle line:84% good thermal conductivity, and not something that 01:12:03.860 --> 01:12:07.380 align:middle line:84% absorbs neutrons or undergoes nuclear reactions. 01:12:07.380 --> 01:12:14.600 align:middle line:84% So the coolant of choice is sodium metal, which is, 01:12:14.600 --> 01:12:17.380 align:middle line:84% of course-- has anyone thrown sodium into water? 01:12:17.380 --> 01:12:21.820 align:middle line:84% Knows that sodium spontaneously combust in air when water vapor 01:12:21.820 --> 01:12:23.700 align:middle line:90% or when you put it into water. 01:12:23.700 --> 01:12:26.740 align:middle line:84% So you have to then put a cover gas over this thing 01:12:26.740 --> 01:12:29.720 align:middle line:84% to make sure that no water vapor gets in there. 01:12:29.720 --> 01:12:32.220 align:middle line:84% Otherwise this reactor spontaneously 01:12:32.220 --> 01:12:33.280 align:middle line:90% will catch on fire. 01:12:33.280 --> 01:12:36.160 align:middle line:90% 01:12:36.160 --> 01:12:39.040 align:middle line:84% Has anyone heard of Oklo, the startup? 01:12:39.040 --> 01:12:42.380 align:middle line:84% This is the concept that they're now going with. 01:12:42.380 --> 01:12:45.300 align:middle line:84% Of course, we built these reactors, too. 01:12:45.300 --> 01:12:51.420 align:middle line:84% The Russians are still kind of a fan of this reactor concept. 01:12:51.420 --> 01:12:55.420 align:middle line:84% They tend to have a lot of problems because of the sodium. 01:12:55.420 --> 01:12:58.380 align:middle line:84% The Russians have the highest success with it 01:12:58.380 --> 01:13:01.860 align:middle line:84% because every time their reactor catches on fire, 01:13:01.860 --> 01:13:03.780 align:middle line:90% they don't turn off the reactor. 01:13:03.780 --> 01:13:07.093 align:middle line:84% They just fight the fire while the reactor is running, 01:13:07.093 --> 01:13:09.260 align:middle line:84% which I think it's something you could only possibly 01:13:09.260 --> 01:13:11.620 align:middle line:90% do in Russia. 01:13:11.620 --> 01:13:15.040 align:middle line:84% And they have multiple of these cooling systems. 01:13:15.040 --> 01:13:17.260 align:middle line:84% So it has to get a fire over here, 01:13:17.260 --> 01:13:20.020 align:middle line:84% and they have another one over here that they can keep running, 01:13:20.020 --> 01:13:24.020 align:middle line:90% keep cooling the reactor down. 01:13:24.020 --> 01:13:26.260 align:middle line:84% The main advantage of a fast reactor, so this 01:13:26.260 --> 01:13:28.060 align:middle line:84% is a fast reactor, so it doesn't rely 01:13:28.060 --> 01:13:29.900 align:middle line:90% on slowing down the neutrons. 01:13:29.900 --> 01:13:33.780 align:middle line:84% In fact, if the neutrons slow down, you have a problem. 01:13:33.780 --> 01:13:35.103 align:middle line:90% So you have a lot more fuel. 01:13:35.103 --> 01:13:36.520 align:middle line:84% You have the fuel closer together. 01:13:36.520 --> 01:13:38.460 align:middle line:90% You use weapons grade fuel. 01:13:38.460 --> 01:13:40.650 align:middle line:84% But one of the features of doing it 01:13:40.650 --> 01:13:44.610 align:middle line:84% is that you then have a lot of extra neutrons, 01:13:44.610 --> 01:13:49.090 align:middle line:84% because you don't lose them to absorption in your moderator 01:13:49.090 --> 01:13:50.290 align:middle line:90% material. 01:13:50.290 --> 01:13:51.770 align:middle line:90% And so you can do things. 01:13:51.770 --> 01:13:55.690 align:middle line:84% You can put some waste or something else in here, 01:13:55.690 --> 01:14:00.730 align:middle line:84% and you can use those extra neutrons to transmute 01:14:00.730 --> 01:14:03.370 align:middle line:90% one isotope to another isotope. 01:14:03.370 --> 01:14:06.370 align:middle line:84% So one of the ways these reactors operates 01:14:06.370 --> 01:14:09.450 align:middle line:84% are they start out with weapons grade uranium. 01:14:09.450 --> 01:14:14.470 align:middle line:84% You put low enriched uranium or natural uranium, 01:14:14.470 --> 01:14:17.670 align:middle line:84% which contains mostly uranium 238 on the outside. 01:14:17.670 --> 01:14:20.250 align:middle line:84% That's not useful as fuel, but it's 01:14:20.250 --> 01:14:22.290 align:middle line:90% absorbing the extra neutrons. 