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>> Thorium is a kind of miraculous element.
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Thorium found in nature isn't fissile.
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The atom's nucleus won't split when it
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absorbs a neutron. And yet, if you put a
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chunk of this same thorium in a special
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nuclear reactor, after a while, most of
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the thorium will be gone. A whole bunch
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of energy will have been generated, and
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you'll be left with typical byproducts
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of fission. It's as if thorium is
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fissile, even though it's not. This is
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the genius of thorium breeder reactors.
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Oh, and I should disclose here that this
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video is sponsored by Copenhagen Atomic,
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who are working to make thorium power a
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reality. But they didn't get any say in
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the video and didn't get to review it
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before posting. The standard
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oversimplified picture of a fission
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reactor is a uranium nucleus splits
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apart, or fissions, releasing heat
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energy and two or three neutrons. And
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those neutrons go on to be captured by
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more uranium nuclei and cause them to
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fission, releasing more heat energy and
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more neutrons, and so on. The heat is
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used to generate electricity, the
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neutrons to maintain the fission chain
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reaction. However, the actual story is
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more complicated. When a nucleus splits,
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there are actually four things that can
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happen to the neutrons it emits. One,
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like we've already mentioned, they can
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be captured by a fissile atom like
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causing it to fission and release more
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neutrons. And this part has to happen on
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average at least once per fission to
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sustain the chain reaction. Two,
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neutrons can be captured by the nuclei
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of other atoms in the reactor without
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causing fission, like maybe the metal
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case, or the moderator, or the control
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rods, or whatever. Three, [music]
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neutrons can escape and leave the
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reactor entirely. Or four, a neutron can
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be captured by an atom that's not
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fissile and transmute it into an atom
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that is fissile. Because remember, these
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are atomic nuclei we're dealing with.
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Absorption of a neutron will turn
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uranium 238 into uranium 239. The
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[music] number is just the total number
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of protons and neutrons. And uranium 239
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can then radioactively decay into
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[music] neptunium 239, which can then
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decay into plutonium 239, which is
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fissile. If the capture of a neutron
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transforms a non-fissile element into a
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fissile one, it's called a fertile
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capture. And fertile capture is what
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makes thorium [music] useful. In fact,
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even in a normal uranium reactor,
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fertile capture accounts for over a
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third of the energy generated by the
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reactor. A normal nuclear reactor uses
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uranium 235, which is fissile. But
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naturally occurring uranium ore contains
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only 0.7% uranium 235. Almost all the
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rest is uranium 238, which is
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essentially non-fissile, but it is
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fertile. Even when using fuels with
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enriched levels of uranium 235
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undergoing fission, there's so much
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non-fissile U-238 around that some of
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the chain reaction neutrons instead
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transform U-238 into plutonium 239,
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which can then [music] fission. But
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U-235 doesn't make enough neutrons, and
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U-238 doesn't turn into plutonium easily
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enough that you can both sustain a
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fission chain reaction and continue to
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transform new fissile fuel. So at the
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end you're left with a big chunk of
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unfissioned but still full of
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radioactive waste uranium 238. There's a
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different kind of reactor called a fast
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breeder reactor that uses plutonium as
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the primary fissile fuel and uranium 238
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as a fertile secondary source. This
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combination can both sustain the fission
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chain reaction and transform new fuel in
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a self-sustaining way. But fast breeder
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reactors are less researched, more
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expensive, and harder to run effectively
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for now. This is where thorium comes in.
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The same route used in the
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transformation of uranium 238 [music] to
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plutonium 239 can be replicated down
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here starting instead with thorium 232.
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By adding a neutron, we get thorium 233,
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which decays to protactinium 233, which
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decays to uranium 233, which is fissile
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and can be used to generate energy. So,
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if you load your reactor with thorium
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232, which remember is not fissile, and
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you throw in some starter fissile fuel,
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then for each fission reaction the
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number of new fissile atoms created is
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more than one on average, and the number
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of new atoms split is more than one on
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average, and remember those atoms give
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you more neutrons. So the transformation
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of thorium and the fission of uranium
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can keep going and going and going until
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in principle all of the thorium is gone.
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And crucially, thorium transformation
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can happen in a reactor that doesn't
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have the same challenges as a fast
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breeder reactor. And it gets rid of most
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of the long-lived radioactive waste.
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[music] And thorium is more abundant
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than uranium and doesn't need the
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expensive refining process to
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concentrate the fissile uranium 235. And
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so you can see why people get excited
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about thorium. There are of course
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challenges and downsides to making
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thorium reactors, which is why we don't
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have and so far have never had
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commercial energy generation from
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thorium. But that's fertile material for
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Making commercial power from thorium may
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soon be possible thanks to the work of
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organizations such as this video's
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sponsor, Copenhagen Atomics. Copenhagen
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Atomics is building compact modular
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thorium reactors to produce cheap
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energy. Unlike traditional nuclear power
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stations, which are giant infrastructure
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projects, Copenhagen Atomics are
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designing a self-contained reactor unit
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that can fit inside a shipping
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container. The reactors are based on a
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design pioneered over 50 years ago that
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uses molten salt to carry the fuel,
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resulting in fewer lost neutrons and
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more complete combustion. So you get
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more energy for less waste. These
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reactors can also use plutonium waste
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from classic nuclear reactors as fuel,
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extracting 10 times more energy out of
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spent nuclear fuel than the initial
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reactor did in the first place. And in
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doing so, converting long-lived
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radioactive waste into short-lived
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radioactive waste. In theory, these
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reactors could run anything from grids
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to ships to moon bases. Check out
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Copenhagen Atomics' website to learn
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more about their work.
