Chernobyl Visually Explained

Chernobyl Accident - the Physics Clearly Explained: a simulation and visualization of the Chernobyl disaster, breaking down the physics behind the accident. It explains nuclear fission, focusing on Uranium-235 and how reactors maintain a controlled chain reaction using control rods and moderators like graphite. The RBMK reactor’s dangerous positive void coefficient is highlighted, showing how overheating increases reactivity.

The simulation shows the events leading to the disaster: a safety test reduced power, causing Xenon-135 buildup and a power drop. Operators attempted to raise power by removing control rods, but reactivity surged when they pressed the SCRAM (AZ-5) button due to a design flaw with graphite tips. This led to the reactor’s explosion.

0:00 Introduction
0:32 Basic Fission
2:57 Control Rods
4:03 Water
5:58 Xenon135
7:05 Moderation
9:04 Event 1: Reactor normal
10:02 Event 2: Power Reduction
10:55 Event 3: Power drop
11:46 Event 4: Power up attempted
12:44 Event 5: Test starts
13:33 Event 6: SCRAM

[r.b.ratieta]

"The computer told them to turn off the reactor. They turned off the computer instead." Those words are equally funny as they are horrifying.

[BjarturMortensen]

The runaway reaction at 15:09 was very very satisfying after all that buildup.

[WisdomVendor]

Moral of the story.... Don't get your rod stuck in the wrong place.

[danwhite]

Being someone who has studied and has great interest in nuclear physics, as well as working in the nuclear and particle accelerator industry, your reactor simulation is very well made and accurate, whilst being very easy to understand. Great job!

[unityxg]

This has to be the most comprehensive, laymen explained video of Chernobyl I have ever seen. Well done and I finally understand fully (for the most part) how this accident happened.

[derekparent]

Human error, the fact that this rundown test must be done is the main factor for this disaster. It was the Russian culture at that time, the inability to be able to question authority is and will be the making of a disaster again.

[TravelGeeq]

Anyone that gets me to watch 15+ minutes while holding my interest earns my sub. Looking forward to learning more.

[jkzero]

"Ridiculously Badly Made Kettle" that should be on a T-shirt, it is brilliant!
Fantastic video, I recall trying to model chain reactions for my video critical mass but gave up. Your is just great. And the runaway reaction was the cherry on top. Great video!

[Higgsinophysics]

Please note, I've found sources in the literature saying reactor was going supercritical before pushing the SCRAM button, while other sources claim after SCRAM button was pushed it caused the reactor going supercritical. In my video, Ive assumed the first is true. It's a historical detail (or advanced Nuclear reactor detail) I'm not sure which is correct.

[AbideByReason]

Really great video! Loved all the animations and the build up with each one. The final runaway reaction was well worth the wait.

[erdngtn]

Dude the simulation video concept is dope. Looking forward to more please

[TanToRza]

This has to be one of the best explanations and visual effects I’ve seen for explaining how the reactor works and how it failed.

[vivekanandan]

After watching all Chernobyl explanied videos, I'm happy to report that "Now I'm a NewClear Scientist"

[MMuraseofSandvich]

I would totally play a game with these graphics. But perhaps in dark mode instead. This is a fantastic simulation that should be shown to all chemistry and physics students.

[fieryweasel]

The xenon aspect of reactors is just fundamentally fascinating. The idea that you're generating Xe at the previous rate, and burning it off at the "power now" rate makes it an interesting twist to reactions.

