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MARKUS KLUTE: Welcome
back to 8.701.

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So in this lecture, we
talk about nuclear fission.

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Already seen the process
when we discussed

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empirical mass formula.

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Nuclear fission occurs
in very heavy nuclei,

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as you can see in this plot
here, fission processes,

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and this part of the spectrum.

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But what happens, and what
can happen spontaneously,

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is that the parent nuclei
just simply breaks up

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into daughter nuclei and maybe
some additional neutrons.

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One example is the
decay of uranium 238.

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One can also induce fission.

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Here, is a plot of the nuclear
potential with the Coulomb

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[? law ?] here.

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And for example, you have
this nuclei which sits here--

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and this could be uranium 238--

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and you are able to bring
it above this activation

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and energy.

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Again, this can, in some nuclei,
occur spontaneously, in others,

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it's being induced.

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And then you just
break up the two,

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break up the nuclei into
two daughter particles.

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So if you, for example,
start with a neutron

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and you just bring
it close-- let's

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say you bring a neutron
close to uranium 235.

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This forms uranium 236.

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And the fact that the neutron
is being absorbed excites,

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then, this zero-kinetic
energy neutron

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excite the daughter
compound nucleus.

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There's an excitation
[INAUDIBLE] in this case,

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of 6.5 MeV.

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And because of that, it then
quickly undergoes fission.

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So we basically add its thermal,
or zero-kinetic energy neutron,

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and when it's being
absorbed it immediately

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causes the fission process.

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The fission fragments then
carry away some energy--

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in this example, here,
it's about 180 MeV--

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and additional prompt neutrons.

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And so those additional
prompt neutrons,

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depending on the
specific decay process,

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the number can be varying
between zero and six,

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and for uranium 235, the
average number is 2.5.

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And then the
fragments, they might

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undergo additional decay
process, maybe better decays,

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alpha decays.

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And when they do
that, they can also

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release additional neutrons.

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So interesting, now, is
to see whether or not

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there can be a self-sustained
reaction, a chain reaction.

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And whether or not this
occurs depends on the number

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of neutrons being emitted.

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So if the number of
neutrons produced

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in the n plus first stage
of the fission process

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is greater than the number of
neutrons produced in the nth

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stage of the
process, the process

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is either critical
or super critical,

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which means that it is able
to, because it produces more

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neutrons and it
needs to continue

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to have a fission process,
it will add or create

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a chain reaction.

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If the number is less than
1, the process will die out.

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And this is exactly what's used
in nuclear fission reactors.

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There's several types
of reactors available.

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The example I want
to discuss here

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very briefly is the one
of a thermal reactor which

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uses uranium as fuel
and low-energy neutrons

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to establish the chain
reaction, as we just discussed.

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And so this is a sketch here.

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And the sketch has three
different elements.

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The first one is a fuel element.

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The fuel element can be
naturally-occurring uranium.

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And then, we have a
moderator material.

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And the purpose of
the moderator material

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is to slow the neutrons down.

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So if you really start from
naturally-occurring uranium,

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you might have to find
some sort of material which

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allows you to efficiently
slow down the neutrons

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when they're being emitted
in the fusion process.

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And this could be,
so-called, heavy water,

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where the hydrogen in
water is replaced in part

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that leaves this deuterium.

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But there's other
examples for this as well.

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And then, you have those very
important retractable rods.

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They are materials which
have a large cross-section

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to capture neutrons.

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And so what you can do with
them by mechanically adding them

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or removing them from
this environment,

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is you can remove
additional neutrons.

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So what you're trying to do
is control this number k here,

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control whether or not there's
enough neutrons available

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in order to sustain
the chain reaction.

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And then, the excess energy
is converted in heat.

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You can, for example, heat
up water and then just

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have a turbine run in
order to produce energy.