A nuclear reactor at Chernobyl exploded.

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

and it wasn't supposed to be possible.

But why?

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

There are simplified answers out there.

I'm not gonna get deep into physics,

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

understand what happened here and why.

But first, an introduction of terms:

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?

Well, why does anything 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.

Why?

Forces? I guess?

Isotope.

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

Six protons? It's carbon, every time.

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.

Unstable isotope.

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

that's because they're stable.

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.

What happened to that mass?

Well, it became energy.

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...

special ability.

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.

Now U-235 is a rare isotope;

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,

which is causing more neutrons.

So this, instantaneously, all of the uranium fizzes.

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

and you can do this in two ways:

You can absorb them, which slows the reaction down

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

That sounds a little counterintuitive...

It is, I'll explain why.

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.

Mostly it will just bounce off.

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

but the nice thing is -

if the reactor starts to get hot, the heavy water starts to boil,

and then there's less of it around to slow down the neutrons,

so the reaction starts to slow down.

It's a negative feedback loop, which is the kind that you want inside of a nuclear reactor.

Now the RBMK reactors used regular water as a coolant because that's cheaper

but regular water is actually a neutron absorber.

That's gonna become really important later.

So instead of heavy water, RBMK reactors use big, heavy blocks of graphite to slow down the neutrons.

That's the neutron moderator.

So to sum up, you've got moderators which help speed things up

and you've got absorbers, which help slow things down.

These are your gas pedal and your brakes in your nuclear reactor.

And you need these things because there's other stuff that affects the rate of the reaction,

that changes as the fuel is used,

it changes depending how long the reaction has been going.

And then all reactors - like in the fuel - you have the build up of different byproducts of the fission.

And some of those absorb neutrons, some of them slow neutrons down.

There's a big one, xenon-135, which is part of the decay pathway after uranium breaks apart.

So it doesn't show up immediately, it just like ramps up slowly and it starts absorbing neutrons,

so you have to pull your control rods out some.

There's ways to control all this stuff but it's complicated.

So you need ways to be able to put on the brakes or put on the accelerator.

The xenon thing is what happened on the day of the accident.

They had basically run the reactor all day, in a sort of a perfect procedure for what you would do -

if you wanted to increase the amount of xenon-135 in your reactor.

They didn't do this on purpose,

it's just what happened.

And so when they started to decrease the power for the test,

the xenon started to suck up all the neutrons,

and the reactor dropped into this situation where they just weren't getting any power out of the reactor.

Now the normal thing to do with an RBMK reactor, because of all these positive feedback loops,

is to very slowly bring the power back up.

But they wanted to get the power back up fast.

And so what they did is they pulled all of the control rods out -

pretty much all the control rods out of the reactor.

The only things left absorbing neutrons in this reactor now

are water and xenon-135.

If one or both of those things go away,

then you would have a nuclear reactor that is in a very difficult to control state.

They talk in the show, and a lot of science communication talks about,

(and I completely understand this because like you don't have however long this video is to explain all of this)

they talk about how the control rods were tipped with graphite.

So when they came back into the reactor, the first thing that happened -

is that this moderator, the things that speed up the reaction, was the first thing that came into the reactor.

Now you might, having heard that version (which is the simplified version),

be like, "Why on earth would you tip a control rod with graphite?"

Like there's easier, cheaper stuff to tip a control rod with.

The simple answer to that question... is that they didn't?

Let's explain.

So when you're using normal water as your coolant, it also is absorbing neutrons.

So if you take your control rod out, which is the thing that absorbs neutrons,

and all that's left, like water, is just taking that space -

you have a less good but still a control rod made of water in that space absorbing neutrons.

So in RBMK reactors, they have control rods that control in both directions.

The top of the control rod is made to absorb neutrons

and then as that pulls out, it pulls in a graphite rod and that is made to increase the reaction.

This is the "graphite tip".

That's not actually a graphite tip, it's part of what makes the reactor work.

So you have, in every control rod, a brake and an accelerator.

And you can't do one without the other.

So on the day of the accident, they hadn't just pulled out all of the brakes -

they'd pulled in all of their accelerators.

Pretty much all of them.

This is not the biggest problem.

The biggest problem is neutron flux,

which is basically the movement of neutrons throughout different parts of the reactor

and you want to control the movement of neutrons throughout the reactors,

so that all of the fuel burns evenly,

so you don't waste anything,

and so there aren't areas of the reactor that are hotter than some other areas.

To accomplish this,

the graphite moderator rods are actually shorter, both at the top and the bottom, than the fuel rods.

So in those spaces, there's some water

and that water is absorbing neutrons.

And in the middle, you get a nice flat curve of neutron flux,

evenly distributed throughout the reactor.

And this is fine. This is great.

Unless, you take all of the control rods out at the same time

and then put them all back in at the exact same time.

And we'll see why.

So on the day of the accident,

they had pulled out all of the absorbers, pulled in all of the accelerators

and then you have your positive feedback loops kicking in.

Specifically, because the flow of water through the reactor had been slowed down, the water started to boil.

Water is usually there absorbing neutrons.

So it's boiling, and suddenly there isn't as much water there absorbing neutrons,

which is increasing the rate of the reaction,

and finally, all these neutrons that are suddenly flowing around eats up all of the xenon-135

So that control, that had been there the whole time, is suddenly gone.

And then you have the final fatal decision,

the thing that you're *supposed* to do in this situation.

When the control rods started into come back to the reactor,

the graphite rods displaced the water at the bottom of the reactor,

until suddenly there was nothing in the bottom row of the reactor

that, in any way, throughout the entire reactor, was controlling neutrons at all.

So the reactor is already dangerously overpowered

and then, localized in the bottom of this reactor, you have this spike of neutron flux

that's beyond anything the reactor is designed for.

That very rapidly increased the amount of energy being produced

and something somewhere broke.

It could have been a lot of different things that in this situation broke.

It might have been multiple things at once.

But when it broke, it locked those graphite rods into this most dangerous of positions.