Name: チェルノブイリで実際に起きたこと (What /Actually/ Happened at Chernobyl)
Uploaded: 2021-01-14T10:29:31.000Z
Duration: 13 min 30 s
Description: VoiceTubeの動画で発音を聞きながら英語表現を覚えよう！学べる英語：

過去完了形

This kind of explosion had never happened before and hasn't happened since

Why was it supposed to be impossible, and why wasn't it?

but I am gonna be a little bit more robust in how I talk about it because I think it's really interesting to actually

Nuclear: pertaining to the nucleus, which is the center of the atom that contains the protons and the neutrons.

Protons: the nuclear particle that has a positive charge.

The number of protons defines what element an atom is.

Neutrons: the neutral particle. Why does it even exist?

But basically, for, like, subatomic particle physics reasons.

If you have too few neutrons in your nucleus, the nucleus will fall apart.

If you have too many, nucleus will fall apart.

So, the number of protons an atom has always determines what element that atom is.

Now, carbon usually has six neutrons, but sometimes it has five or seven or eight or nine,

and so we have this word "isotope" to describe these different forms of carbon.

So the normal carbon is carbon-12 and then there's carbon-13 and carbon-11, etc.

Now, almost all naturally existing elements on Earth are stable isotopes;

Unstable isotopes generally have too many or too few neutrons,

and so they will emit energy or particles on their path to becoming a more stable nucleus.

And that process of emitting energy or particles is called "radioactive decay."

Radioactive decay is one kind of nuclear reaction. There are two other kinds:

there's fusion, where two atoms come together to become one atom,

and then there's fission, when one atom breaks into two atoms.

During fission the leftover fission products, which is all the particles and atoms that are left over, are always a little bit lighter than the original atom.

As described by a pretty famous equation.

That energy is most of what we ultimately capture in a nuclear reactor and turn into electricity.

The products of fission themselves are unstable isotopes,

so they continue to decay inside the nuclear reactor.

So heat in nuclear reactors is actually created in two ways:

One, the vast majority of it is created through the fission of the atoms into smaller atoms.

Secondarily, though, heat is produced by the radioactive decay of these unstable isotopes

that are the product of the fission reactions.

So even after the nuclear reaction stopped, this radioactive decay continues to produce heat.

That heat has to go somewhere; it goes into the fuel,

and if you can't keep cooling off the fuel, it will melt itself.

And then it keeps getting hotter and hotter;

it makes itself hotter as long as these isotopes are unstable and emitting radiation.

They heat themselves up and that can melt through just about anything, which is what we call a "meltdown."

Now there is one single naturally occurring isotope on our planet that has a very...

Uranium-235, if you hit it with a neutron, not only will it split apart, but it will create more neutrons.

And those neutrons, if they hit another uranium-235, that will split apart,

and it will cause this nuclear chain reaction.

This is the reason why we have nuclear power on this planet.

it's about 0.7% of naturally occurring uranium.

Uranium-238 is much more common and much more stable.

So usually when we're making nuclear power or a nuclear bomb,

we have to enrich the uranium so there's much more U-235.

In, like, a nuclear bomb, it's like 80% U-235.

In that case, pretty much every neutron flying around in the thing is causing another fission reaction,

So this, instantaneously, all of the uranium fizzes.

It becomes a bomb. Like it's a bomb. That's the whole point of the bomb.

This is, of course, not what we want in nuclear reactors.

We want to be able to very carefully control the speed of the chain reaction.

Usually a super important part of this is enriching the uranium -

not all the way up to 80%, but to some more enriched version than naturally occurring uranium.

Just that enriching uranium, it turns out, is extremely expensive.

So the genius-ish thing about the RBMK reactor design, the thing that people liked about it,

was that it was a cheap way to use unenriched uranium or very slightly enriched uranium

to create a self-sustaining nuclear reaction.

Now to do this, because the fissionable atoms of u-235 are farther apart

and there's other stuff in there that can absorb neutrons,

you have to do more with the neutrons you have.

A primary goal of all nuclear reactors is controlling neutrons

You can absorb them, which slows the reaction down

or you can moderate them, which speeds the reaction up.

When a uranium atom splits apart, the neutrons that come out are going extremely fast.

They're going too fast actually, for quantum mechanics reasons that I don't understand,

to actually hit another uranium atom and break it apart.

And then it'll fly out of the reactor and it won't get used.

So counter intuitively, you have to slow down the neutrons to speed up the rate of reaction.

To do this, you use something called a "moderator" - they moderate the speed of neutrons,

they do not moderate the speed of the reaction,

they *increase* the speed of the reaction.

Now there are some reactors that are able to do this with heavy water,

which is like isotopically enriched water

that actually makes it really good at neutrons bouncing off of it and slowing down some.

That's expensive because heavy water is expensive

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チェルノブイリで実際に起きたこと (What /Actually/ Happened at Chernobyl)

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scenario

extremely

situation