01:14:22.290 --> 01:14:25.810 align:middle line:84% And for a reaction that I will show you later on in class, 01:14:25.810 --> 01:14:28.530 align:middle line:84% that then turns into plutonium, which you then 01:14:28.530 --> 01:14:33.050 align:middle line:84% extract and use to make new plutonium fuel, which makes 01:14:33.050 --> 01:14:34.890 align:middle line:90% even more neutrons per fission. 01:14:34.890 --> 01:14:40.810 align:middle line:84% And then this thing becomes this fuel making machine, 01:14:40.810 --> 01:14:46.130 align:middle line:84% where it converts non-fuel uranium into fuel plutonium, 01:14:46.130 --> 01:14:50.097 align:middle line:84% and therefore, uses very efficiently the uranium 01:14:50.097 --> 01:14:51.430 align:middle line:90% that you pull out of the ground. 01:14:51.430 --> 01:14:53.910 align:middle line:84% Because most of the time we're doing this-- 01:14:53.910 --> 01:14:56.490 align:middle line:90% 01:14:56.490 --> 01:15:00.050 align:middle line:84% most of the time, we were doing this-- where is it? 01:15:00.050 --> 01:15:01.790 align:middle line:84% We were doing this isotope separation, 01:15:01.790 --> 01:15:05.450 align:middle line:84% where we had to get rid of these grade 238 atoms. 01:15:05.450 --> 01:15:07.810 align:middle line:84% Now instead, we're going to convert them electronically 01:15:07.810 --> 01:15:10.890 align:middle line:90% with extra neutrons into fuel. 01:15:10.890 --> 01:15:13.470 align:middle line:84% And that means you get more uranium utilization. 01:15:13.470 --> 01:15:17.090 align:middle line:84% That would be important if the world had very little uranium 01:15:17.090 --> 01:15:19.170 align:middle line:84% in it, but it turns out not to be the case. 01:15:19.170 --> 01:15:22.170 align:middle line:84% But nevertheless, people still like the idea 01:15:22.170 --> 01:15:24.570 align:middle line:90% of efficient usage of uranium. 01:15:24.570 --> 01:15:27.490 align:middle line:84% But it depends, of course, all on economics. 01:15:27.490 --> 01:15:32.790 align:middle line:84% If this process is not efficient, then economically, 01:15:32.790 --> 01:15:36.530 align:middle line:84% then who cares if it's efficient on a per atom basis? 01:15:36.530 --> 01:15:39.780 align:middle line:84% So we'll talk more about these reactors later on in the class. 01:15:39.780 --> 01:15:42.060 align:middle line:84% But this is also how the thorium reactors work. 01:15:42.060 --> 01:15:44.520 align:middle line:90% Thorium is this gray stuff. 01:15:44.520 --> 01:15:48.320 align:middle line:84% It's not a fuel, but you can convert it into a fuel 01:15:48.320 --> 01:15:51.480 align:middle line:84% if you stick it around the core, you absorb neutrons, 01:15:51.480 --> 01:15:56.077 align:middle line:84% and then you can get uranium 233 out of it and make fuel of that. 01:15:56.077 --> 01:15:57.660 align:middle line:84% So that's how a thorium reactor works. 01:15:57.660 --> 01:16:00.200 align:middle line:84% It's the same way as a fast reactor. 01:16:00.200 --> 01:16:00.700 align:middle line:90% All right. 01:16:00.700 --> 01:16:09.000 align:middle line:84% So that's all I want to say on terms of fission reactors. 01:16:09.000 --> 01:16:12.360 align:middle line:84% We have only a few moments left, so let me just briefly talk 01:16:12.360 --> 01:16:14.120 align:middle line:90% about fusion. 01:16:14.120 --> 01:16:15.840 align:middle line:84% Remember, the key thing about fusion 01:16:15.840 --> 01:16:18.840 align:middle line:84% is that in order to overcome the coulombic repulsion, 01:16:18.840 --> 01:16:21.560 align:middle line:84% we have to get these atoms moving very fast. 01:16:21.560 --> 01:16:25.840 align:middle line:84% And so we need some way to heat them up and keep 01:16:25.840 --> 01:16:27.300 align:middle line:90% them contained in a space. 01:16:27.300 --> 01:16:31.160 align:middle line:84% So lots of different concepts for doing this. 01:16:31.160 --> 01:16:35.920 align:middle line:84% One is that we could put a little pellet inside a chamber, 01:16:35.920 --> 01:16:39.080 align:middle line:84% and then we could shoot a whole bunch of laser beams at it. 01:16:39.080 --> 01:16:43.440 align:middle line:84% And if we get all the laser beams incident on it 01:16:43.440 --> 01:16:48.360 align:middle line:84% symmetrically, then it will be inertially confined, 01:16:48.360 --> 01:16:50.780 align:middle line:90% and heat up and undergo fusion. 