WEBVTT 00:00:00.000 --> 00:00:02.460 align:middle line:90% [SQUEAKING] 00:00:02.460 --> 00:00:04.420 align:middle line:90% [RUSTLING] 00:00:04.420 --> 00:00:05.400 align:middle line:90% [CLICKING] 00:00:05.400 --> 00:00:10.800 align:middle line:90% 00:00:10.800 --> 00:00:15.140 align:middle line:84% PROFESSOR: Today I just want to review some radiation basics. 00:00:15.140 --> 00:00:21.080 align:middle line:84% Now, it used to be that there was a big pset on calculating 00:00:21.080 --> 00:00:24.600 align:middle line:84% the number of people who die from a nuclear accident 00:00:24.600 --> 00:00:28.300 align:middle line:84% but I have, this semester, taken out the psets. 00:00:28.300 --> 00:00:30.120 align:middle line:84% And so what we will do is we will 00:00:30.120 --> 00:00:36.480 align:middle line:84% do this calculation in class next Wednesday as a whole class. 00:00:36.480 --> 00:00:41.000 align:middle line:84% And this lecture was previously to prepare 00:00:41.000 --> 00:00:46.760 align:middle line:84% you to do the pset because there's a mix of backgrounds. 00:00:46.760 --> 00:00:49.440 align:middle line:84% I think I'm still going to give it. 00:00:49.440 --> 00:00:52.900 align:middle line:84% And for a lot of you, this will be a lot of review. 00:00:52.900 --> 00:00:55.860 align:middle line:84% There might be some new information. 00:00:55.860 --> 00:00:58.300 align:middle line:90% And that's great. 00:00:58.300 --> 00:01:01.230 align:middle line:84% And maybe you learned something, and maybe you know it all. 00:01:01.230 --> 00:01:07.110 align:middle line:84% All right, so let's just have a quick review. 00:01:07.110 --> 00:01:09.030 align:middle line:84% Just to first remind you, there's 00:01:09.030 --> 00:01:12.070 align:middle line:84% something called non-ionizing radiation 00:01:12.070 --> 00:01:13.450 align:middle line:90% versus ionizing radiation. 00:01:13.450 --> 00:01:18.590 align:middle line:84% Non-ionizing is stuff like radio waves and microwaves and so on, 00:01:18.590 --> 00:01:19.662 align:middle line:90% visible light. 00:01:19.662 --> 00:01:21.370 align:middle line:84% And when you get towards the ultraviolet, 00:01:21.370 --> 00:01:23.950 align:middle line:84% the energies of the photons becomes energetic enough 00:01:23.950 --> 00:01:27.210 align:middle line:84% that you can actually liberate electrons from atoms. 00:01:27.210 --> 00:01:28.970 align:middle line:84% And that is what makes it ionizing, 00:01:28.970 --> 00:01:31.870 align:middle line:84% and that's when you start to get radiation effects that break 00:01:31.870 --> 00:01:35.350 align:middle line:90% down DNA and cause cancer. 00:01:35.350 --> 00:01:38.710 align:middle line:84% So photons are where we normally start thinking, 00:01:38.710 --> 00:01:41.557 align:middle line:84% but there's also charged particles. 00:01:41.557 --> 00:01:42.890 align:middle line:90% There's a lot of different ones. 00:01:42.890 --> 00:01:44.765 align:middle line:84% You have alpha particles, which are basically 00:01:44.765 --> 00:01:47.430 align:middle line:90% a helium-4 nucleus, protons. 00:01:47.430 --> 00:01:48.890 align:middle line:84% Muons we don't think of very much, 00:01:48.890 --> 00:01:52.630 align:middle line:90% but they show up in cosmic rays. 00:01:52.630 --> 00:01:56.670 align:middle line:84% And beta particles, which are just basically-- beta plus 00:01:56.670 --> 00:02:00.040 align:middle line:84% and beta minus, either free electrons or free positrons, 00:02:00.040 --> 00:02:07.318 align:middle line:84% positrons being the antimatter analog of an electron. 00:02:07.318 --> 00:02:08.860 align:middle line:84% And you say, well, where in the world 00:02:08.860 --> 00:02:09.985 align:middle line:90% is that going to come from? 00:02:09.985 --> 00:02:13.000 align:middle line:84% And actually you do get it in these reactions. 00:02:13.000 --> 00:02:13.780 align:middle line:90% It does show up. 00:02:13.780 --> 00:02:19.260 align:middle line:84% These positrons, antimatter does show up in these reactions. 00:02:19.260 --> 00:02:22.340 align:middle line:84% So these things are being charged, 00:02:22.340 --> 00:02:25.260 align:middle line:84% have an electric field associated with them. 00:02:25.260 --> 00:02:28.220 align:middle line:84% And as they move through matter, those electric fields 00:02:28.220 --> 00:02:30.620 align:middle line:84% interact with the electric fields of the atoms, 00:02:30.620 --> 00:02:32.120 align:middle line:90% of the electrons. 00:02:32.120 --> 00:02:39.340 align:middle line:84% And that allows them to interact in a variety of ways, typically 00:02:39.340 --> 00:02:43.660 align:middle line:84% off the electron structure, but sometimes off 00:02:43.660 --> 00:02:46.500 align:middle line:90% the nucleus itself. 00:02:46.500 --> 00:02:50.860 align:middle line:84% And then you have charged neutral particles, namely 00:02:50.860 --> 00:02:54.140 align:middle line:84% the neutron, which is, of course, only going 00:02:54.140 --> 00:02:56.210 align:middle line:84% to interact through the strong force 00:02:56.210 --> 00:02:59.450 align:middle line:84% and thus will only interact with the nucleus of the atom. 00:02:59.450 --> 00:03:02.770 align:middle line:84% But when those interactions do happen and they're not very-- 00:03:02.770 --> 00:03:04.810 align:middle line:84% these particles, they fly through matter, 00:03:04.810 --> 00:03:06.553 align:middle line:84% and they're interacting instantaneously. 00:03:06.553 --> 00:03:08.470 align:middle line:84% And they're slowing down and depositing energy 00:03:08.470 --> 00:03:10.830 align:middle line:84% in just the thinnest layers of matter. 00:03:10.830 --> 00:03:13.090 align:middle line:84% But these things can penetrate quite deeply 00:03:13.090 --> 00:03:15.810 align:middle line:90% because they're not charged. 00:03:15.810 --> 00:03:18.570 align:middle line:84% But when they interact, they're very dramatic. 00:03:18.570 --> 00:03:19.250 align:middle line:90% All right. 00:03:19.250 --> 00:03:23.010 align:middle line:84% So you've probably all seen this from high school. 00:03:23.010 --> 00:03:26.390 align:middle line:84% Heavy charged particles get stopped by things like paper. 00:03:26.390 --> 00:03:29.752 align:middle line:84% Lighter, thinner smaller ones can go into your hand. 00:03:29.752 --> 00:03:31.210 align:middle line:84% But they'll be stopped by something 00:03:31.210 --> 00:03:33.230 align:middle line:84% that has a high density of electrons, 00:03:33.230 --> 00:03:37.090 align:middle line:84% so like metal, which has a sea of electrons. 00:03:37.090 --> 00:03:40.890 align:middle line:84% Neutrons are absorbed in water because they scatter off 00:03:40.890 --> 00:03:45.490 align:middle line:84% of like hydrogen. And gamma radiation can easily 00:03:45.490 --> 00:03:46.670 align:middle line:90% penetrate all these things. 00:03:46.670 --> 00:03:52.250 align:middle line:84% And you need wind up basically needing lead as a shield. 00:03:52.250 --> 00:03:53.830 align:middle line:90% It's a very simplified view. 00:03:53.830 --> 00:03:56.330 align:middle line:84% In reality, this is what it looks like. 00:03:56.330 --> 00:04:00.770 align:middle line:84% You have just crazy number of reactions occurring 00:04:00.770 --> 00:04:02.310 align:middle line:90% with matter of all kinds. 00:04:02.310 --> 00:04:05.130 align:middle line:84% Let's just talk about these a little bit more. 00:04:05.130 --> 00:04:11.490 align:middle line:84% So the three principal reactions that dominate 00:04:11.490 --> 00:04:16.010 align:middle line:84% are the photoelectric effect, the Compton effect, 00:04:16.010 --> 00:04:17.490 align:middle line:90% and pair production. 00:04:17.490 --> 00:04:22.170 align:middle line:84% And all of these depend on the energy of the incoming particle 00:04:22.170 --> 00:04:26.570 align:middle line:84% and on the z, or the atomic number of the nucleus. 00:04:26.570 --> 00:04:29.530 align:middle line:84% So the first process there on the top left 00:04:29.530 --> 00:04:31.770 align:middle line:84% is photoelectric effect, and this is usually 00:04:31.770 --> 00:04:34.550 align:middle line:84% a dominant process when you have a low-energy photon. 00:04:34.550 --> 00:04:41.210 align:middle line:84% So the photon is usually below a few hundred keV in energy. 00:04:41.210 --> 00:04:49.590 align:middle line:84% And what it does is it transfers all of its energy to an electron 00:04:49.590 --> 00:04:53.860 align:middle line:84% and kicks out the electron, usually from an inner shell. 00:04:53.860 --> 00:04:56.300 align:middle line:84% And so now you have this high-energy electron, 00:04:56.300 --> 00:04:58.060 align:middle line:84% which is traveling, which is also charged. 00:04:58.060 --> 00:05:02.160 align:middle line:84% And that's going to transfer further energy and so on. 00:05:02.160 --> 00:05:07.840 align:middle line:84% So this is what I think Einstein received his Nobel Prize for. 00:05:07.840 --> 00:05:12.680 align:middle line:84% The probability of this increases dramatically 00:05:12.680 --> 00:05:16.520 align:middle line:84% with atomic number, basically somewhere between z cubed and z 00:05:16.520 --> 00:05:17.960 align:middle line:90% to the fifth. 00:05:17.960 --> 00:05:22.600 align:middle line:84% And it decreases dramatically with the energy of the photon, 00:05:22.600 --> 00:05:25.600 align:middle line:90% about 1 over E to the-- 00:05:25.600 --> 00:05:30.640 align:middle line:84% or E to the minus, dependence on E being energy, obviously. 00:05:30.640 --> 00:05:34.160 align:middle line:84% The Compton effect is the most common effect. 00:05:34.160 --> 00:05:37.420 align:middle line:84% So in the Compton effect, you have a photon coming in. 00:05:37.420 --> 00:05:40.680 align:middle line:84% It looks like it's going to be photoelectric, but actually what 00:05:40.680 --> 00:05:44.480 align:middle line:84% it does is it scatters off of this so-called quasi-free 00:05:44.480 --> 00:05:45.480 align:middle line:90% electron. 00:05:45.480 --> 00:05:48.640 align:middle line:84% And it just gives up some of the energy to the electron 00:05:48.640 --> 00:05:50.200 align:middle line:90% and kicks out the electron. 00:05:50.200 --> 00:05:54.390 align:middle line:84% And then a lower energy photon continues, 00:05:54.390 --> 00:05:56.830 align:middle line:90% so you get this branching. 00:05:56.830 --> 00:06:02.750 align:middle line:84% And this is something that basically leads 00:06:02.750 --> 00:06:04.330 align:middle line:90% to these kinds of showers. 00:06:04.330 --> 00:06:09.710 align:middle line:84% You see these branching events in matter because of this. 00:06:09.710 --> 00:06:12.230 align:middle line:84% So essentially the Compton cross-section is essentially 00:06:12.230 --> 00:06:15.230 align:middle line:84% proportional to the number of atoms-- 00:06:15.230 --> 00:06:17.290 align:middle line:84% sorry, the number of electrons in an atom. 00:06:17.290 --> 00:06:23.870 align:middle line:84% So it basically just scales with the atomic number. 00:06:23.870 --> 00:06:26.950 align:middle line:84% If you have a very high-energy photon, 00:06:26.950 --> 00:06:31.710 align:middle line:84% it can interact with the coulombic field of the nucleus 00:06:31.710 --> 00:06:38.050 align:middle line:84% and split into an electron and a positron. 