ฝึกพูดภาษาอังกฤษด้วยเทคนิค Shadowing จากวิดีโอ: The Magic of Thorium Nuclear Reactors

Thorium is a kind of miraculous element.

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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 Atomics,

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,

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.

3 Neutrons can escape and leave the reactor entirely.

Or 4 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 number is just the total number of protons and neutrons.

And uranium-239 can then radioactively decay into 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 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 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 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.

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.

Getting 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 ten 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.

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ฝึกพูดภาษาอังกฤษด้วยเทคนิค Shadowing จากวิดีโอ: The Magic of Thorium Nuclear Reactors

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