01:16:50.780 --> 01:16:53.680 align:middle line:84% And that is what's happening at NIF, the National Ignition 01:16:53.680 --> 01:16:57.920 align:middle line:84% Facility that has operated at Lawrence Livermore National 01:16:57.920 --> 01:16:59.320 align:middle line:90% Laboratory. 01:16:59.320 --> 01:17:01.880 align:middle line:84% Doing this has taken decades of research 01:17:01.880 --> 01:17:05.520 align:middle line:84% because it turns out heating things perfectly symmetrically 01:17:05.520 --> 01:17:07.960 align:middle line:90% is not easy. 01:17:07.960 --> 01:17:11.960 align:middle line:90% So that is NIF. 01:17:11.960 --> 01:17:14.480 align:middle line:90% There are other concepts. 01:17:14.480 --> 01:17:18.960 align:middle line:84% You can heat it using RF energy when you vibrate, 01:17:18.960 --> 01:17:22.040 align:middle line:84% and you try to contain it somehow. 01:17:22.040 --> 01:17:25.960 align:middle line:84% So one way you contain it is that you could put it 01:17:25.960 --> 01:17:31.860 align:middle line:84% in a magnetic field, so that if these are atoms are charged, 01:17:31.860 --> 01:17:34.270 align:middle line:84% then they will circle back on themselves 01:17:34.270 --> 01:17:37.230 align:middle line:90% and stay in one place. 01:17:37.230 --> 01:17:39.053 align:middle line:90% This is how tokamaks work. 01:17:39.053 --> 01:17:40.970 align:middle line:84% And then there are other concepts for heating. 01:17:40.970 --> 01:17:44.550 align:middle line:84% You can shoot neutrons and other neutral beams 01:17:44.550 --> 01:17:46.890 align:middle line:84% in there to kinetically collide with the BT gas, 01:17:46.890 --> 01:17:49.150 align:middle line:84% and try to heat them up through kinetic collision. 01:17:49.150 --> 01:17:51.710 align:middle line:90% These are very inefficient. 01:17:51.710 --> 01:17:56.830 align:middle line:84% This is a photograph of one of the laser rooms at NIF. 01:17:56.830 --> 01:17:58.470 align:middle line:90% These are all lasers. 01:17:58.470 --> 01:18:02.070 align:middle line:84% You see a picture of a guy, and there's just lots, and lots, 01:18:02.070 --> 01:18:04.790 align:middle line:84% and lots of terawatts of laser energy 01:18:04.790 --> 01:18:06.450 align:middle line:90% going and heating these things. 01:18:06.450 --> 01:18:11.640 align:middle line:90% 01:18:11.640 --> 01:18:15.230 align:middle line:84% I don't really have time to talk about Q. 01:18:15.230 --> 01:18:17.630 align:middle line:90% I think I will wrap it up there. 01:18:17.630 --> 01:18:18.810 align:middle line:90% Maybe I'll leave it here. 01:18:18.810 --> 01:18:21.710 align:middle line:84% This just gives you a sense of what actually 01:18:21.710 --> 01:18:23.590 align:middle line:90% happens in the laser version. 01:18:23.590 --> 01:18:26.390 align:middle line:84% This is the amount of energy that goes in. 01:18:26.390 --> 01:18:29.350 align:middle line:84% Some gets scattered, some causes electrons to bounce off 01:18:29.350 --> 01:18:31.510 align:middle line:90% a material from work functions. 01:18:31.510 --> 01:18:34.573 align:middle line:84% You get some plasmas, you get some X-rays. 01:18:34.573 --> 01:18:35.990 align:middle line:84% A lot of it just goes into heating 01:18:35.990 --> 01:18:39.270 align:middle line:84% the walls of the thing that's holding the fuel. 01:18:39.270 --> 01:18:42.830 align:middle line:84% Some of it just leaks out, and then some of it 01:18:42.830 --> 01:18:45.710 align:middle line:84% goes into actually compressing the fuel, 01:18:45.710 --> 01:18:47.810 align:middle line:90% and then you get energy. 01:18:47.810 --> 01:18:49.990 align:middle line:90% So all of these losses-- 01:18:49.990 --> 01:18:53.870 align:middle line:84% fine tuning this process so these losses are controlled 01:18:53.870 --> 01:18:57.550 align:middle line:84% are what was so hard and what took me 01:18:57.550 --> 01:19:00.470 align:middle line:90% so long to be successful at it. 01:19:00.470 --> 01:19:02.250 align:middle line:84% And with that, I think we're out of time. 01:19:02.250 --> 01:19:05.530 align:middle line:84% So that's the 101 on how the technology works. 01:19:05.530 --> 01:19:08.780 align:middle line:84% We'll get back to policy issues next class. 01:19:08.780 --> 01:19:31.000 align:middle line:90%