00:06:38.050 --> 00:06:42.830 align:middle line:84% So it goes from being a mass-free, matter-less thing, 00:06:42.830 --> 00:06:44.750 align:middle line:90% and it creates matter. 00:06:44.750 --> 00:06:47.470 align:middle line:84% And then the positron will touch something else 00:06:47.470 --> 00:06:49.820 align:middle line:84% and then annihilate and create a photon back. 00:06:49.820 --> 00:06:52.020 align:middle line:90% And this is pair production. 00:06:52.020 --> 00:06:57.260 align:middle line:84% To do that, you have to have an incoming photon whose energy is 00:06:57.260 --> 00:07:01.660 align:middle line:84% high enough that it's higher than the rest mass of these two 00:07:01.660 --> 00:07:07.500 align:middle line:84% particles, and that is 1.022 MeV. 00:07:07.500 --> 00:07:09.620 align:middle line:84% The probability of this happening 00:07:09.620 --> 00:07:12.100 align:middle line:84% increases dramatically as the photon energy 00:07:12.100 --> 00:07:15.700 align:middle line:90% goes above 1.02 MeV. 00:07:15.700 --> 00:07:21.100 align:middle line:84% And it increases roughly as z squared. 00:07:21.100 --> 00:07:25.940 align:middle line:84% So this really is something that happens in high-z materials, 00:07:25.940 --> 00:07:28.580 align:middle line:90% gold and so on. 00:07:28.580 --> 00:07:32.180 align:middle line:84% It's not going to be a dominant process in human tissue, 00:07:32.180 --> 00:07:33.500 align:middle line:90% for example. 00:07:33.500 --> 00:07:37.340 align:middle line:84% So these things all combine in this array. 00:07:37.340 --> 00:07:42.860 align:middle line:84% And you get basically two types of behavior. 00:07:42.860 --> 00:07:47.570 align:middle line:84% If you have energies-- if you have radiation 00:07:47.570 --> 00:07:55.090 align:middle line:84% that originates either from gamma radiation or electrons, 00:07:55.090 --> 00:07:58.730 align:middle line:84% you'll get these branching phenomenon, where 00:07:58.730 --> 00:08:01.330 align:middle line:84% you have an interaction, and then the particle 00:08:01.330 --> 00:08:02.710 align:middle line:90% travel some distance and so on. 00:08:02.710 --> 00:08:06.730 align:middle line:84% And we call this low LET, which stands for linear energy 00:08:06.730 --> 00:08:08.290 align:middle line:90% transfer. 00:08:08.290 --> 00:08:10.290 align:middle line:84% And so you'll have these events separated 00:08:10.290 --> 00:08:14.730 align:middle line:84% by hundreds of nanometers, and you get these branches. 00:08:14.730 --> 00:08:18.650 align:middle line:84% If instead you have a particle like a muon 00:08:18.650 --> 00:08:23.170 align:middle line:84% or an alpha particle, which is heavy, it has a large field, 00:08:23.170 --> 00:08:26.610 align:middle line:84% you will get what we call high LET event, radiation 00:08:26.610 --> 00:08:27.190 align:middle line:90% interaction. 00:08:27.190 --> 00:08:30.930 align:middle line:84% So basically this is just like this 00:08:30.930 --> 00:08:33.970 align:middle line:84% has it's such a huge massive particle 00:08:33.970 --> 00:08:35.990 align:middle line:90% and it has so much field. 00:08:35.990 --> 00:08:39.254 align:middle line:84% And it's just slowly interacting, interacting, 00:08:39.254 --> 00:08:43.490 align:middle line:84% interacting and just leaves this wake of destruction. 00:08:43.490 --> 00:08:44.970 align:middle line:90% And it doesn't go very far. 00:08:44.970 --> 00:08:48.320 align:middle line:84% But the damage it does do is very dense. 00:08:48.320 --> 00:08:51.480 align:middle line:84% And so this kind of radiation tends 00:08:51.480 --> 00:08:56.840 align:middle line:84% to lead to more localized cell damage 00:08:56.840 --> 00:09:01.053 align:middle line:84% and is more likely to kill a cell or do DNA damage. 00:09:01.053 --> 00:09:02.720 align:middle line:84% So actually that's very useful if you're 00:09:02.720 --> 00:09:04.520 align:middle line:90% trying to treat cancers. 00:09:04.520 --> 00:09:07.000 align:middle line:84% And you can put that radiation source like that 00:09:07.000 --> 00:09:08.360 align:middle line:90% right next to the tumor. 00:09:08.360 --> 00:09:11.440 align:middle line:84% Then it's really good at not going very far 00:09:11.440 --> 00:09:13.600 align:middle line:84% into healthy tissue and also just really doing 00:09:13.600 --> 00:09:17.960 align:middle line:84% a bad job on the unhealthy tissue. 00:09:17.960 --> 00:09:19.240 align:middle line:90% All right. 00:09:19.240 --> 00:09:25.860 align:middle line:84% So these are obviously basic, different rate of each. 00:09:25.860 --> 00:09:30.880 align:middle line:84% So radiation from the decay of nucleotides, a key thing 00:09:30.880 --> 00:09:33.720 align:middle line:84% to remember is that the activity level 00:09:33.720 --> 00:09:36.440 align:middle line:84% is equal to the rate of decay for the isotope. 00:09:36.440 --> 00:09:38.840 align:middle line:84% And that means that you can specify 00:09:38.840 --> 00:09:41.620 align:middle line:84% these things are all proportional, the activity, 00:09:41.620 --> 00:09:45.870 align:middle line:84% the number of atoms present in the isotope, 00:09:45.870 --> 00:09:48.210 align:middle line:90% and the mass of that isotope. 00:09:48.210 --> 00:09:50.370 align:middle line:84% If someone gives you one-- they say, oh, 00:09:50.370 --> 00:09:54.030 align:middle line:84% there was a 100 becquerel-- that's a measurement 00:09:54.030 --> 00:09:55.350 align:middle line:90% of activity-- 00:09:55.350 --> 00:09:56.470 align:middle line:90% of iodine. 00:09:56.470 --> 00:09:57.870 align:middle line:84% You can calculate either of these 00:09:57.870 --> 00:10:00.650 align:middle line:84% because they're all directly proportional. 00:10:00.650 --> 00:10:02.070 align:middle line:90% Does anyone know how to-- 00:10:02.070 --> 00:10:03.570 align:middle line:84% I assume people know how to do this. 00:10:03.570 --> 00:10:05.770 align:middle line:84% I will do it for if you don't know how to do it. 00:10:05.770 --> 00:10:09.650 align:middle line:84% But basically, you have these equations 00:10:09.650 --> 00:10:11.150 align:middle line:84% that give you the rate change, which 00:10:11.150 --> 00:10:13.990 align:middle line:84% is the activity proportional to the decay constant, 00:10:13.990 --> 00:10:16.190 align:middle line:90% times the number of atoms. 00:10:16.190 --> 00:10:18.130 align:middle line:84% The decay constant is related to the half life 00:10:18.130 --> 00:10:19.590 align:middle line:90% by this relationship. 00:10:19.590 --> 00:10:21.350 align:middle line:84% You should basically know all this. 00:10:21.350 --> 00:10:24.750 align:middle line:84% And then you can just calculate forward. 00:10:24.750 --> 00:10:29.230 align:middle line:84% I think, unless someone has not seen this, don't be shy. 00:10:29.230 --> 00:10:33.670 align:middle line:84% All right, I will just zoom through this. 00:10:33.670 --> 00:10:40.670 align:middle line:84% All right, so we have two principal units for decay. 00:10:40.670 --> 00:10:45.052 align:middle line:84% In case you don't remember, the CGS unit is a decay per second. 00:10:45.052 --> 00:10:46.010 align:middle line:90% We call it a Becquerel. 00:10:46.010 --> 00:10:48.558 align:middle line:84% Sometimes you'll see this older unit called a Curie. 00:10:48.558 --> 00:10:50.350 align:middle line:84% And you have to remember this number, which 00:10:50.350 --> 00:10:55.390 align:middle line:84% is really annoying, 3.7 times 10 to the 10. 00:10:55.390 --> 00:10:56.970 align:middle line:90% Decays per second is a Curie. 00:10:56.970 --> 00:10:58.950 align:middle line:84% So Yes, a Curie is a lot of radiation 00:10:58.950 --> 00:11:01.830 align:middle line:90% compared to a Becquerel. 00:11:01.830 --> 00:11:09.970 align:middle line:84% All right, so sometimes, you might think about-- 00:11:09.970 --> 00:11:12.790 align:middle line:84% so in movies, you often hear people say, oh, it 00:11:12.790 --> 00:11:16.130 align:middle line:90% has a half life of 30 years. 00:11:16.130 --> 00:11:19.550 align:middle line:84% Therefore it won't be safe to be there for 30 years. 00:11:19.550 --> 00:11:23.610 align:middle line:84% No, only half of the atoms are decaying in 30 years. 00:11:23.610 --> 00:11:25.550 align:middle line:84% How long does it take to get safe? 00:11:25.550 --> 00:11:27.850 align:middle line:90% Strictly speaking, almost never. 00:11:27.850 --> 00:11:32.430 align:middle line:84% But a nice couple orders of magnitude reduction 00:11:32.430 --> 00:11:35.330 align:middle line:90% is often practical. 00:11:35.330 --> 00:11:37.710 align:middle line:84% And so 7 half lives is kind of a good rule 00:11:37.710 --> 00:11:40.500 align:middle line:84% of thumb for when you're thinking 00:11:40.500 --> 00:11:43.500 align:middle line:90% of something being irrelevant. 00:11:43.500 --> 00:11:45.980 align:middle line:84% So you're just thinking ahead, about contamination 00:11:45.980 --> 00:11:46.660 align:middle line:90% from fallout. 00:11:46.660 --> 00:11:50.820 align:middle line:84% And you think, oh, cesium-137, big isotope in fallout, 00:11:50.820 --> 00:11:53.140 align:middle line:90% has a half life of 30 years. 00:11:53.140 --> 00:11:56.580 align:middle line:84% How long do I have to worry about it? 00:11:56.580 --> 00:12:00.980 align:middle line:84% Something like 30 times 7, a couple hundred years, 00:12:00.980 --> 00:12:02.900 align:middle line:90% something like that. 00:12:02.900 --> 00:12:06.500 align:middle line:84% All right, so of course, there's a lot 00:12:06.500 --> 00:12:09.580 align:middle line:90% of radiation in the environment. 00:12:09.580 --> 00:12:11.480 align:middle line:84% There's basically, in the United States, 00:12:11.480 --> 00:12:13.280 align:middle line:84% roughly-- this is the gamma dose. 00:12:13.280 --> 00:12:18.220 align:middle line:84% This is measured 1 meter above the ground, from all isotopes. 00:12:18.220 --> 00:12:21.500 align:middle line:84% It comes from-- we call this ground shine. 00:12:21.500 --> 00:12:25.180 align:middle line:84% Ground shine also includes fallout. 00:12:25.180 --> 00:12:26.740 align:middle line:84% So when we do our calculation class, 00:12:26.740 --> 00:12:31.800 align:middle line:84% we will calculate the ground shine from a nuclear accident. 00:12:31.800 --> 00:12:34.380 align:middle line:84% But there's a naturally occurring ground shine. 00:12:34.380 --> 00:12:38.300 align:middle line:84% And this comes from, basically, three sources, uranium 00:12:38.300 --> 00:12:43.250 align:middle line:84% and its daughters, thorium and its daughters, and potassium-40. 00:12:43.250 --> 00:12:45.270 align:middle line:84% And these are the principal things. 00:12:45.270 --> 00:12:47.650 align:middle line:84% So potassium-40 tends to aggregate 00:12:47.650 --> 00:12:56.010 align:middle line:84% in places that have silica, or mica, or salt deposits, 00:12:56.010 --> 00:12:56.850 align:middle line:90% and so on. 00:12:56.850 --> 00:12:59.490 align:middle line:84% And then, you might find uranium and thorium 00:12:59.490 --> 00:13:02.310 align:middle line:84% in things like granite, and so on and so forth. 00:13:02.310 --> 00:13:04.850 align:middle line:90% 00:13:04.850 --> 00:13:12.410 align:middle line:84% So what can I say about all these? 