Why Chernobyl Exploded - The Real Physics Behind The Reactor

With the TV show doing a great job at delivering its explanation in a manner that most people can easily understand, I felt I wanted to do a more detailed description. So I cover basic reactor physics, explain how the RBMK reactor works, how Xenon 135 works, Why the control rods included graphite tips, and why the reactor became unstable and ran away.

And of course I highly recommend the TV show:

[neon-john]

Hello Scott. Retired nuclear engineer here. I was on the DOE's emergency response team, bags packed and ready to fly to Chernobyl but as you know, in the end Detente' had not progressed enough for them to trust and accept us. Anyway, This is probably the best explanation of what happened I've seen or read. First a small thing. In the US, the xenon buildup is referred to as the "Xenon well". You should witness the scramble in a power plant if someone accidentally trips the reactor by, for example, valving in a water level transmitter in the wrong sequence. EVERYBODY is running, trying to get through the restart procedure before the reactor sinks too far into the well.

There is one critical part that you left out which is probably the major reason the transient was so large. In each control rod, below the bottom of the boron carbide is about a foot long void, filled with air or whatever they fill the control rod with. When this void passed by a section of fuel, there was no moderation and no neutron absorption which let the fuel go prompt critical. This prompt critical reaction continued until disassembly started. At the very beginning of the prompt critical reaction, the rod channel was distorted enough to freeze the rod in place.

This was NOT a small nuclear explosion. This was easily determined by looking at the fission product profile from air samples. A nuclear device profile is much different than a prompt critical excursion. This is because the device begins disassembly so rapidly that the first generation fission products are not in the neutron field long enough to either be burned or transmuted to another isotope. A prompt critical excursion, by contrast, lasts relatively forever. Some first generation fission products are burned and others transmuted to other isotopes.

Eyewitness testimonies I've read from operators who survived stated that the first explosion was large enough to rattle the fuel insertion shield plugs and shake some out but that the big one happened maybe a minute or two later. The delay was long enough for operators to run to an observation deck and see the shield plugs rattling. That's consistent with a buildup of H2/O2 and then detonation.

This difference in air samples over Europe is the first indication that there had been a reactor accident and not the Sovs having conducted a very small above-ground nuclear device test.

My guess, based both on what I know and from what I've seen of small samples of graphite smuggled back from Chernobyl is that this was a hydrogen-oxygen explosion. Of course, like everyone else, mine is only a guess.

There are two catalogs on the net. One is the catalog of the isotopes using an NaI detector and the other is the same but using a GeLi detector. On the last page of the GeLi catalog is a spectrum of fission products of an atmospheric explosion taken seconds after an atmospheric test. It's so dense it looks like white noise on a spectrum analyzer.

John

[jerry]

I’d say the reactor worked better than expected. It completed the 5 year heat generation plan in under 10 minutes!

[Jack-ecii]

I think you are mistaken comrade. RBMK reactors don’t explode.

[jkfilms]

Scott: *Explains Coefficients.*
Me: "I like your funny words, magic man."

[Betterhose]

It is so tragically ironic that the reactor had been running under high load flawlessly, but failed when they tried to prove it's safety.

[tomstech]

Me: [has seen every nuclear documentary he can in the last 15 years, half of which on chernobyl]
Scott: Wanna watch yet another explanation on chernobyl?
Me: Yes.

[jimfrazier]

I'm a former nuclear operator, and your explanation (while somewhat simplified), is one of the better ones I've seen on Youtube. Well done, sir.

[gregtegreg]

I was 7 years old when the explosion happened. We were a group of kids playing outside, having fun, suddenly our parents came out and started screaming at us to come inside quickly. I got a pill that I had to swallow and I asked why. My mother said "it's for something in the air".
I was like wtf is in the air - she said nevermind that. When I saw this series I got shivers down my spine. I remeber reading about it in my teens but never put much thought about it. Really weird feeling. But later on, in 1989, the revolution started and we were hearing bullets hitting the trees around us while playing so a bit of radiation was nothing. What a childhood...

