[As Valery Legasov and Boris Shcherbina make their way to Chernobyl in a helicopter...]
Boris Shcherbina: How does a nuclear reactor work?
Valery Legasov: What?
Shcherbina: It's a simple question.
Legasov: It's hardly a simple answer.
Shcherbina: Of course, you presume I'm too stupid to understand. So I'll restate: Tell me how a nuclear reactor works, or I'll have one of these soldiers throw you out of the helicopter.
Legasov: A nuclear reactor makes electricity with steam. The steam turns a turbine which generates electricity. Where a typical power plant makes steam by burning coal, a nuclear plant...
[Legasov tries to find a pen in his pocket to draw a diagram. Scherbina hands his pen and a scratch paper to Legasov]
Legasov: [Draws diagram] In a nuclear plant, we use something called fission. We take an unstable element like Uranium-235, which has too many neutrons. A neutron is, uh—
Shcherbina: The bullet.
Legasov: [Impressed] Yes, the bullet. So, bullets are flying off of the uranium. Now...if we put enough uranium atoms close together, the bullets from one atom will eventually strike another atom. The force of this impact splits that atom apart, releasing a tremendous amount of energy, fission.
Shcherbina: And the graphite?
Legasov: [Exhales] Ah, yes. The neutrons are actually traveling so fast we call this ‘flux’—it's relatively unlikely that the uranium atoms will ever hit one another. In RBMK reactors, we surround the fuel rods with graphite to moderate—slow down—the neutron flux.
Shcherbina: Good. [Takes his pen back] I know how a nuclear reactor works. Now I don't need you.