PHYS 273 Energy and Environment ©
Lesson 8 Transcript: Introduction to Nuclear Energy
In this lecture, we're going to look at nuclear reactions, the basic mechanisms of nuclear
fission reactions, and the energy that comes out of that. So, a very brief bit of history
here. So, in the late 30s, 38 and 39, the physicists Hahn, Strassman, Meitner, and
Frisch discovered, while they were looking at uranium nuclei, that they did a remarkable
thing, which was when the nuclei were bombarded with neutrons, so you fire a bunch of
neutrons at these nuclei, the nuclei fission, so as they split into two fragments, at the
same time, so they split into two fragments and more neutrons were emitted. Also, the
fragments had a certain amount of energy. So it's 160 mega electron volts of kinetic
energy, which is divided between the two. And so there was some energy, there was
neutrons, and these heavy nuclei were splitting into two fragments. So we're not going
to look in detail too much into the specifics of nuclear physics, but I'm going to look at
the reactions and kind of the general outline of how they work. So, we're going to start
with a nucleus of uranium 235. So that's this here and then, I mentioned already, this
uranium is being bombarded by neutrons. So, uranium is the element 235, it means
there's a total of 235 neutrons and protons in this nucleus. Then, when a neutron
interacts with it, it becomes uranium 236, which is an unstable nucleus. So unstable
means that it spontaneously turns into something else. In this case, what happens is the
unstable uranium 236 almost immediately breaks into two smaller nuclei. So, these are,
in this case barium and krypton. And so what I get is, I have uranium 235 to start with,
the neutron uranium 236, then this fissions into barium and krypton, and then what I
have is gamma rays, which are high-energy light, and more neutrons. Let's look a little
more detail here. Now let's look at the results from this reaction, so as I said, they're
gamma rays, two heavy nuclei, krypton and barium, and neutrons. So, first looking at
the gamma rays. So, these gamma rays, as I mentioned, are photons, or high-energy
light, they carry some energy with them and this needs to be accounted for in the design
of the reactor. So, this is not mostly how we're getting the energy out of a reactor the
useful energy anyway, but we have these high-energy gamma rays, I don't want them
coming out of my reactor. So, it needs to have shielding to contain these high-energy
gamma rays, or this high-energy light, or high-energy photons. That's all saying the
same thing. The next product from this reaction to consider are the neutrons. So I also
have a number of neutrons coming out of this process and so what I get is, I started
remember with uranium 235, I needed a high energy neutron to start this reaction, and
now what I have is all my products, the krypton, the barium, the gamma rays, and then
more neutrons, and so this causes a chain reaction. If I have more neutrons coming out,
then those neutrons can be incident on other uranium 235 nuclei and caused this to
happen again. So, this is how a chain reaction works and this chain reaction has to be
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regulated. In a nuclear reactor, there's neutron absorbers that if they're inserted into the
reaction, they'll absorb the neutrons stopping this chain reaction, if they come out then
the chain reaction can grow, and if there's not enough neutron-absorbing material, then
this chain reaction can grow out of hand. The final of these products I want to talk about
are the barium and krypton nuclei. This is actually where we're getting our useful energy
that we used to produce electricity later in our nuclear reactor. And so about 85% of the
energy released in this reaction is in the form of kinetic energy of the krypton and
barium nuclei. So, they're moving, they have kinetic energy; eventually, they come to
rest and they do this by colliding with other particles. What this does is it gives them
kinetic energy, so essentially what I'm doing is I'm taking their kinetic energy and giving
that into thermal energy of the particles, and so this thermal energy is what we use later
for power generation. Let's consider where this energy comes from. So, if I look at my
initial thing, this is the uranium 236, which fission into barium, krypton, gamma rays and
three neutrons. So, if I look at the mass of the uranium 236 and compare this with the
masses of the barium nucleus, the krypton nucleus, and these three neutrons, the
gamma rays have no mass, so I add up the mass of all these and compare it to the
mass of my original uranium 236, what I get is the uranium 236 has more mass than
these final products. So, there's some missing mass. Now, the missing mass is not very
much, it's about that many kilograms. So a very, very, very small amount. But we've all
seen this famous E equals MC squared equation, we talked about this a little bit when
we first introduced energy, and what this tells me is that mass can be converted directly
into energy. So this is what's happening, this missing mass, the mass that disappears, I
can put here, multiply it by the speed of light squared and that's the amount of energy
which we get. So, I'm losing mass and converting that mass into energy. There's
missing mass and turn into kinetic energy of these two, mostly has been turned into the
kinetic energy of these two products, and that is that 160 mega electron volts of energy,
what it's saying is that, if I lose this much, sorry, if I lose this much mass, it's this many
joules of energy, which is equivalent to the 160 mega electron volts of energy we talked
about, at the beginning. It seems like not very much, right? It's this many joules is
00000, however, many joules of energy, which doesn't seem like a lot. However, if we
compare it to chemical reactions, like if I were to burn fossil fuels, for example, then this
is actually about 100 million times less energy per atom, then this nuclear energy. So, in
fact, nuclear energy, the E equals MC squared energy, is much, much more than the
energy you get when you break chemical bonds in a chemical reaction. Here, we're
going to follow the energy flow in a power plant. So, what we start with is we have the
nuclear energy, so we've talked about that, and this is in the core of the reactor here,
then that nuclear energy, so this is E equals MC squared energy, as we mentioned, is
turned into the kinetic energy of the fragments, then those fragments stop and they
transfer their energy into heat essentially, into thermal energy. So, what I have is some
thermal energy, essentially, this is heating up and then I have this heat exchanger. So I
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have this hot coil here from the core, and then I have some fluid here that passes
through, heats it up, and turns into steam, and so what I have is this hot steam, that I've
gotten the thermal energy from the reactor core here, then that steam fires a turbine.
So, it turns this turbine, so I have kinetic energy of the turbine, which then in turn, turns
an electric generator and gives me electrical energy. My flow is nuclear energy, to
kinetic energy of the fragments, to the thermal energy, to kinetic energy of the
generator, to electrical energy. So finally, I just want to talk a little bit about the resource
itself. So, uranium ore is found in the earth, it's mined, it's mostly in the form of an oxide
u308, or called yellowcake or yellowcake uranium. The deposits of uranium, or the
known resources are in order of how much there is in of the known resources Australia,
Kazakhstan, and then Canada and Russia about the same. So Australia has the most
by a pretty sizable chunk, about 29% of the world's total. Canada, Russia, both with
about 9% of the world's total. Now uranium is mined as I mentioned, but it's very dense
in energy, like a small amount of uranium has much, much more energy than the same
amount of coal, for example, and so the mining required and the drawbacks associated
with mining are smaller because it's less amount of mining per amount of energy.
Nuclear energy is the only fundamentally new source of energy in more than 100 years.
So fossil fuels, wind, all of that stuff has been used before but this is a new form of
energy.
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