He fixes the RED placard - "Nuclear Fission" (R1) to the top of the RIGHT COLUMN of the board. The words on the placard are in Cyrillic. Doesn't matter.
All that matters is the BRIGHT RED COLOR.
LEGASOV
Uranium fuel. As uranium atoms split apart and collide, reactivity goes up. But if you don't balance the reactivity, it never stops rising.
Next, Legasov puts the BLUE PLACARD - "Control Rods" (B1), in the LEFT COLUMN. Balancing it out. Blue/cool, red/hot.
LEGASOV
Boron control rods. They reduce reactivity, like brakes on a car.
He places another BLUE PLACARD - "Water" (B2) on the left.
LEGASOV
But there's a third factor to consider. Water. Cool water takes heat out of the system. But as it does, it turns to steam, or what we call a void.
He places two RED PLACARDS - "Positive Void Coefficient" (R2) and another "Nuclear Fission" (R3) - on the right side.
LEGASOV
In an RBMK reactor of the type used at Chernobyl, there is something called a "positive void coefficient." What does that mean? It means the more steam, the higher the reactivity, which means more heat, which means more steam-- it would appear we have a vicious cycle on our hands. And we would, if not for this.
Legasov picks up a BLUE PLACARD - "Negative Temperature Coefficient" (B3) and places it on the left column.
LEGASOV
Negative temperature coefficient. When nuclear fuel gets hotter, it gets less reactive. So—
He stares at the board. Even after all this time, he still can't help but marvel at the science. He moves his hands between the columns of RED and BLUE...
LEGASOV
—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...
He turns back to the room.
LEGASOV When things are normal.
Legasov lifts up a BLUE PLACARD. "Xenon Poisoning" (B4),
LEGASOV
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 4 was held at half power for ten hours. The xenon did not burn away. It built up, poisoning the core.
He places "Xenon Poisoning" (B4) on the board.
LEGASOV
We're starting to lose balance.
The BLUE column is longer than the RED. The words don't matter. The colors tell the tale.
LEGASOV
At 28 past midnight, the reactor is now primed to slow down. And yet, in less than an hour, it will explode. (beat)
If you can't understand how a stalled reactor could lead to an explosion, I do not blame you. After all, you don't work in the control room of a nuclear power plant.
He turns to the defendants.
LEGASOV
But as it turned out, the men who did... did not understand it either.
530 INT. CONTROL ROOM - REACTOR #4 - NIGHT 530
THE LED DISPLAY: 1600. And then... it begins lowering. 1590. 1570. 1510...
Akimov and Toptunov stare up from the panel-- still shoulder to shoulder, hands on buttons.
THE DISPLAY - 1450... 1395...
AKIMOV Easy now. Slow it down.
Toptunov nods. Wipes sweat from his head. Makes an adjustment.
AKIMOV Good. Like that.
In the B.G. - Dyatlov PACES. Shaking his head.
DYATLOV
You should have been finished by now.
AKIMOV
We're following protocol for reduction rate.
DYATLOV
You're procrastinating. There are ten other men in this plant who would have done it already.
Toptunov glances at Akimov, but Akimov doesn't look back. Just keeps his eyes on the panel.
AKIMOV (to Toptunov) Keep working. You're doing fine.
Dyatlov takes a cigarette from his pack. Tosses the pack on to the table. Annoyed.
DYATLOV
Kirschenbaum. Come get me when these old women are ready.
KIRSCHENBAUM Yes, Comrade Dyatlov.
Dyatlov EXITS. Slams the door behind him.
531 INT. CORRIDOR OUTSIDE CONTROL ROOM 4 - CONTINUOUS 531
Dyatlov steps into the corridor. Lights his cigarette. Checks his watch. Then looks through the window by the landing, which faces out toward:
THE LIGHTS OF PRIPYAT - just a few on at this late hour.
532 INT. CONTROL ROOM - REACTOR #4 - MINUTES LATER 532
Kirschenbaum and Stolyarchuk wait quietly. Stolyarchuk checks a meter... makes an adjustment to his panel.
Toptunov and Akimov continue to work their panel.
THE DISPLAY: 775... 765... 755...
AKIMOV
Okay. Very slow now. Let's coast down to 700.
THE DISPLAY: 745... 725... 715...
AKIMOV Whoa whoa whoa... slow!
TOPTUNOV (confused) I didn't move any rods there...
THE DISPLAY: 700... 680... 640... 590...
Akimov looks at the display. Utterly bewildered.
AKIMOV What is this?
Toptunov puts his hands up.
TOPTUNOV I'm not even touching it.
THE DISPLAY: 570... 550... 540...
INT. CORRIDOR OUTSIDE CONTROL ROOM 4 - CONTINUOUS 533
As Dyatlov finishes his cigarette, Akimov emerges from the control room. Dyatlov turns to him and can immediately see from Akimov's face that something's gone wrong. Again.
All he wanted was one fucking moment of quiet, and now...?
INT. CONTROL ROOM - REACTOR #4 - CONTINUOUS 534
Dyatlov storms in, followed by Akimov. And the first thing Dyatlov sees is:
THE DISPLAY: 520... 515... 510...
AKIMOV
We did everything right. I think maybe the core is poisoned...
DYATLOV
If you thought the core was poisoned then you DIDN'T do everything right, because you're CHOKING MY REACTOR! Get it back up!
Akimov knows there's no sense in debating. Just:
AKIMOV
If we switch off LAC, it's possible we could get more control...
DYATLOV
Do it. Go! Akimov rushes back to Toptunov.
AKIMOV
Disable local automatic control, go to global...
Toptunov hesitates, nervous—
AKIMOV
Leonid.
Right. Okay. Toptunov turns a series of switches. Presses some buttons.
TOPTUNOV LAC disabled. Global control activated.
Everyone raises their head to:
THE DISPLAY: 500... 440... 260... 220...
Toptunov backs away from the panel in disbelief.
DYATLOV What did you do?
TOPTUNOV
I did what you said! I switched-- DYATLOV
LOOK AT IT!
THE DISPLAY: 110... 75... 55.
AKIMOV I don't understand....
THE DISPLAY: 35 32 30 and then holds.
DYATLOV
You fucking amateurs. You stalled the reactor. HOW THE FUCK DID YOU GET THIS JOB?
Toptunov puts his head in his hands. He looks like he's about to cry. Akimov turns back to Dyatlov.
AKIMOV
Comrade--
DYATLOV
You're going to tell me you did everything right again, you incompetent arsehole?
Akimov slumps. It's over.
AKIMOV
I apologize for this unsatisfactory result.
"Unsatisfactory result?" The phrase only serves to disgust Dyatlov even further.
Akimov turns back to the panel.