00:13:12.410 --> 00:13:15.850 align:middle line:84% I think I'm going to go through a level-- 00:13:15.850 --> 00:13:18.850 align:middle line:90% this is a level diagram. 00:13:18.850 --> 00:13:21.190 align:middle line:84% But let me-- I'm going to have one a little bit later. 00:13:21.190 --> 00:13:26.170 align:middle line:84% I think I want to talk about that a little bit later. 00:13:26.170 --> 00:13:28.640 align:middle line:84% Basically, here's, just briefly, an example 00:13:28.640 --> 00:13:31.430 align:middle line:84% of potassium-40 decays into argon and calcium. 00:13:31.430 --> 00:13:34.130 align:middle line:90% 00:13:34.130 --> 00:13:37.400 align:middle line:84% Here are the uranium decay chains. 00:13:37.400 --> 00:13:39.720 align:middle line:90% You've probably seen these. 00:13:39.720 --> 00:13:43.820 align:middle line:84% The issue with these is, being a heavy isotope, 00:13:43.820 --> 00:13:47.240 align:middle line:84% it, of course, has a lot of things it can decay into. 00:13:47.240 --> 00:13:50.480 align:middle line:84% Thorium goes to-- starts out as uranium-238, becomes 00:13:50.480 --> 00:13:54.060 align:middle line:84% thorium, protactinium, protactinium, uranium, thorium, 00:13:54.060 --> 00:14:00.880 align:middle line:84% radon, radium, polonium, and so on and so forth, bismuth. 00:14:00.880 --> 00:14:05.560 align:middle line:90% The worst of these is radon. 00:14:05.560 --> 00:14:10.680 align:middle line:84% It has the issue that it has a half life of almost four days, 00:14:10.680 --> 00:14:12.200 align:middle line:90% and it's a gas. 00:14:12.200 --> 00:14:15.320 align:middle line:84% So all this stuff is typically happening underground, 00:14:15.320 --> 00:14:16.880 align:middle line:90% in the rocks. 00:14:16.880 --> 00:14:17.900 align:middle line:90% And it's fine. 00:14:17.900 --> 00:14:23.540 align:middle line:84% But then, it produces this radon, and that seeps out. 00:14:23.540 --> 00:14:25.280 align:middle line:84% And actually, this is responsible 00:14:25.280 --> 00:14:28.240 align:middle line:84% for-- it's the biggest single source of natural radiation 00:14:28.240 --> 00:14:30.440 align:middle line:90% exposure for most Americans. 00:14:30.440 --> 00:14:33.720 align:middle line:84% And it's comparable to the average dose 00:14:33.720 --> 00:14:35.470 align:middle line:84% that an average American receives 00:14:35.470 --> 00:14:38.790 align:middle line:84% from diagnostic medical X-rays and procedures. 00:14:38.790 --> 00:14:40.470 align:middle line:90% It's a big source of dose. 00:14:40.470 --> 00:14:43.270 align:middle line:84% And depending on where you live, and what kind of rock 00:14:43.270 --> 00:14:46.870 align:middle line:84% is under you, and how close it is to the surface, 00:14:46.870 --> 00:14:48.570 align:middle line:90% this can be a big problem. 00:14:48.570 --> 00:14:50.130 align:middle line:90% What happens is it's a gas. 00:14:50.130 --> 00:14:51.310 align:middle line:90% It floats into the air. 00:14:51.310 --> 00:14:52.890 align:middle line:90% It collects in your basement. 00:14:52.890 --> 00:14:53.910 align:middle line:90% You Inhale it. 00:14:53.910 --> 00:14:58.470 align:middle line:84% And then, it decays in your lungs. 00:14:58.470 --> 00:15:01.690 align:middle line:84% And the rest of this decay chain then happens in your lungs. 00:15:01.690 --> 00:15:04.050 align:middle line:90% And that's not a great thing. 00:15:04.050 --> 00:15:08.110 align:middle line:90% 00:15:08.110 --> 00:15:10.710 align:middle line:84% The other thing to point out, I guess, with these 00:15:10.710 --> 00:15:15.310 align:middle line:90% is that-- this is thorium. 00:15:15.310 --> 00:15:16.670 align:middle line:90% Thorium is common. 00:15:16.670 --> 00:15:20.870 align:middle line:84% You find it a lot in different kinds of sand. 00:15:20.870 --> 00:15:22.830 align:middle line:84% It's not such a big thing for Americans 00:15:22.830 --> 00:15:25.130 align:middle line:84% unless you have a brick house or something. 00:15:25.130 --> 00:15:27.830 align:middle line:90% 00:15:27.830 --> 00:15:33.140 align:middle line:84% But it goes to show how the chemistry really matters. 00:15:33.140 --> 00:15:35.040 align:middle line:84% Actually, there's also a radon here. 00:15:35.040 --> 00:15:38.900 align:middle line:84% But this is less of a problem, this one. 00:15:38.900 --> 00:15:41.060 align:middle line:84% The chemistry of these things affect 00:15:41.060 --> 00:15:44.540 align:middle line:84% where they are aggregated, how they are transported 00:15:44.540 --> 00:15:46.420 align:middle line:90% into different kinds of water-- 00:15:46.420 --> 00:15:49.740 align:middle line:84% into different kinds of minerals by water-- and so forth. 00:15:49.740 --> 00:15:51.740 align:middle line:84% And I think this is a good opportunity 00:15:51.740 --> 00:15:55.220 align:middle line:84% to bring up the idea of secular equilibrium. 00:15:55.220 --> 00:15:59.540 align:middle line:84% So if we just look at the half lives, at the top there, 00:15:59.540 --> 00:16:02.300 align:middle line:84% you see that the half life of thorium-232 00:16:02.300 --> 00:16:06.120 align:middle line:84% is 10 to the 10 years, 1.4 times 10 to the 10. 00:16:06.120 --> 00:16:08.060 align:middle line:90% It's a very long time. 00:16:08.060 --> 00:16:12.200 align:middle line:84% And then, everything else is short-- five years, six hours, 00:16:12.200 --> 00:16:16.840 align:middle line:84% one year, three days, 55 seconds, tenth of a second, 00:16:16.840 --> 00:16:21.900 align:middle line:84% 10 hours, 60 minutes, 0.229 microseconds. 00:16:21.900 --> 00:16:25.140 align:middle line:84% So what does this look like in practice? 00:16:25.140 --> 00:16:33.010 align:middle line:84% In practice, you have this reservoir of thorium. 00:16:33.010 --> 00:16:34.890 align:middle line:90% Nothing is happening. 00:16:34.890 --> 00:16:38.570 align:middle line:84% And then, if we just speed up time, 00:16:38.570 --> 00:16:40.195 align:middle line:84% one thorium atom decays, and then goes, 00:16:40.195 --> 00:16:42.237 align:middle line:84% doo, doo, doo, doo, doo, doo, doo, doo, doo, doo. 00:16:42.237 --> 00:16:43.850 align:middle line:84% And you have all these other decays. 00:16:43.850 --> 00:16:47.010 align:middle line:84% So essentially, all of these isotopes 00:16:47.010 --> 00:16:52.570 align:middle line:84% are present with the same rate as the thorium atom. 00:16:52.570 --> 00:16:55.130 align:middle line:84% That's the rate at which they show up. 00:16:55.130 --> 00:16:58.130 align:middle line:84% And you can treat all of these subsequent decays 00:16:58.130 --> 00:17:02.570 align:middle line:84% as basically happening almost simultaneously 00:17:02.570 --> 00:17:05.450 align:middle line:90% with the rate of thorium. 00:17:05.450 --> 00:17:08.290 align:middle line:84% So this is an important factor when you're-- or an important 00:17:08.290 --> 00:17:11.170 align:middle line:84% concept when you're doing calculations of dose. 00:17:11.170 --> 00:17:14.410 align:middle line:84% Because you might have one really long-lived isotope, which 00:17:14.410 --> 00:17:17.339 align:middle line:84% is really controlling everything that's going on. 00:17:17.339 --> 00:17:19.089 align:middle line:84% And so you just want to keep this in mind, 00:17:19.089 --> 00:17:22.450 align:middle line:84% because it can ease the calculations. 00:17:22.450 --> 00:17:26.609 align:middle line:84% You would add up all of these energies 00:17:26.609 --> 00:17:28.850 align:middle line:84% and all of these decay paths, and just 00:17:28.850 --> 00:17:35.090 align:middle line:84% attribute it to that mother isotope decay time. 00:17:35.090 --> 00:17:41.450 align:middle line:84% You can, I guess-- yes, there it is, written out. 00:17:41.450 --> 00:17:46.530 align:middle line:84% You can figure out the exact amount 00:17:46.530 --> 00:17:49.808 align:middle line:84% of every one of these given this-- 00:17:49.808 --> 00:17:51.850 align:middle line:84% think of this as a set of differential equations, 00:17:51.850 --> 00:17:55.730 align:middle line:84% where you have one bucket slowly filling into another bucket, 00:17:55.730 --> 00:17:56.950 align:middle line:90% pouring into another bucket. 00:17:56.950 --> 00:18:00.410 align:middle line:84% And you can solve that bucket for an arbitrary 00:18:00.410 --> 00:18:02.130 align:middle line:90% number of buckets. 00:18:02.130 --> 00:18:04.930 align:middle line:84% And this is called the Bateman equation. 00:18:04.930 --> 00:18:07.450 align:middle line:90% So there it is. 00:18:07.450 --> 00:18:10.930 align:middle line:84% And this will tell you the activity of any given isotope 00:18:10.930 --> 00:18:14.630 align:middle line:90% I at time t for a chain. 00:18:14.630 --> 00:18:17.270 align:middle line:84% And you would follow this-- you have to work out this equation. 00:18:17.270 --> 00:18:19.020 align:middle line:84% But you don't have to write this down now. 00:18:19.020 --> 00:18:21.730 align:middle line:84% If you really want to know this, just remember the name "Bateman 00:18:21.730 --> 00:18:23.410 align:middle line:90% equation," and go look it up. 00:18:23.410 --> 00:18:28.080 align:middle line:84% And there is a closed form way of figuring all this out. 00:18:28.080 --> 00:18:30.160 align:middle line:90% All right? 00:18:30.160 --> 00:18:34.000 align:middle line:84% So secular equilibrium, important concept to just keep 00:18:34.000 --> 00:18:36.520 align:middle line:90% in the back of your head. 00:18:36.520 --> 00:18:41.197 align:middle line:84% All right, so next week, when we do the calculation, 00:18:41.197 --> 00:18:43.780 align:middle line:84% we're not going to be interested in naturally occurring stuff. 00:18:43.780 --> 00:18:46.000 align:middle line:84% We're going to be interested in radiation that 00:18:46.000 --> 00:18:48.120 align:middle line:90% comes from nuclear reactors. 00:18:48.120 --> 00:18:55.985 align:middle line:84% And most of that comes from fission. 00:18:55.985 --> 00:18:57.360 align:middle line:84% Of course, when you have fission, 00:18:57.360 --> 00:19:01.680 align:middle line:84% you have two radioactive nuclear fragments. 00:19:01.680 --> 00:19:05.880 align:middle line:84% And if you remember back to your basics, 00:19:05.880 --> 00:19:11.240 align:middle line:84% this isotope is stabilized by a high number of neutrons 00:19:11.240 --> 00:19:13.000 align:middle line:90% because it's a heavy isotope. 00:19:13.000 --> 00:19:18.120 align:middle line:84% And you need those extra neutrons to hold it together, 00:19:18.120 --> 00:19:22.320 align:middle line:84% more strong force against the coulombic repulsion. 00:19:22.320 --> 00:19:25.020 align:middle line:84% What happens is, now these fragments are smaller, 00:19:25.020 --> 00:19:27.230 align:middle line:90% there's an excess of neutrons. 00:19:27.230 --> 00:19:32.470 align:middle line:84% And so you have, principally, a mode 00:19:32.470 --> 00:19:39.490 align:middle line:84% of decay that is beta radiation, where these neutrons convert-- 00:19:39.490 --> 00:19:42.310 align:middle line:84% they pop and they convert themselves into protons. 00:19:42.310 --> 00:19:48.030 align:middle line:84% And then, the charge balance is maintained 00:19:48.030 --> 00:19:52.590 align:middle line:84% by then ejecting an electron and an antineutrino. 