[IanSmithKSP]

This is the best explanation of the Chernobyl engineering mistakes on YouTube. Some of the explanations on YouTube are so dumbed down they’re offensive.

[mikicerise]

Basically, they swerved left, then they swerved right trying to compensate, then they swerved left again trying to compensate for that, then they flew right off the road.

[dave_in_florida]

I get that feeling "getting stuck in the xenon pit" around 3pm every day

[billhiggins-haall]

As a submarine officer and nuclear engineer, this was a great description of the accident, in terms that many can understand. All US and most other country's reactors are designed with far greater safety and a negative temperature coefficient. The control rods are completely released and fall rapidly into the reactor core. For a submarine, we immediately shut the throttle reducing energy taken from the core. We also shift power supply from the electric turbine generators to the ship's batteries. We shift to reduced electric load for the batteries to last longer. If needed, we come to periscope depth and use the diesel and the ship snorkel mast. Control room crew immediately bring the ship up to 150 feet. All these actions are immediate after the Engineering Officer of the watch announces "Reactor Scram". We practice the event often in ship drills.

[PlanetFrosty]

Excellent presentation. As an engineer I’m always fascinated with how convoluted events are presented understandably! Good job!

[dreadengineer]

Former nuclear control systems engineer here -- this is an accurate explanation of what went wrong at Chernobyl. Scott, I am legit impressed how you're able to totally switch fields and still be technically competent. You're like a Scottish Neil deGrasse Tyson!

The tldr in case the video wasn't clear enough: in certain situations, a scram (emergency drop of the control rods to kill the reaction) actually boosted reactivity (i.e. neutron multiplication rate) instead of killing it. Which turned the RBMK "emergency shutdown" system into an "initiate steam explosion" system. It was just 100% pure engineering failure.

Separately, I love those USSR corrective actions after the disaster: "Prevention of the emergency safety systems from being bypassed while the reactor is operating." Just... profound.

[LudwigSC]

I now understand what my dog goes through when I'm talking to him.

[bobchurch]

My favorite thing about the series is that it explained why the test was gone ahead with despite all the negative conditions. It was necessary if they were to sign off on the reactor by May Day, and if that happened several higher ups would get promotions and move up to nicer offices.

[MatthiasEdling]

"When did u became an expert in thermo nuclear science?"

Last night

[torybruno]

This was a classic Soviet shortcut. Western reactors use fuels enriched to 3 to 5% U235. This is very expensive and time consuming. In fact, getting to 5% U235 is a significant portion of the effort required to reach all the way to the 80 to 90% of weapons grade (kind of the way LEO is most of the way to anywhere in the solar system). RBMK's used around 2% enrichment. While this sounds like small difference, it is not. It meant much cheaper, faster fuel production and an increase in the enrichment industrial capacity available for weapons production.

Unfortunately, is also meant a relatively sluggish fuel. So, the RMBKs use fixed channels literally made from a reaction accelerating (neutron moderating) material: graphite. This "juices" the reactor in order to make the low quality fuel work. This is the primary driver underlying the positive void coefficient (unstable) nature of the RBMK.

The Soviets also chose this design, so that it would be common with those dedicated entirely to weapons grade plutonium production. Plutonium slowly builds up as a byproduct in fission reactors and is harvested from fuel rods. The RBMK has the unique ability of replacing its fuel rods quickly without shutting down the reactor, making plutonium production more efficient.

(And yes, Scott... this is actually me: @torybruno. Great job on the video!)

[sergey]

Imagine explaining all of this to some politician who ask you why the hell you couldn't run the test today.

[onlinevorlesung]

Thank you this in depth explanation!
Another video suggestion:
The plugging of the 2011 Deepwater oil spill was basically a space mission on earth. Really fascinating engineering designs were proposed to stop it, but no good YouTube video exists on the matter because 11 years ago, ScienceTube was still too niche. I'd be so happy to see this subject covered by you.