Chernobyl scene #1: Valery Legasov explains, how an RBMK reactor works

Scene: Episode 5 from the miniseries called Chernobyl.
All rights belongs to HBO.

[yacineassas]

everyone is a nuclear scientist until Professor Legassov walks in

[comradedyatlov]

You're confused.

RBMK reactor cores don't explode. He's in shock, get him out of here.

[Deltlead]

The soldier crouch-running to adjust the mic is one of the most honest and subtle details that really adds character to the scene. It really grounds it in reality

[johnd]

This scene should be called “Nuclear Reactors for dummies.”

[slitor]

I love how the court offical rushes infront of the camera to adjust the mic.

[RedApeAndrew]

Dyatlov is learning just as much as everyone else.

[shadowthrunight]

"but as it turned out the man who did, didnt understand it either". Such a well written sentence, it really sums up the whole thing.

[TheSecondVersion]

"And that is how an RBMK nuclear reactor works."

"Sir, this is a Wendy's"

[danieldevito]

I love how they showed Boris' transition from dedicated party zombie to honest, caring human being throughout the entire series...

[alhussainalrawi]

Not great not terrible explanation, I rate 3.6/ 15000

[someguy]

From now on, I want everything explained to me by Jared Harris

[MrJustonemorevoice]

(After watching this episode)
"You know I'm something of a scientist myself"

[Glive]

"This is the invisible dance that power's major cities, and it is beautiful"

[rahiabrar]

"When did you become and expert on thermonuclear physics?"
Me after spending the night watching Chernobyl: "Last night."

[gigstar]

"The man who did didn't understand it either"
that shit hurts

[demonhunter]

I'm a nuclear reactor operator, working on a pressurized water reactor (PWR). Truly, I tell you, this is one of the most scientifically accurate scenes you'll ever see in TV or cinema. I'll do my best to explain the science.


We measure reactor power by the neutron population in the reactor. Neutrons split uranium atoms, which creates heat, fission products, and more neutrons. But a lot of neutrons move too fast to hit uranium. It needs to be moderated. (more on this later)

Critical- the reactor power is stable. Supercritical- the reactor power is rising. Subcritical- the reactor power is lowering.


Reactivity- how much the reactor departs from criticality. Another way to define reactivity is the generational change in neutron population.


Scram- full automatic insertion of control rods. In the RBMK reactor, the AZ-5 button slowly lowers all control rods into the core. In my reactor, rods are unlatched and instantly dropped to the bottom.


Boron is used for control rods because they absorb neutrons, thus lowering reactivity. Simple enough.


Water is used as a coolant, transferring heat from the core, and turning into steam to turn electric turbines to produce electricity. As you know, water is made up of 2 hydrogen and 1 oxygen atoms. The hydrogen atoms are the same atomic weight as neutrons. So, neutrons can elastically collide with water and transfer its energy, and slow down (becomes thermalized). These thermal neutrons are far more likely to hit uranium atoms and cause fission.


Positive Void Coefficient- the mathematical explanation of water when it converts to steam, it raises the reactivity, thus reactor power goes up


Negative Temperature Coefficient- the mathematical explanation of how water affects reactivity. As you know, colder water is denser than hotter water. Because water is more dense when colder, it has a greater chance of moderating neutrons, therefore causes reactor power to go up. Therefore, hotter water will actually cause reactivity to go down.


Xenon- this is a very important fission product. It is called a "poison" because it absorbs neutrons, thus reduces reactivity. Some xenon is produced directly from fission, but most comes from the decay of iodine, another fission product. Xenon is removed from the core from either absorbing neutrons (called neutron burnout) or from decaying into cesium. So when a reactor's control rods are inserted or scrammed, the neutrons are quickly absorbed by the rods, which causes xenon to rise significantly. If you try to prematurely restart a reactor from shutdown, you have a lot of xenon to overcome. The Chernobyl reactor operators did not understand how xenon behaved, and so were confused when the power tanked and then panicked when they tried to quickly raise the power again.


Graphite- the RBMK reactor uses graphite as its moderator. In comparison to water inside the reactor, graphite moderates neutrons far better, causing more fission reactions. This is a design feature of the RMBK reactor.


On the night of the accident, the operators lowered the power, which allowed the xenon concentration to rise. To overcome the xenon, they raised control rods all the way to the top.They started the safety test, which stopped the pump supplying water to the reactor. Thus all the water in the reactor turned to steam, which caused power to rise due to the positive void coefficient, even faster with the rods at the top. When Akimov hit AZ-5 (scram), the graphite tipped rods inserted, which caused the power to skyrocket. All these combined factors caused Reactor 4 to burst open. The superheated graphite, when exposed to the open air, instantly caught fire, which created the explosion.

This is a lot of info and there's a lot more to explain. If you have further questions, I'll be happy to answer. =)

[hiderunbride]

Now I know how a Nuclear Reactor works. Now I don't need you.

[hotelmario]

I love the genuine passion he has for nuclear power despite the horrific accident. I don't see Chernobyl as an anti-nuclear-power show, I see it as a show about the importance of accountability when it comes to things that can hurt people.

[marshallkarp]

For you teachers, instructors, and educators out there. This is excellent educational instructional methodology. He is taking something unimaginably complicated and making it simple with no PowerPoint, CGI, video, or any other 23rd century technology.

[Whatever]

People are saying him dropping the cards wasn’t scripted... I refuse to believe that cause him dropping the cards just added so much more to the scene! It’s crazy.