00:19:52.590 --> 00:19:54.770 align:middle line:90% And that is beta decay. 00:19:54.770 --> 00:19:56.670 align:middle line:84% And that's the principal mode of decay 00:19:56.670 --> 00:20:00.910 align:middle line:84% that we really see from these fission products. 00:20:00.910 --> 00:20:03.510 align:middle line:84% So instantaneously, fission is just 00:20:03.510 --> 00:20:06.350 align:middle line:84% giving off a few neutrons, which is obviously radiation. 00:20:06.350 --> 00:20:08.730 align:middle line:84% But it's actually the decay of the fission products, 00:20:08.730 --> 00:20:12.950 align:middle line:84% which happens later, that is the concern for reactors. 00:20:12.950 --> 00:20:18.790 align:middle line:84% There are, in spent fuel, still sources of neutrons. 00:20:18.790 --> 00:20:21.870 align:middle line:84% Some of these fission products have a mode where 00:20:21.870 --> 00:20:24.180 align:middle line:90% they will eject the neutron. 00:20:24.180 --> 00:20:29.180 align:middle line:84% And also, you have neutron capture, where you have 00:20:29.180 --> 00:20:30.780 align:middle line:90% what are called transuranics. 00:20:30.780 --> 00:20:35.020 align:middle line:84% So you start with uranium-235 or 238, and it absorbs a neutron, 00:20:35.020 --> 00:20:36.880 align:middle line:90% and becomes uranium-239. 00:20:36.880 --> 00:20:41.260 align:middle line:84% You can keep absorbing neutrons, and just by luck, 00:20:41.260 --> 00:20:45.900 align:middle line:84% not get fissioned, and be in the spent fuel. 00:20:45.900 --> 00:20:49.220 align:middle line:84% And then, some of those transuranics 00:20:49.220 --> 00:20:51.600 align:middle line:90% want to decay by a neutron. 00:20:51.600 --> 00:20:54.820 align:middle line:84% So you will have a neutron source in spent nuclear fuel 00:20:54.820 --> 00:20:56.820 align:middle line:84% that you have to worry about when you're 00:20:56.820 --> 00:21:00.680 align:middle line:84% talking about dealing with spent nuclear fuel, 00:21:00.680 --> 00:21:04.220 align:middle line:84% not so much, as it turns out, dealing with reactor accidents. 00:21:04.220 --> 00:21:08.380 align:middle line:84% And that's because those heavy isotopes don't typically 00:21:08.380 --> 00:21:11.020 align:middle line:90% come out during the accident. 00:21:11.020 --> 00:21:15.100 align:middle line:84% It's really the lighter fission products. 00:21:15.100 --> 00:21:17.280 align:middle line:84% So which fission products are there? 00:21:17.280 --> 00:21:20.000 align:middle line:84% Well, in principle, almost anything can be there. 00:21:20.000 --> 00:21:21.940 align:middle line:90% Here's the table of isotopes. 00:21:21.940 --> 00:21:25.930 align:middle line:84% And you break up that uranium, and you just wind up 00:21:25.930 --> 00:21:30.330 align:middle line:84% in two spots on the chart, where the number of protons 00:21:30.330 --> 00:21:34.610 align:middle line:84% and neutrons adds up to the number that 00:21:34.610 --> 00:21:38.730 align:middle line:84% is in u-235 minus 2 or 3 neutrons that came out 00:21:38.730 --> 00:21:40.050 align:middle line:90% during fission. 00:21:40.050 --> 00:21:43.810 align:middle line:84% And that makes everything complicated. 00:21:43.810 --> 00:21:47.290 align:middle line:84% Because you have basically every single element 00:21:47.290 --> 00:21:52.710 align:middle line:84% in there, and that makes it a pain to work with. 00:21:52.710 --> 00:21:55.187 align:middle line:84% There is a reliable statistical distribution. 00:21:55.187 --> 00:21:56.270 align:middle line:90% Have you probably seen it? 00:21:56.270 --> 00:21:59.330 align:middle line:84% It looks like an M. I should have put a plot of it 00:21:59.330 --> 00:22:02.290 align:middle line:84% in here, that tells you which isotopes 00:22:02.290 --> 00:22:05.090 align:middle line:90% are more likely to come out. 00:22:05.090 --> 00:22:09.250 align:middle line:84% But basically, you do have to deal with everything. 00:22:09.250 --> 00:22:16.170 align:middle line:84% All right, so let's just look at the situation in what's 00:22:16.170 --> 00:22:18.090 align:middle line:90% going on in the fuel. 00:22:18.090 --> 00:22:24.100 align:middle line:84% So because of this, all of these fission fragments, 00:22:24.100 --> 00:22:31.440 align:middle line:84% which are undergoing decay over time scales of between minutes 00:22:31.440 --> 00:22:35.320 align:middle line:84% and hundreds of years, thousands of years, 00:22:35.320 --> 00:22:39.160 align:middle line:84% we have to keep the cool-- sorry, 00:22:39.160 --> 00:22:46.160 align:middle line:84% keep the fuel cool so it doesn't heat up from the decay and melt. 00:22:46.160 --> 00:22:49.800 align:middle line:84% And so all of this, of course, happens under water. 00:22:49.800 --> 00:22:54.600 align:middle line:84% Here's a picture of a fuel machine unloading a PWR pressure 00:22:54.600 --> 00:22:55.600 align:middle line:90% vessel. 00:22:55.600 --> 00:23:00.160 align:middle line:84% Here is a fuel bundle being inserted into the spent fuel 00:23:00.160 --> 00:23:00.960 align:middle line:90% pond. 00:23:00.960 --> 00:23:05.560 align:middle line:84% These barriers here, which make these little casks, 00:23:05.560 --> 00:23:10.400 align:middle line:84% are neutron-absorbing so that we can squeeze more fuel bundles, 00:23:10.400 --> 00:23:14.960 align:middle line:84% and they won't start reacting with each other. 00:23:14.960 --> 00:23:16.480 align:middle line:84% Most of the blue light in this photo 00:23:16.480 --> 00:23:19.970 align:middle line:84% is just blue because of Rayleigh scattering of the light. 00:23:19.970 --> 00:23:25.350 align:middle line:84% But you can see, right here, the Cherenkov radiation 00:23:25.350 --> 00:23:29.030 align:middle line:84% on this freshly unloaded fuel bundle. 00:23:29.030 --> 00:23:33.190 align:middle line:90% So how spicy are these things? 00:23:33.190 --> 00:23:37.390 align:middle line:84% So here's a good way to understand that. 00:23:37.390 --> 00:23:40.790 align:middle line:84% This is a function of the time after discharge 00:23:40.790 --> 00:23:42.350 align:middle line:90% of a single bundle. 00:23:42.350 --> 00:23:45.090 align:middle line:90% And we have two bundles for you. 00:23:45.090 --> 00:23:49.050 align:middle line:84% The old fuel, 33 megawatt days per kilogram, 00:23:49.050 --> 00:23:53.190 align:middle line:84% that's kind of where we were in the '70s in terms of how far we 00:23:53.190 --> 00:23:54.550 align:middle line:90% would burn the fuel. 00:23:54.550 --> 00:23:57.330 align:middle line:84% Now we burn the fuel much further, 00:23:57.330 --> 00:24:01.050 align:middle line:84% so there's more radiation in it, 50 megawatt days per kilogram. 00:24:01.050 --> 00:24:03.750 align:middle line:90% So that's the solid green line. 00:24:03.750 --> 00:24:07.190 align:middle line:84% And you can just see, immediately, 00:24:07.190 --> 00:24:09.950 align:middle line:84% you have a dose rate of something like 100, 00:24:09.950 --> 00:24:14.630 align:middle line:84% or more than 100, sieverts per hour 1 meter 00:24:14.630 --> 00:24:16.770 align:middle line:84% from the center of the fuel bundle. 00:24:16.770 --> 00:24:21.310 align:middle line:84% So it's like, if the bundle is there, and I'm standing here, 00:24:21.310 --> 00:24:23.990 align:middle line:84% I'm going to get 100 sieverts per hour. 00:24:23.990 --> 00:24:25.990 align:middle line:90% The LD50 for that is-- 00:24:25.990 --> 00:24:28.270 align:middle line:84% I will have a 50% probability of dying 00:24:28.270 --> 00:24:31.390 align:middle line:90% if I stand there for 2 minutes. 00:24:31.390 --> 00:24:35.250 align:middle line:84% But you can see it cools down pretty fast. 00:24:35.250 --> 00:24:38.630 align:middle line:84% It's down to a factor of 10 times cooler 00:24:38.630 --> 00:24:40.950 align:middle line:90% after about 15 years. 00:24:40.950 --> 00:24:45.830 align:middle line:84% And it gets to another 10 times cooler after about 100 years. 00:24:45.830 --> 00:24:47.510 align:middle line:84% But even then, we're still talking 00:24:47.510 --> 00:24:50.830 align:middle line:90% about an LD50 in 3 to 4 hours. 00:24:50.830 --> 00:24:55.870 align:middle line:84% So if I were trying to steal this fuel bundle 00:24:55.870 --> 00:25:00.510 align:middle line:84% and manipulate it, I don't have a good chance of surviving. 00:25:00.510 --> 00:25:02.890 align:middle line:84% No matter what, I'm going to get radiation illness. 00:25:02.890 --> 00:25:05.430 align:middle line:90% 00:25:05.430 --> 00:25:06.650 align:middle line:90% It's a big thing. 00:25:06.650 --> 00:25:11.270 align:middle line:84% And it's actually part of our protection against theft. 00:25:11.270 --> 00:25:14.470 align:middle line:84% The IAEA has this definition of self-protecting. 00:25:14.470 --> 00:25:21.900 align:middle line:84% And I think the self-protecting definition is roughly out here. 00:25:21.900 --> 00:25:25.800 align:middle line:84% So what we don't want, of course, 00:25:25.800 --> 00:25:29.660 align:middle line:84% is to get any of this inside of our body, 00:25:29.660 --> 00:25:34.020 align:middle line:84% because that's really seriously problematic. 00:25:34.020 --> 00:25:36.980 align:middle line:84% And so we don't want any of this being dispersed. 00:25:36.980 --> 00:25:39.980 align:middle line:84% So how does this stuff get dispersed? 00:25:39.980 --> 00:25:43.720 align:middle line:84% Mostly, this happens because the field bundles get too hot, 00:25:43.720 --> 00:25:46.580 align:middle line:84% because they haven't been maintained cool in the pool, 00:25:46.580 --> 00:25:49.740 align:middle line:84% or some event happens, which melts the fuel. 00:25:49.740 --> 00:25:54.380 align:middle line:84% And that causes the isotopes, which are more volatile-- 00:25:54.380 --> 00:25:57.380 align:middle line:84% which is to say they have a lower melting and vaporization 00:25:57.380 --> 00:26:00.120 align:middle line:84% temperature-- to start boiling away. 00:26:00.120 --> 00:26:03.260 align:middle line:84% And then, they vaporize, and they travel in the air. 00:26:03.260 --> 00:26:04.680 align:middle line:84% And this can happen, for example, 00:26:04.680 --> 00:26:08.260 align:middle line:84% if have a fire, various things like that. 00:26:08.260 --> 00:26:16.630 align:middle line:84% So the principal isotopes that we worry about in an accident 00:26:16.630 --> 00:26:24.890 align:middle line:84% are these, cesium-137, americium, cesium-134, 00:26:24.890 --> 00:26:28.050 align:middle line:90% and then these transuranics-- 00:26:28.050 --> 00:26:30.490 align:middle line:84% or americium is a transuranic-- plutonium, americium, 00:26:30.490 --> 00:26:33.070 align:middle line:84% and these other plutonium isotopes. 00:26:33.070 --> 00:26:36.490 align:middle line:84% These are the ones that really contribute 00:26:36.490 --> 00:26:40.090 align:middle line:90% a lot to nuclear accident dose. 00:26:40.090 --> 00:26:42.190 align:middle line:84% There are some other things in here as well, 00:26:42.190 --> 00:26:47.050 align:middle line:84% but they tend to be daughters of these things, bismuth and so on. 00:26:47.050 --> 00:26:50.610 align:middle line:90% Barium is another one. 00:26:50.610 --> 00:26:53.190 align:middle line:84% So when we do our calculation next Wednesday, 00:26:53.190 --> 00:26:58.730 align:middle line:84% we're going to really focus on cesium-137 00:26:58.730 --> 00:27:00.390 align:middle line:90% as the principal isotope. 