The Magic of Thorium Nuclear Reactors

[music]

>> Thorium is a kind of miraculous element.

Thorium found in nature isn't fissile.

The atom's nucleus won't split when it

absorbs a neutron. And yet, if you put a

chunk of this same thorium in a special

nuclear reactor, after a while, most of

the thorium will be gone. A whole bunch

of energy will have been generated, and

you'll be left with typical byproducts

of fission. It's as if thorium is

fissile, even though it's not. This is

the genius of thorium breeder reactors.

Oh, and I should disclose here that this

video is sponsored by Copenhagen Atomic,

who are working to make thorium power a

reality. But they didn't get any say in

the video and didn't get to review it

before posting. The standard

oversimplified picture of a fission

reactor is a uranium nucleus splits

apart, or fissions, releasing heat

energy and two or three neutrons. And

those neutrons go on to be captured by

more uranium nuclei and cause them to

fission, releasing more heat energy and

more neutrons, and so on. The heat is

used to generate electricity, the

neutrons to maintain the fission chain

reaction. However, the actual story is

more complicated. When a nucleus splits,

there are actually four things that can

happen to the neutrons it emits. One,

like we've already mentioned, they can

be captured by a fissile atom like

uranium 235, [music]

causing it to fission and release more

neutrons. And this part has to happen on

average at least once per fission to

sustain the chain reaction. Two,

neutrons can be captured by the nuclei

of other atoms in the reactor without

causing fission, like maybe the metal

case, or the moderator, or the control

rods, or whatever. Three, [music]

neutrons can escape and leave the

reactor entirely. Or four, a neutron can

be captured by an atom that's not

fissile and transmute it into an atom

that is fissile. Because remember, these

are atomic nuclei we're dealing with.

Absorption of a neutron will turn

uranium 238 into uranium 239. The

[music] number is just the total number

of protons and neutrons. And uranium 239

can then radioactively decay into

[music] neptunium 239, which can then

decay into plutonium 239, which is

fissile. If the capture of a neutron

transforms a non-fissile element into a

fissile one, it's called a fertile

capture. And fertile capture is what

makes thorium [music] useful. In fact,

even in a normal uranium reactor,

fertile capture accounts for over a

third of the energy generated by the

reactor. A normal nuclear reactor uses

uranium 235, which is fissile. But

naturally occurring uranium ore contains

only 0.7% uranium 235. Almost all the

rest is uranium 238, which is

essentially non-fissile, but it is

fertile. Even when using fuels with

enriched levels of uranium 235

undergoing fission, there's so much

non-fissile U-238 around that some of

the chain reaction neutrons instead

transform U-238 into plutonium 239,

which can then [music] fission. But

U-235 doesn't make enough neutrons, and

U-238 doesn't turn into plutonium easily

enough that you can both sustain a

fission chain reaction and continue to

transform new fissile fuel. So at the

end you're left with a big chunk of

unfissioned but still full of

radioactive waste uranium 238. There's a

different kind of reactor called a fast

breeder reactor that uses plutonium as

the primary fissile fuel and uranium 238

as a fertile secondary source. This

combination can both sustain the fission

chain reaction and transform new fuel in

a self-sustaining way. But fast breeder

reactors are less researched, more

expensive, and harder to run effectively

for now. This is where thorium comes in.

The same route used in the

transformation of uranium 238 [music] to

plutonium 239 can be replicated down

here starting instead with thorium 232.

By adding a neutron, we get thorium 233,

which decays to protactinium 233, which

decays to uranium 233, which is fissile

and can be used to generate energy. So,

if you load your reactor with thorium

232, which remember is not fissile, and

you throw in some starter fissile fuel,

then for each fission reaction the

number of new fissile atoms created is

more than one on average, and the number

of new atoms split is more than one on

average, and remember those atoms give

you more neutrons. So the transformation

of thorium and the fission of uranium

can keep going and going and going until

in principle all of the thorium is gone.

And crucially, thorium transformation

can happen in a reactor that doesn't

have the same challenges as a fast

breeder reactor. And it gets rid of most

of the long-lived radioactive waste.

[music] And thorium is more abundant

than uranium and doesn't need the

expensive refining process to

concentrate the fissile uranium 235. And

so you can see why people get excited

about thorium. There are of course

challenges and downsides to making

thorium reactors, which is why we don't

have and so far have never had

commercial energy generation from

thorium. But that's fertile material for

another time.

Making commercial power from thorium may

soon be possible thanks to the work of

organizations such as this video's

sponsor, Copenhagen Atomics. Copenhagen

Atomics is building compact modular

thorium reactors to produce cheap

energy. Unlike traditional nuclear power

stations, which are giant infrastructure

projects, Copenhagen Atomics are

designing a self-contained reactor unit

that can fit inside a shipping

container. The reactors are based on a

design pioneered over 50 years ago that

uses molten salt to carry the fuel,

resulting in fewer lost neutrons and

more complete combustion. So you get

more energy for less waste. These

reactors can also use plutonium waste

from classic nuclear reactors as fuel,

extracting 10 times more energy out of

spent nuclear fuel than the initial

reactor did in the first place. And in

doing so, converting long-lived

radioactive waste into short-lived

radioactive waste. In theory, these

reactors could run anything from grids

to ships to moon bases. Check out

Copenhagen Atomics' website to learn

more about their work.