[ddopson]

So, the visual simulation is interesting, but it runs the risk of having wildly different time-constants compared to the real-world system. In particular, Xenon burn-off has a time constant several orders-of-magnitude too slow to explain the rapid acceleration in Chernobyl-4's reaction rate. Xenon build-up played a key role in setting up the accident, by getting the operators to remove all but a handful of control rods, and by unbalancing the reaction so that it was proceeding entirely at the top and bottom of the reactor, and was almost totally stalled in the center of the reactor. But Xenon burn-off was NOT an important feedback factor in the reactor's rapid power rise. In addition to the slow time constants included in several simulation papers as well as the INSAG accident reports, it's relatively straight-forward to prove this from first-principles by observing that Xenon's absorption cross-section, of 2 million barns, or 2e-18 cm^2, times the neutron flux of an RBMK-1000, which was, IIRC, on the order of 10^14 /s/cm^2 at full power. So that tells us that at full power, your average Xenon-135 atom is going to take on the order of 500 seconds to absorb a neutron. Even during the transient, at 10X or 100X the normal reactor power, the transient was so rapid that only a small fraction of the Xenon atoms could be expected to burn off. It's a relatively slow feedback factor.

Also, the accident can't be properly understood without understanding the role of the asymmetric power distribution prior to the accident. With the middle of the reactor was in the depths of a Xenon pit shut-down, there were really two separate, entirely decoupled reactions, one at the top of the reactor, and the other at the bottom. Thus pushing the control rods further into the middle of the reactor had little effect because the middle of the reactor was already dead, but pushing the graphite segments further down had a very significant effect. The SCRAM effectively shifted reactivity from the dead middle of the reactor to the over-active bottom of the reactor, further accelerating the reaction.

Void reactivity was a known consideration to the reactor designers, and RBMK-1000 doesn't have a universally positive void coefficient in all scenarios. In particular the void coefficient is negative early in the fueling cycle and becomes positive later as more fuel is burned up and more control rods have to be removed to maintain reactivity. Chernobyl-4 was late in the fueling cycle, and the Xenon pit caused the operators to pull out even more control rods until the void coefficient became terrifyingly positive. That was a transient property of Chernobyl-4, not a universal condition of the RBMK-1000 in all conditions.

One of the mitigations for void reactivity and other positive feedback factors is the even faster negative feedback from doppler broadening as the fuel temperature rises. That's one of the fastest feedback mechanisms and is supposed to help in stabilizing the reactor, but between putting the reactor into a configuration where it had a huge void coefficient, much larger than even a late-cycle RBMK-1000 normally would, and generating an asymmetric reaction that would be pushed over the cliff by the SCRAM event's modest reactivity spatial shift from the dead middle to the overheating bottom, the operators managed to thoroughly overwhelm doppler broadening's ability to limit a rapid transient.

Regarding it "going supercritical", what you really mean is prompt thermal supercritical, as being delayed supercritical isn't all that exciting and merely implies that the reactor's power level is increasing, possibly very slowly. When the reaction needs to wait for the slower decay groups to achieve supercriticality, power rises very slowly, with time constants in the tens of seconds. When exceeding a dollar of supercriticality, reaction growth only needs to wait for the next neutron generation, and thing get out of hand very quickly, on the order of 10^-4 seconds, which is still wildly slower than prompt fast supercriticality in a bomb, 10^-8 seconds per generation; even in an out of control Chernobyl-4, the reaction still has to wait for graphite moderation between neutron generations, preventing it from significantly exceeding the level of energy release required to disassemble the reactor. For most of Chernobyl's power transient, it probably wasn't prompt thermal supercritical, and was probably growing at a rate throttled by of one of the fastest two delay groups, but when all the water flashed to steam, things got pretty crazy and none of the papers I've read can agree on the exact course of events. I'm still trying to understand the simulations myself and when exactly they may have reached prompt thermal supercriticality.

[gavinbannister]

Hi! This was such a fascinating video to watch! I randomly saw this in my feed, and now I'm going to learn more about nuclear physics and the history of Chernobyl, thanks!

Just one small thing, the graph at the end ( 16:28 ) is partially covered by the next watch recommendation.

Looking forward to seeing more of your videos, subscribed!

[moonstruckmoth]

Very well explained capturing all the important contributing factors

[powerupminion]

Very nice presentation. I like the simulation a lot! 💯

[trefferquotehoch]

Amazing!
Best most compact explanation ever.
A masterpiece!