00:27:00.390 --> 00:27:04.530 align:middle line:84% Because you can see how This is something 00:27:04.530 --> 00:27:07.070 align:middle line:90% that is high for a long time. 00:27:07.070 --> 00:27:09.470 align:middle line:90% This is a log chart. 00:27:09.470 --> 00:27:13.020 align:middle line:84% So big difference if you're a little further up on this chart. 00:27:13.020 --> 00:27:15.080 align:middle line:84% This thing tends to really dominate. 00:27:15.080 --> 00:27:18.680 align:middle line:84% So it's kind of a good first order calculation. 00:27:18.680 --> 00:27:21.560 align:middle line:84% But when you do talk about spent fuel, 00:27:21.560 --> 00:27:25.840 align:middle line:84% and you're talking about hundreds or thousands of years, 00:27:25.840 --> 00:27:28.840 align:middle line:84% then these transuranics begin to matter more. 00:27:28.840 --> 00:27:33.360 align:middle line:84% And that's what matters in the very long run. 00:27:33.360 --> 00:27:39.660 align:middle line:84% All right, so how does this accident occur? 00:27:39.660 --> 00:27:42.040 align:middle line:84% So this is the inside of a reactor. 00:27:42.040 --> 00:27:46.340 align:middle line:84% The pressure vessel, the reactor core is over here. 00:27:46.340 --> 00:27:48.560 align:middle line:84% That's just the edge of the well. 00:27:48.560 --> 00:27:50.280 align:middle line:90% It goes like this. 00:27:50.280 --> 00:27:52.160 align:middle line:84% And the reactor is roughly there. 00:27:52.160 --> 00:27:53.600 align:middle line:84% So the first thing that can happen 00:27:53.600 --> 00:27:56.340 align:middle line:84% is you have some kind of loss of cooling accident. 00:27:56.340 --> 00:28:00.120 align:middle line:84% And inside the core, you have some fuel that is exposed. 00:28:00.120 --> 00:28:03.220 align:middle line:84% And that fuel will then not be locally cooled. 00:28:03.220 --> 00:28:07.120 align:middle line:84% And just from the radiation decay, it will heat up, 00:28:07.120 --> 00:28:10.400 align:middle line:90% and the cladding can blister. 00:28:10.400 --> 00:28:13.217 align:middle line:84% And you can have melting of the fuel, 00:28:13.217 --> 00:28:15.550 align:middle line:84% and-- we talked about this in the last class-- something 00:28:15.550 --> 00:28:18.190 align:middle line:84% called corium, which is basically a molten fuel, 00:28:18.190 --> 00:28:21.290 align:middle line:84% dripping down to the bottom of the pressure vessel. 00:28:21.290 --> 00:28:25.190 align:middle line:90% That's your principal concern. 00:28:25.190 --> 00:28:28.270 align:middle line:84% But we also have to worry about this thing. 00:28:28.270 --> 00:28:31.750 align:middle line:84% This is the spent fuel pool, where all the spent fuel goes. 00:28:31.750 --> 00:28:36.390 align:middle line:84% And there can be decades of fuel in here. 00:28:36.390 --> 00:28:37.930 align:middle line:90% This fuel has been cooling. 00:28:37.930 --> 00:28:40.390 align:middle line:84% This fuel is fresh and extra spicy. 00:28:40.390 --> 00:28:45.110 align:middle line:84% But there's a lot more fuel here than there is over here. 00:28:45.110 --> 00:28:48.630 align:middle line:84% So what happens is you have this machine that pulls out the fuel, 00:28:48.630 --> 00:28:51.390 align:middle line:84% and it transfers it through this sluice gate. 00:28:51.390 --> 00:28:55.130 align:middle line:84% And then it brings it and sticks it into one of these slots, 00:28:55.130 --> 00:28:57.670 align:middle line:90% as I showed you earlier. 00:28:57.670 --> 00:29:01.190 align:middle line:84% And then it sits in this pool for some period of time, 00:29:01.190 --> 00:29:02.250 align:middle line:90% until it's cool enough. 00:29:02.250 --> 00:29:04.670 align:middle line:84% And then you put a concrete cask in here. 00:29:04.670 --> 00:29:07.910 align:middle line:84% And then you transfer the fuel into the cask, seal the cask, 00:29:07.910 --> 00:29:08.910 align:middle line:90% and you can take it out. 00:29:08.910 --> 00:29:11.780 align:middle line:84% And then once it's cool enough, it can go into this cask 00:29:11.780 --> 00:29:13.280 align:middle line:90% and just be air cooled. 00:29:13.280 --> 00:29:17.660 align:middle line:84% And then you stick it outside the reactor on a platform. 00:29:17.660 --> 00:29:22.860 align:middle line:84% So there's about 6.5 meters of water, 20 feet of water, 00:29:22.860 --> 00:29:24.580 align:middle line:90% above this fuel. 00:29:24.580 --> 00:29:26.520 align:middle line:90% And it's there for two reasons. 00:29:26.520 --> 00:29:29.900 align:middle line:84% One is, it's a lot of mass that acts as a biological shield-- it 00:29:29.900 --> 00:29:32.160 align:middle line:84% protects the people who are in the plant-- and two, 00:29:32.160 --> 00:29:36.260 align:middle line:84% obviously, to keep that water cool. 00:29:36.260 --> 00:29:42.360 align:middle line:84% In 1979, Sandia National Laboratory, 00:29:42.360 --> 00:29:44.660 align:middle line:84% which one of our government labs, 00:29:44.660 --> 00:29:46.500 align:middle line:84% realized that one of the scenarios 00:29:46.500 --> 00:29:48.460 align:middle line:84% for a potential reactor accident was 00:29:48.460 --> 00:29:52.220 align:middle line:84% not just what was happening over here in the core, 00:29:52.220 --> 00:29:57.860 align:middle line:84% but that you could have the water leak out of this pool 00:29:57.860 --> 00:29:59.220 align:middle line:90% somehow. 00:29:59.220 --> 00:30:01.780 align:middle line:84% And that could cause the zirconium 00:30:01.780 --> 00:30:05.980 align:middle line:84% clad on the outside of the fuel to heat up to the point 00:30:05.980 --> 00:30:08.910 align:middle line:84% where it would spontaneously ignite. 00:30:08.910 --> 00:30:12.170 align:middle line:84% It would undergo an autocatalytic reaction. 00:30:12.170 --> 00:30:16.570 align:middle line:84% Even if there's some water, and there's steam coming up, 00:30:16.570 --> 00:30:21.010 align:middle line:84% at a certain temperature, it will rip off the oxygen molecule 00:30:21.010 --> 00:30:23.690 align:middle line:84% from the water atoms in the steam 00:30:23.690 --> 00:30:27.970 align:middle line:84% and basically have a runaway oxidation 00:30:27.970 --> 00:30:30.390 align:middle line:90% reaction, which is burning. 00:30:30.390 --> 00:30:34.570 align:middle line:84% So it will spontaneously catch on fire if it gets too hot. 00:30:34.570 --> 00:30:36.810 align:middle line:84% And then, of course, you will boil away 00:30:36.810 --> 00:30:40.350 align:middle line:84% all of these encased radionuclides. 00:30:40.350 --> 00:30:43.930 align:middle line:84% And you could have a very big problem. 00:30:43.930 --> 00:30:46.850 align:middle line:84% That, so far, has never happened. 00:30:46.850 --> 00:30:51.170 align:middle line:84% But it is considered to be a hypothetical concern. 00:30:51.170 --> 00:30:53.810 align:middle line:90% It receives some attention. 00:30:53.810 --> 00:30:56.570 align:middle line:84% But actually, during the Fukushima accident, 00:30:56.570 --> 00:30:59.490 align:middle line:84% we had a loss of offsite power event 00:30:59.490 --> 00:31:04.850 align:middle line:84% that got us pretty close to this particular scenario happening. 00:31:04.850 --> 00:31:08.080 align:middle line:84% So let me just tell you a little bit more of what happened. 00:31:08.080 --> 00:31:12.400 align:middle line:84% Basically, because there was no offsite power to the reactor, 00:31:12.400 --> 00:31:16.640 align:middle line:84% all the systems that regulate the water level 00:31:16.640 --> 00:31:21.040 align:middle line:84% and pump in the water to maintain the safety of this pool 00:31:21.040 --> 00:31:22.520 align:middle line:90% were disabled. 00:31:22.520 --> 00:31:27.360 align:middle line:84% And very slowly, the pool began to heat up. 00:31:27.360 --> 00:31:32.040 align:middle line:84% And you can see, in this photo, this steam coming off 00:31:32.040 --> 00:31:33.680 align:middle line:90% of the reactor pool. 00:31:33.680 --> 00:31:35.920 align:middle line:84% It's boiling the water in the pool 00:31:35.920 --> 00:31:40.200 align:middle line:84% because there's no cool water being circulated into the pool. 00:31:40.200 --> 00:31:44.320 align:middle line:84% This photo was actually taken after TEPCO 00:31:44.320 --> 00:31:49.760 align:middle line:84% was able to deliver fresh emergency water into the pool 00:31:49.760 --> 00:31:50.940 align:middle line:90% using fire trucks. 00:31:50.940 --> 00:31:52.660 align:middle line:84% So they were able to raise the level. 00:31:52.660 --> 00:31:54.520 align:middle line:90% But you can see here-- 00:31:54.520 --> 00:31:58.760 align:middle line:84% you can see kind of see the rust line here. 00:31:58.760 --> 00:32:03.800 align:middle line:84% And so the level is still kind of like 8 to 10 inches below 00:32:03.800 --> 00:32:05.220 align:middle line:90% where it normally was. 00:32:05.220 --> 00:32:10.280 align:middle line:90% 00:32:10.280 --> 00:32:12.280 align:middle line:84% The other thing to see in this photo 00:32:12.280 --> 00:32:15.920 align:middle line:84% is you can see daylight, up here. 00:32:15.920 --> 00:32:23.040 align:middle line:84% Because the reactor building had exploded because of hydrogen. 00:32:23.040 --> 00:32:27.140 align:middle line:84% And so what happened is this radiolysis of water 00:32:27.140 --> 00:32:28.900 align:middle line:90% produces hydrogen gas. 00:32:28.900 --> 00:32:34.320 align:middle line:84% It accumulates in the building, and then it ignites. 00:32:34.320 --> 00:32:37.000 align:middle line:84% And it blew the reactor building apart. 00:32:37.000 --> 00:32:41.240 align:middle line:84% So this spent fuel pool was just open to the outside environment, 00:32:41.240 --> 00:32:43.740 align:middle line:90% and this boiling off the water. 00:32:43.740 --> 00:32:47.640 align:middle line:84% And they were worried that the water 00:32:47.640 --> 00:32:51.240 align:middle line:84% was going to reach the point where the fuel would 00:32:51.240 --> 00:32:52.680 align:middle line:90% become uncovered. 00:32:52.680 --> 00:32:54.840 align:middle line:84% So they kept bringing in fire trucks, 00:32:54.840 --> 00:32:56.942 align:middle line:84% and shooting hoses, and hoping that-- 00:32:56.942 --> 00:32:59.400 align:middle line:84% on the top of the building, and hoping that-- it would rain 00:32:59.400 --> 00:33:02.550 align:middle line:90% into this pool to refill it. 00:33:02.550 --> 00:33:05.750 align:middle line:84% Despite all that effort, actually, they 00:33:05.750 --> 00:33:08.270 align:middle line:84% thought they were hosed, no pun intended. 00:33:08.270 --> 00:33:13.390 align:middle line:84% So this is a chart from the National Academies 00:33:13.390 --> 00:33:15.870 align:middle line:84% that shows, basically, what happened. 00:33:15.870 --> 00:33:20.190 align:middle line:84% The official prediction of the water level was the black line. 00:33:20.190 --> 00:33:23.830 align:middle line:90% So here's the accident. 