RBMK

I want you to think 

of Yuri Gagarin. 

I want you to imagine that he has been 

told nothing of his mission into space 

until the moment that he is on the launch pad. 

I want you to imagine that all he has is a list of instructions 

that he has never seen before, some of which have been crossed out. 

This is exactly 

what was happening in 

The Control Room of Reactor-3

The night shift had not 

been trained to perform 

The Experiment. 

They hadn't even been warned it was happening. 

Leonid Toptunov, The Operator 

responsible for controlling and stabilising 

The Reactor that night was... all of 25 years old. 

And his total experience 

on The Job? Four months. 

This is The Human Problem 

created by The Delay. 

But inside The Reactor Core, 

in the space between the atoms 

themselves, something far more 

dangerous is forming. A poison. 

The Time is 28 past Midnight. 

Comrade Legasov. 

(BREATHES DEEPLY) 

(PUSHCART ROLLING) 

I'm pleased to see some of my colleagues here from the Kurchatov Institute and Minenergo. 

But you don't need to be 

a nuclear scientist to understand What Happened at Chernobyl. 

You only need to know this : 

there are essentially two things that happen inside a nuclear reactor. 

The reactivity which 

generates Power either 

goes up, or it goes down. 

That's it. 

All The Operators do 

is maintain balance. 

Uranium fuel. 

As uranium atoms split apart 

and collide, reactivity goes up. 

But if you don't 

balance the reactivity,
it never stops rising. So... 

Boron control rods. 

They reduce reactivity 

like brakes on a car. 

But there's a third 

factor to consider : water. 

Cool water takes heat 

out of The System. 

As it does, it turns to steam,
or what we call A "Void." 

In an RBMK reactor of the type 

used at Chernobyl, there's something 

called a "positive void-coefficient." 

What does that mean? 

It means that the 

more steam present 

within The System, 

the higher The Reactivity, 

which means more heat, which means 

more steam, which means... 

It would appear we have 

a vicious cycle on our hands. 

And we would, were it not for this... 

(CLATTERS) 

And we would, were it not for this : 

the negative temperature coefficient. 

When nuclear fuel gets hotter, it gets

less reactive, so...fuel increases reactivity. 

Control rods and water reduce it. 

Steam increases it, and 

the rise in temperature 

reduces it. 

This is the invisible dance that powers 

entire cities without smoke or flame. 

And it is, beautiful when things are normal. 

As uranium splits apart to release energy, 

it breaks down into a new element, xenon. 

Xenon reduces reactivity. 

This is The Poison 

Comrade Khomyuk mentioned. 

When The Core is running at full power, 

it burns the xenon away before it can 

cause a problem. 

But because of The Delay, 

Chernobyl Reactor-3 has been 

held at half power for ten hours. 

The xenon did not burn away. 

It built up, poisoning The Core. 

We're starting to lose balance. 

- (COUGHING)

- At minutes past midnight, 

The Reactor is now primed to slow down. 

And yet, in less than an hour, it will explode. 

If you can't understand how 

a stalled nuclear reactor could 

lead to an explosion, I don't blame you. 

After all, you don't work 

in the control room of a 

nuclear power plant. 

But as it turned out, 

the men who did 

didn't understand it either.