00:33:23.830 --> 00:33:26.310 align:middle line:84% They thought that the water was basically 00:33:26.310 --> 00:33:29.310 align:middle line:84% evaporating at a constant rate because of the heat. 00:33:29.310 --> 00:33:32.388 align:middle line:84% And then they realized that they need to do something about it, 00:33:32.388 --> 00:33:33.930 align:middle line:84% so they kept bringing in fire trucks. 00:33:33.930 --> 00:33:36.390 align:middle line:84% And they got really desperate, and they kept hosing in. 00:33:36.390 --> 00:33:38.223 align:middle line:84% And they thought they had gotten some water, 00:33:38.223 --> 00:33:40.110 align:middle line:90% but then there were some issues. 00:33:40.110 --> 00:33:42.810 align:middle line:84% And once it reached this red line-- 00:33:42.810 --> 00:33:46.030 align:middle line:90% this is the height of the water. 00:33:46.030 --> 00:33:49.190 align:middle line:84% So 4 meters is the height of the fuel bundle. 00:33:49.190 --> 00:33:53.470 align:middle line:84% The water is at 6.5 meters above that. 00:33:53.470 --> 00:33:57.470 align:middle line:84% And they thought, basically, they were exposing the fuel, 00:33:57.470 --> 00:33:59.390 align:middle line:90% and things were going wrong. 00:33:59.390 --> 00:34:02.620 align:middle line:84% And they were waiting for the fire to start. 00:34:02.620 --> 00:34:05.300 align:middle line:90% That, turns out, didn't happen. 00:34:05.300 --> 00:34:11.900 align:middle line:84% The reality is that this sluice gate 00:34:11.900 --> 00:34:17.659 align:middle line:84% here, that is supposed to seal off this chamber that contains 00:34:17.659 --> 00:34:20.179 align:middle line:84% the reactor pressure vessel, had gotten 00:34:20.179 --> 00:34:22.500 align:middle line:90% damaged in the earthquake. 00:34:22.500 --> 00:34:28.020 align:middle line:84% And as a result, water was leaking from that vessel 00:34:28.020 --> 00:34:31.780 align:middle line:84% into the reactor, cooling-- into the spent fuel pond. 00:34:31.780 --> 00:34:35.340 align:middle line:84% And that is what-- this is the actual water level. 00:34:35.340 --> 00:34:38.500 align:middle line:90% And so they were able to-- 00:34:38.500 --> 00:34:43.820 align:middle line:84% they basically got lucky because of a leak that leaked water 00:34:43.820 --> 00:34:46.300 align:middle line:90% into the spent fuel pool. 00:34:46.300 --> 00:34:49.880 align:middle line:84% You can imagine what would have happened if, in fact, 00:34:49.880 --> 00:34:53.672 align:middle line:84% the leak occurred the other way, and there was a crack somewhere 00:34:53.672 --> 00:34:55.380 align:middle line:84% in the structure of that spent fuel pool, 00:34:55.380 --> 00:34:56.820 align:middle line:90% and it was leaking out. 00:34:56.820 --> 00:34:59.990 align:middle line:84% Then it would have been a much worse situation. 00:34:59.990 --> 00:35:01.210 align:middle line:90% So here you can see-- 00:35:01.210 --> 00:35:03.390 align:middle line:84% these are photographs from the reactor accident. 00:35:03.390 --> 00:35:07.170 align:middle line:84% You can see a bunch of broken concrete sitting on top 00:35:07.170 --> 00:35:09.290 align:middle line:90% of the fuel bundles there. 00:35:09.290 --> 00:35:10.490 align:middle line:90% Here's a top view. 00:35:10.490 --> 00:35:13.530 align:middle line:90% 00:35:13.530 --> 00:35:17.170 align:middle line:84% That's what, basically, the fuel looked like in the pool. 00:35:17.170 --> 00:35:19.110 align:middle line:84% And this is why the leak occurred. 00:35:19.110 --> 00:35:22.570 align:middle line:84% This green thing here is the fuel handling machine 00:35:22.570 --> 00:35:25.830 align:middle line:84% that reaches into the pond and moves the bundles around. 00:35:25.830 --> 00:35:29.430 align:middle line:84% So right here is where the pond is, 00:35:29.430 --> 00:35:31.730 align:middle line:84% and that's the steam coming off the pond. 00:35:31.730 --> 00:35:36.370 align:middle line:90% So we got really lucky. 00:35:36.370 --> 00:35:38.330 align:middle line:84% You could ask, well, how bad would this 00:35:38.330 --> 00:35:45.410 align:middle line:84% have been if this had become uncovered and caught on fire? 00:35:45.410 --> 00:35:48.850 align:middle line:84% So the Nuclear Regulatory Commission did a calculation. 00:35:48.850 --> 00:35:51.410 align:middle line:84% And they said that the radiation releases 00:35:51.410 --> 00:35:56.130 align:middle line:84% would have been about 100 times higher than they were, 00:35:56.130 --> 00:35:59.480 align:middle line:84% which, if you do the math, is about 25 times worse 00:35:59.480 --> 00:36:01.400 align:middle line:90% than Chernobyl. 00:36:01.400 --> 00:36:04.560 align:middle line:84% So even though this is a reactor that 00:36:04.560 --> 00:36:09.000 align:middle line:84% has a containment and everything else, 00:36:09.000 --> 00:36:11.320 align:middle line:84% it still could have been really bad. 00:36:11.320 --> 00:36:15.840 align:middle line:84% And just, we got lucky because of the broken gate, 00:36:15.840 --> 00:36:19.800 align:middle line:84% and no cracks anywhere else in the pool. 00:36:19.800 --> 00:36:25.680 align:middle line:84% So do not take away the idea that Chernobyl was bad 00:36:25.680 --> 00:36:29.600 align:middle line:84% because it was a bad design, and all modern reactors can't 00:36:29.600 --> 00:36:30.900 align:middle line:90% have those types of accidents. 00:36:30.900 --> 00:36:34.360 align:middle line:90% That's not the case. 00:36:34.360 --> 00:36:39.640 align:middle line:84% A really bad accident was basically just luck away. 00:36:39.640 --> 00:36:50.280 align:middle line:84% So let's talk about how you would calculate dose 00:36:50.280 --> 00:36:52.720 align:middle line:90% if such a thing were to occur. 00:36:52.720 --> 00:36:56.930 align:middle line:90% So we talked about these units. 00:36:56.930 --> 00:36:59.470 align:middle line:90% Those are units of decay. 00:36:59.470 --> 00:37:03.990 align:middle line:84% What we want to know for dose is energy deposited per unit mass, 00:37:03.990 --> 00:37:05.510 align:middle line:90% per unit matter. 00:37:05.510 --> 00:37:10.030 align:middle line:84% And so we have these other units of a gray and a rad. 00:37:10.030 --> 00:37:12.910 align:middle line:90% The gray is the kgs unit. 00:37:12.910 --> 00:37:19.710 align:middle line:84% And in the United States, historically, we used CGS units. 00:37:19.710 --> 00:37:21.990 align:middle line:84% They're not imperial units, but it's 00:37:21.990 --> 00:37:25.670 align:middle line:84% ergs per gram, which is CGS version. 00:37:25.670 --> 00:37:33.630 align:middle line:84% And then, you can calculate, essentially, the dose per decay. 00:37:33.630 --> 00:37:35.570 align:middle line:90% And that gives you a dose rate. 00:37:35.570 --> 00:37:38.430 align:middle line:90% So you have your decay rate. 00:37:38.430 --> 00:37:43.510 align:middle line:84% And then, you need some measure of your dose per decay. 00:37:43.510 --> 00:37:47.030 align:middle line:84% And we'll talk about how we get that in a second. 00:37:47.030 --> 00:37:52.950 align:middle line:84% So this energy is not just random kinetic energy 00:37:52.950 --> 00:37:56.780 align:middle line:84% that is just heating things up homogeneously. 00:37:56.780 --> 00:38:00.220 align:middle line:84% This energy is being emitted in these charged particles, which 00:38:00.220 --> 00:38:03.300 align:middle line:84% interact in all those photoelectric and other 00:38:03.300 --> 00:38:05.760 align:middle line:84% phenomenon that I showed you earlier. 00:38:05.760 --> 00:38:09.380 align:middle line:90% So the simple way-- 00:38:09.380 --> 00:38:11.900 align:middle line:84% basically, it's very complex to calculate it exactly, 00:38:11.900 --> 00:38:15.180 align:middle line:84% so we have developed these simple, back of the envelope 00:38:15.180 --> 00:38:16.202 align:middle line:90% ways of doing it. 00:38:16.202 --> 00:38:17.660 align:middle line:84% And what we use is something called 00:38:17.660 --> 00:38:21.260 align:middle line:84% the relative biological effectiveness. 00:38:21.260 --> 00:38:24.220 align:middle line:84% And so what this does is it just attributes 00:38:24.220 --> 00:38:29.780 align:middle line:84% a number that allows us to convert from actual energy 00:38:29.780 --> 00:38:33.260 align:middle line:84% deposited to effective energy deposited 00:38:33.260 --> 00:38:39.760 align:middle line:84% depending on how the radiation interacts with the matter. 00:38:39.760 --> 00:38:44.260 align:middle line:84% So the dose will be based just on the electron energy, 00:38:44.260 --> 00:38:46.060 align:middle line:90% or on the photon energy. 00:38:46.060 --> 00:38:49.820 align:middle line:84% But then, we multiply it by some effectiveness number. 00:38:49.820 --> 00:38:53.340 align:middle line:84% And we have something called equivalent dose, which 00:38:53.340 --> 00:38:56.580 align:middle line:84% has exactly the same units, because we are multiplying it 00:38:56.580 --> 00:38:58.490 align:middle line:90% by a unitless scaling factor. 00:38:58.490 --> 00:39:00.240 align:middle line:84% But it doesn't at all mean the same thing. 00:39:00.240 --> 00:39:01.780 align:middle line:84% Once we're in effective dose, this 00:39:01.780 --> 00:39:06.320 align:middle line:84% is basically a proportionality to dose from photons. 00:39:06.320 --> 00:39:07.120 align:middle line:90% That's the idea. 00:39:07.120 --> 00:39:09.260 align:middle line:84% We'll set photons at 1, and then we'll 00:39:09.260 --> 00:39:11.580 align:middle line:84% have these multiplying factors for everything other 00:39:11.580 --> 00:39:12.740 align:middle line:90% than photons. 00:39:12.740 --> 00:39:15.580 align:middle line:90% And that's what we do. 00:39:15.580 --> 00:39:18.340 align:middle line:84% So generally, electrons are also 1, and then 00:39:18.340 --> 00:39:20.780 align:middle line:84% neutrons of different energy, 5, 10, 20. 00:39:20.780 --> 00:39:22.240 align:middle line:90% You can see how crude this is. 00:39:22.240 --> 00:39:25.980 align:middle line:84% It's very rough-- heavier ions, and so on. 00:39:25.980 --> 00:39:29.040 align:middle line:84% So once we do that, in order to prevent confusion, 00:39:29.040 --> 00:39:30.720 align:middle line:90% we change the name of the unit. 00:39:30.720 --> 00:39:35.740 align:middle line:84% We go from grays, which is literally joules per kilogram, 00:39:35.740 --> 00:39:39.720 align:middle line:84% to sieverts, which has units of joules per kilogram, 00:39:39.720 --> 00:39:42.500 align:middle line:84% but it also contains this multiplier. 00:39:42.500 --> 00:39:46.400 align:middle line:84% So if it's sieverts of neutron dose, in fact, 00:39:46.400 --> 00:39:51.050 align:middle line:84% the actual energy deposited would be a lot less. 00:39:51.050 --> 00:39:54.770 align:middle line:84% And the equivalent one for rad is rem, which 00:39:54.770 --> 00:39:57.690 align:middle line:90% stands for rad equivalent man. 00:39:57.690 --> 00:40:00.010 align:middle line:84% And you have this conversion factor, 00:40:00.010 --> 00:40:05.010 align:middle line:84% because we're going from CGS to KGS, of, basically, 00:40:05.010 --> 00:40:09.750 align:middle line:84% 100 rem is 1 sievert, 100 rad is 1 gray. 00:40:09.750 --> 00:40:12.570 align:middle line:90% 00:40:12.570 --> 00:40:14.010 align:middle line:84% If you can never-- sometimes it's 00:40:14.010 --> 00:40:15.510 align:middle line:90% hard to remember what that is. 00:40:15.510 --> 00:40:18.170 align:middle line:84% So a stupid thing you can remember 00:40:18.170 --> 00:40:21.010 align:middle line:84% is, you should have 100 rad experiences before you're 00:40:21.010 --> 00:40:25.970 align:middle line:84% gray, 1 per year, just to help get your unit 00:40:25.970 --> 00:40:29.930 align:middle line:90% conversion in your mind. 00:40:29.930 --> 00:40:34.010 align:middle line:84% All right, so the average annual dose 00:40:34.010 --> 00:40:35.730 align:middle line:84% that we accumulate from background 00:40:35.730 --> 00:40:38.770 align:middle line:84% is about a millisievert, excluding 00:40:38.770 --> 00:40:43.930 align:middle line:90% radon, which is a big source. 00:40:43.930 --> 00:40:46.530 align:middle line:84% This is a very crude way of doing it. 00:40:46.530 --> 00:40:50.240 align:middle line:84% It's not how you would do it if you worked in a hospital 00:40:50.240 --> 00:40:53.180 align:middle line:84% and you wanted to calculate a dose for a patient 00:40:53.180 --> 00:40:55.680 align:middle line:84% that you were going to give them for some kind of treatment. 00:40:55.680 --> 00:40:58.440 align:middle line:84% In that case, you would do something much more fancy. 00:40:58.440 --> 00:41:01.140 align:middle line:84% You would have a tissue weighting factor. 00:41:01.140 --> 00:41:05.400 align:middle line:84% You would have a dose rate weighting factor. 00:41:05.400 --> 00:41:08.120 align:middle line:84% And you would have all of these adjustments 00:41:08.120 --> 00:41:11.440 align:middle line:84% for how fast the radiation comes in, what tissue. 00:41:11.440 --> 00:41:15.720 align:middle line:84% Different tissues have different vulnerabilities, and so on. 00:41:15.720 --> 00:41:20.080 align:middle line:84% These numbers are very poorly understood. 00:41:20.080 --> 00:41:22.520 align:middle line:84% There's a lot of subjectivity in this. 00:41:22.520 --> 00:41:25.720 align:middle line:84% And basically, this is way too fine of a process 00:41:25.720 --> 00:41:27.580 align:middle line:84% if we're just doing reactor accidents, 00:41:27.580 --> 00:41:30.600 align:middle line:84% and it's just like people being exposed in all different ways. 00:41:30.600 --> 00:41:33.000 align:middle line:84% So we typically don't actually do 00:41:33.000 --> 00:41:37.120 align:middle line:84% this for the types of problems that we're interested in. 00:41:37.120 --> 00:41:40.320 align:middle line:84% All right, let me talk about the level diagram. 00:41:40.320 --> 00:41:44.180 align:middle line:84% All right, so who knows how to read all these things? 00:41:44.180 --> 00:41:47.470 align:middle line:90% 00:41:47.470 --> 00:41:51.150 align:middle line:90% All right, one person. 00:41:51.150 --> 00:41:52.870 align:middle line:90% Let me go through it then. 00:41:52.870 --> 00:41:59.510 align:middle line:84% So this is a level diagram for cesium. 00:41:59.510 --> 00:42:03.550 align:middle line:90% This is the atomic number. 00:42:03.550 --> 00:42:06.030 align:middle line:90% And this is the half life. 00:42:06.030 --> 00:42:13.270 align:middle line:90% A stands for annum, 30.07 years. 00:42:13.270 --> 00:42:15.690 align:middle line:90% It decays via two paths. 00:42:15.690 --> 00:42:18.030 align:middle line:84% Both of them are beta minus, meaning 00:42:18.030 --> 00:42:21.550 align:middle line:90% it gives off an electron. 00:42:21.550 --> 00:42:26.070 align:middle line:84% Beta plus would be giving off a positron. 00:42:26.070 --> 00:42:31.430 align:middle line:84% This path produces an electron with 0.512 MeV. 00:42:31.430 --> 00:42:35.190 align:middle line:84% This path produces an electron with 1.174 MeV. 00:42:35.190 --> 00:42:37.910 align:middle line:84% The probability of it going down this path is that. 00:42:37.910 --> 00:42:40.230 align:middle line:84% Probability of it going down this path is that. 00:42:40.230 --> 00:42:44.110 align:middle line:84% So it's just a roll of the dice depending on where it goes. 00:42:44.110 --> 00:42:45.620 align:middle line:84% If it goes down this path, it turns 00:42:45.620 --> 00:42:51.440 align:middle line:84% into barium-136, atomic number 656, half life of 2.55 minutes. 00:42:51.440 --> 00:42:56.800 align:middle line:84% If it runs up here, then 100%-- well, actually, 85.1%. 00:42:56.800 --> 00:43:00.620 align:middle line:84% I don't know what the rest is-- basically decay by-- 00:43:00.620 --> 00:43:02.020 align:middle line:90% oh, this is actually-- 00:43:02.020 --> 00:43:04.620 align:middle line:90% 00:43:04.620 --> 00:43:07.280 align:middle line:84% I'll have to go double-check what that number is. 00:43:07.280 --> 00:43:11.860 align:middle line:84% But basically, this now decays by gamma, with this energy. 00:43:11.860 --> 00:43:16.860 align:middle line:84% So this energy plus this energy equals that energy. 00:43:16.860 --> 00:43:18.940 align:middle line:84% It all winds up in the same place. 00:43:18.940 --> 00:43:21.540 align:middle line:90% 00:43:21.540 --> 00:43:23.040 align:middle line:90% Those energies, we call them. 00:43:23.040 --> 00:43:23.940 align:middle line:90% Qs. 00:43:23.940 --> 00:43:29.740 align:middle line:90% That's the reaction energy. 00:43:29.740 --> 00:43:33.380 align:middle line:90% So let me ask you a question. 00:43:33.380 --> 00:43:36.740 align:middle line:84% Can I just say, OK, well essentially, they're all 00:43:36.740 --> 00:43:39.460 align:middle line:90% going to wind up as barium-137. 00:43:39.460 --> 00:43:47.050 align:middle line:84% So why don't I just assume that all the decays give 1.174 MeV? 00:43:47.050 --> 00:43:49.030 align:middle line:90% Can I do that? 00:43:49.030 --> 00:43:49.530 align:middle line:90% Why? 00:43:49.530 --> 00:43:57.530 align:middle line:90% 00:43:57.530 --> 00:44:01.150 align:middle line:84% One's a beta, one's an electron, and one is a photon, 00:44:01.150 --> 00:44:02.990 align:middle line:90% but they both have an RBE of 1. 00:44:02.990 --> 00:44:26.530 align:middle line:90% 00:44:26.530 --> 00:44:28.090 align:middle line:90% I start out here. 00:44:28.090 --> 00:44:30.290 align:middle line:84% I either go straight to here, or I go, do, do. 00:44:30.290 --> 00:44:33.970 align:middle line:84% And I have two decays, two particles versus one particle. 00:44:33.970 --> 00:44:36.750 align:middle line:84% But these two particles, the levels add up. 00:44:36.750 --> 00:44:41.680 align:middle line:84% So that level plus that level is the same as this level. 00:44:41.680 --> 00:44:47.435 align:middle line:84% Can I just treat them all as having 1.174? 00:44:47.435 --> 00:44:49.060 align:middle line:84% STUDENT: I'd have to double-check this, 00:44:49.060 --> 00:44:50.810 align:middle line:84% but the lower energy beta probably doesn't 00:44:50.810 --> 00:44:53.420 align:middle line:90% have enough to penetrate that. 00:44:53.420 --> 00:44:54.100 align:middle line:90% Maybe it does. 00:44:54.100 --> 00:44:56.320 align:middle line:90% No, that's 500 [INAUDIBLE]. 00:44:56.320 --> 00:44:59.000 align:middle line:84% R. SCOTT KEMP: Penetration is not really actually considered 00:44:59.000 --> 00:45:01.120 align:middle line:90% at all here. 00:45:01.120 --> 00:45:02.400 align:middle line:90% You still multiply. 00:45:02.400 --> 00:45:04.108 align:middle line:84% And the way you do this is-- you wouldn't 00:45:04.108 --> 00:45:05.608 align:middle line:84% say-- if it touches your skin, if it 00:45:05.608 --> 00:45:08.340 align:middle line:84% was exposed to the outside, then there could be some adjustment. 00:45:08.340 --> 00:45:09.720 align:middle line:84% But let's just say you ingest this. 00:45:09.720 --> 00:45:10.220 align:middle line:90% STUDENT: OK. 00:45:10.220 --> 00:45:11.678 align:middle line:84% R. SCOTT KEMP: Yeah, so penetration 00:45:11.678 --> 00:45:12.620 align:middle line:90% doesn't really matter. 00:45:12.620 --> 00:45:22.572 align:middle line:90% 00:45:22.572 --> 00:45:23.780 align:middle line:90% I'm not going to answer this. 00:45:23.780 --> 00:45:25.113 align:middle line:90% I'm going to wait for an answer. 00:45:25.113 --> 00:45:28.760 align:middle line:90% 00:45:28.760 --> 00:45:29.920 align:middle line:90% All right. 00:45:29.920 --> 00:45:33.743 align:middle line:84% STUDENT: You can treat them as both having the 1.174. 00:45:33.743 --> 00:45:36.160 align:middle line:84% R. SCOTT KEMP: Who agrees that we can treat them the same, 00:45:36.160 --> 00:45:39.360 align:middle line:84% we can treat everything as having 1.174? 00:45:39.360 --> 00:45:41.680 align:middle line:90% All right, who disagrees? 00:45:41.680 --> 00:45:42.975 align:middle line:90% All right, why? 00:45:42.975 --> 00:45:44.600 align:middle line:84% STUDENT: Well, wouldn't it be different 00:45:44.600 --> 00:45:48.353 align:middle line:84% because you have gamma that's on i versus another gamma? 00:45:48.353 --> 00:45:50.020 align:middle line:84% R. SCOTT KEMP: That's what the RBE does. 00:45:50.020 --> 00:45:51.580 align:middle line:90% It's supposed to adjust. 00:45:51.580 --> 00:45:53.560 align:middle line:84% And you see here, the photon and the electron 00:45:53.560 --> 00:45:56.880 align:middle line:84% is both assigned, essentially, unity. 00:45:56.880 --> 00:45:57.580 align:middle line:90% So you're right. 00:45:57.580 --> 00:45:59.460 align:middle line:84% Technically, it's not exactly the same, 00:45:59.460 --> 00:46:02.040 align:middle line:90% but it's almost the same, yeah. 00:46:02.040 --> 00:46:03.180 align:middle line:90% Yeah? 00:46:03.180 --> 00:46:06.920 align:middle line:84% STUDENT: Are we assuming, now, that that 85% is 100%. 00:46:06.920 --> 00:46:10.440 align:middle line:84% R. SCOTT KEMP: Yeah, assume that's 100%, yeah. 00:46:10.440 --> 00:46:13.400 align:middle line:84% Just probably different levels, yeah. 00:46:13.400 --> 00:46:21.200 align:middle line:90% 00:46:21.200 --> 00:46:31.440 align:middle line:84% Antineutrinos-- so this level change is 1.174. 00:46:31.440 --> 00:46:34.480 align:middle line:84% But remember, beta minus decay includes an electron 00:46:34.480 --> 00:46:36.640 align:middle line:90% and an antineutrino. 00:46:36.640 --> 00:46:40.590 align:middle line:84% The antineutrino basically doesn't interact with matter. 00:46:40.590 --> 00:46:46.630 align:middle line:84% And roughly-- we could derive it, but I'm not going to. 00:46:46.630 --> 00:46:53.070 align:middle line:84% Roughly 1/3 of the energy is in the beta minus particle. 00:46:53.070 --> 00:46:57.730 align:middle line:84% So if you were going to do this calculation, you would do. 00:46:57.730 --> 00:47:05.190 align:middle line:90% 00:47:05.190 --> 00:47:33.470 align:middle line:84% 0.054 times 1/3 times 1.17 MeV plus 0.512 times 1/3 plus 0.946 00:47:33.470 --> 00:47:35.330 align:middle line:90% times 1 point-- 00:47:35.330 --> 00:47:49.220 align:middle line:90% 00:47:49.220 --> 00:47:50.620 align:middle line:90% like that. 00:47:50.620 --> 00:47:53.660 align:middle line:84% You would multiply the electron decays by 1/3, 00:47:53.660 --> 00:47:58.260 align:middle line:90% and 100% for the photon decays. 00:47:58.260 --> 00:48:01.860 align:middle line:84% Because the antineutrino is taking away approximately 2/3 00:48:01.860 --> 00:48:02.520 align:middle line:90% of the energy. 00:48:02.520 --> 00:48:05.100 align:middle line:84% So in fact, this decay is a lot less damaging 00:48:05.100 --> 00:48:06.660 align:middle line:90% than this decay path. 00:48:06.660 --> 00:48:11.340 align:middle line:84% And the barium-137m is your enemy. 00:48:11.340 --> 00:48:16.020 align:middle line:90% All right, so don't forget. 00:48:16.020 --> 00:48:18.620 align:middle line:84% You can estimate the beta energy exactly 00:48:18.620 --> 00:48:21.540 align:middle line:84% if you want to, using Fermi theory. 00:48:21.540 --> 00:48:24.660 align:middle line:84% But I assume that this is annoying. 00:48:24.660 --> 00:48:27.420 align:middle line:90% So just use this. 00:48:27.420 --> 00:48:29.240 align:middle line:90% This is good enough, 1/3. 00:48:29.240 --> 00:48:32.500 align:middle line:90% 00:48:32.500 --> 00:48:38.530 align:middle line:84% All right, so cesium-137 is a very big problem. 00:48:38.530 --> 00:48:42.450 align:middle line:84% Another one that we have to worry about is iodine. 00:48:42.450 --> 00:48:45.130 align:middle line:84% Iodine is not typically produced in large quantities 00:48:45.130 --> 00:48:49.290 align:middle line:84% directly from fission products, but it is a decay product 00:48:49.290 --> 00:48:50.810 align:middle line:90% of things that are. 00:48:50.810 --> 00:48:52.730 align:middle line:84% And the reason we worry about iodine 00:48:52.730 --> 00:48:58.410 align:middle line:84% is because when we ingest it, it accumulates in our thyroid. 00:48:58.410 --> 00:49:04.290 align:middle line:84% So it typically comes from either indium or tin, 00:49:04.290 --> 00:49:08.210 align:middle line:90% and there are these decay paths. 00:49:08.210 --> 00:49:11.010 align:middle line:84% Indium has a low melting temperature, 00:49:11.010 --> 00:49:14.050 align:middle line:90% but a very high boiling point. 00:49:14.050 --> 00:49:19.930 align:middle line:84% So it will tend to melt, but not vaporize if there's a fire. 00:49:19.930 --> 00:49:21.470 align:middle line:90% The same is true for tin. 00:49:21.470 --> 00:49:26.810 align:middle line:84% And does anyone remember what Sb is? 00:49:26.810 --> 00:49:28.750 align:middle line:90% What? 00:49:28.750 --> 00:49:32.370 align:middle line:90% Antimony-- wow, that's great. 00:49:32.370 --> 00:49:40.080 align:middle line:84% Yeah, tin and antimony, both are unlikely to vaporize. 00:49:40.080 --> 00:49:47.040 align:middle line:84% But iodine and tellurium-- but iodine vaporizes 00:49:47.040 --> 00:49:52.880 align:middle line:84% at about 184 C, which is like your kitchen oven temperature. 00:49:52.880 --> 00:49:58.840 align:middle line:84% So this stuff all comes boiling out of the reactor. 00:49:58.840 --> 00:50:02.440 align:middle line:84% Even after the accident is over, the fuel 00:50:02.440 --> 00:50:04.640 align:middle line:90% is easily still this hot. 00:50:04.640 --> 00:50:06.760 align:middle line:84% And so this is how-- then it gets 00:50:06.760 --> 00:50:08.360 align:middle line:90% released into the environment. 00:50:08.360 --> 00:50:14.320 align:middle line:84% And you have eight days, 20 hours, and 6.5-hour half lives. 00:50:14.320 --> 00:50:17.060 align:middle line:84% The eight-day stuff is the stuff that's really problematic. 00:50:17.060 --> 00:50:20.840 align:middle line:84% Because eight days is long enough for a plume 00:50:20.840 --> 00:50:24.000 align:middle line:84% to go into the air, to travel through the atmosphere, 00:50:24.000 --> 00:50:26.720 align:middle line:90% and to slowly settle. 00:50:26.720 --> 00:50:29.340 align:middle line:84% And what happens is once it vaporizes, 00:50:29.340 --> 00:50:32.390 align:middle line:84% it cools, but then it forms aerosols. 00:50:32.390 --> 00:50:36.350 align:middle line:84% And then they have a very slow settling based on what-- 00:50:36.350 --> 00:50:38.870 align:middle line:84% you've heard it called the Stokes settling velocity. 00:50:38.870 --> 00:50:41.090 align:middle line:84% So you can calculate, depending on the particle size, 00:50:41.090 --> 00:50:43.110 align:middle line:84% how fast these things will fall to the ground. 00:50:43.110 --> 00:50:45.488 align:middle line:84% And eventually, they'll land on something. 00:50:45.488 --> 00:50:47.030 align:middle line:84% You'll either inhale them, or they'll 00:50:47.030 --> 00:50:48.670 align:middle line:90% land on something you eat. 00:50:48.670 --> 00:50:53.470 align:middle line:84% And because this thing stays around eight days, 00:50:53.470 --> 00:50:56.030 align:middle line:84% there's a good chance, in the immediate aftermath, 00:50:56.030 --> 00:51:00.590 align:middle line:84% that you might ingest some of this iodine. 00:51:00.590 --> 00:51:01.750 align:middle line:90% And that becomes a problem. 00:51:01.750 --> 00:51:05.270 align:middle line:84% Because then it decays inside you. 00:51:05.270 --> 00:51:10.670 align:middle line:84% Once it decays, it typically decays to xenon. 00:51:10.670 --> 00:51:14.910 align:middle line:84% The long-lived xenon isotopes actually are less problematic, 00:51:14.910 --> 00:51:16.430 align:middle line:84% because they're noble gases, and you 00:51:16.430 --> 00:51:19.130 align:middle line:84% have a high chance of them escaping from your body. 00:51:19.130 --> 00:51:22.450 align:middle line:84% But some of these shorter-lived xenons, 00:51:22.450 --> 00:51:26.110 align:middle line:84% whether that atom will escape within 15 minutes, 00:51:26.110 --> 00:51:28.390 align:middle line:90% probably unlikely. 00:51:28.390 --> 00:51:33.740 align:middle line:90% So this is the other source of-- 00:51:33.740 --> 00:51:36.420 align:middle line:90% big source of dose after cesium. 00:51:36.420 --> 00:51:38.020 align:middle line:84% This is really only something we worry 00:51:38.020 --> 00:51:41.740 align:middle line:84% about in the immediate aftermath of an accident. 00:51:41.740 --> 00:51:43.720 align:middle line:84% We don't worry about it when we do long-term. 00:51:43.720 --> 00:51:48.260 align:middle line:84% So we will do an adjustment next Wednesday, for iodine. 00:51:48.260 --> 00:51:51.460 align:middle line:84% But most of the calculation we're going to focus on 00:51:51.460 --> 00:51:54.700 align:middle line:90% is cesium. 00:51:54.700 --> 00:51:59.140 align:middle line:84% That's basically all I have in the review. 00:51:59.140 --> 00:52:00.800 align:middle line:84% Since we have a little bit of time, 00:52:00.800 --> 00:52:04.320 align:middle line:84% I will just show you a couple of things. 00:52:04.320 --> 00:52:07.440 align:middle line:84% So here, if we go back to remembering low-LET radiation, 00:52:07.440 --> 00:52:09.260 align:middle line:84% which is like photons and electrons, 00:52:09.260 --> 00:52:11.933 align:middle line:84% you're going to have this kind of behavior, 00:52:11.933 --> 00:52:13.600 align:middle line:84% where you have some kind of interaction, 00:52:13.600 --> 00:52:16.500 align:middle line:84% and then the particle can travel some distance before it 00:52:16.500 --> 00:52:18.620 align:middle line:90% reaches another interaction. 00:52:18.620 --> 00:52:25.420 align:middle line:84% That distance is large compared to the size of your DNA strands, 00:52:25.420 --> 00:52:28.540 align:middle line:84% which are wrapped onto these things called histones. 00:52:28.540 --> 00:52:32.300 align:middle line:84% And so the DNA is coiled into this coil of coils. 00:52:32.300 --> 00:52:36.300 align:middle line:84% But the high-LET, like alpha particles, 00:52:36.300 --> 00:52:39.980 align:middle line:84% they're just interacting all over the place. 00:52:39.980 --> 00:52:45.165 align:middle line:84% And so you have a high probability of getting what are 00:52:45.165 --> 00:52:47.540 align:middle line:84% called double-strand breaks, which I'll talk a little bit 00:52:47.540 --> 00:52:52.740 align:middle line:84% more about next class, where you basically-- 00:52:52.740 --> 00:52:56.900 align:middle line:84% a single-strand break is where you have-- 00:52:56.900 --> 00:52:58.160 align:middle line:90% there's your DNA. 00:52:58.160 --> 00:53:01.940 align:middle line:90% 00:53:01.940 --> 00:53:04.280 align:middle line:90% And you have all these bases. 00:53:04.280 --> 00:53:06.100 align:middle line:90% So this is your sugar backbone. 00:53:06.100 --> 00:53:07.580 align:middle line:84% So typically, what will happen is 00:53:07.580 --> 00:53:09.700 align:middle line:84% you'll have some reaction come in, 00:53:09.700 --> 00:53:12.580 align:middle line:84% and it'll interact with an atom here. 00:53:12.580 --> 00:53:15.220 align:middle line:90% And it will lyse that. 00:53:15.220 --> 00:53:18.980 align:middle line:84% And then, your body has a process to repair that. 00:53:18.980 --> 00:53:22.100 align:middle line:84% But if you have high-LET radiation, 00:53:22.100 --> 00:53:24.680 align:middle line:84% it's interacting, interacting, interacting, interacting, 00:53:24.680 --> 00:53:26.250 align:middle line:90% interacting, interacting. 00:53:26.250 --> 00:53:29.690 align:middle line:84% And so you can get cuts in multiple places. 00:53:29.690 --> 00:53:33.330 align:middle line:84% And then you can get a bigger, more dramatic separation 00:53:33.330 --> 00:53:34.410 align:middle line:90% of the DNA. 00:53:34.410 --> 00:53:37.890 align:middle line:84% And that typically results in a bigger problem. 00:53:37.890 --> 00:53:40.330 align:middle line:84% The problems don't arise from the cutting 00:53:40.330 --> 00:53:42.450 align:middle line:90% of the DNA themselves. 00:53:42.450 --> 00:53:46.250 align:middle line:84% The problems arise from the DNA repair. 00:53:46.250 --> 00:53:52.630 align:middle line:84% And that's a really, really, really important thing to know. 00:53:52.630 --> 00:53:54.850 align:middle line:84% It's, in fact, the misrepair of DNA 00:53:54.850 --> 00:53:57.030 align:middle line:84% that leads to mutations, aberrations, 00:53:57.030 --> 00:53:59.850 align:middle line:84% genomic instability, and cell death. 00:53:59.850 --> 00:54:04.550 align:middle line:84% So we'll talk a little bit about this in the next class. 00:54:04.550 --> 00:54:07.090 align:middle line:90% 00:54:07.090 --> 00:54:13.310 align:middle line:84% The fact that your body heals itself does not fix the problem, 00:54:13.310 --> 00:54:16.490 align:middle line:90% it is the problem. 00:54:16.490 --> 00:54:18.690 align:middle line:84% Otherwise, if you just slice the genome, 00:54:18.690 --> 00:54:20.850 align:middle line:84% and the DNA did not repair itself, 00:54:20.850 --> 00:54:23.570 align:middle line:90% the cell dies, and that's it. 00:54:23.570 --> 00:54:29.280 align:middle line:84% It either just dies, or becomes senescent and doesn't replicate. 00:54:29.280 --> 00:54:32.920 align:middle line:84% It's only when the DNA repairs itself, basically, 00:54:32.920 --> 00:54:35.000 align:middle line:90% that it's an issue. 00:54:35.000 --> 00:54:37.480 align:middle line:84% All right, I'm going to stop there 00:54:37.480 --> 00:54:41.920 align:middle line:84% because I won't have content for the next class. 00:54:41.920 --> 00:54:48.000 align:middle line:84% But we will pick up on this talk about dose response models, 00:54:48.000 --> 00:54:51.040 align:middle line:84% how to estimate the probability of cancer given 00:54:51.040 --> 00:54:54.630 align:middle line:90% some dose in the next class. 00:54:54.630 --> 00:55:23.000 align:middle line:90%