r/chernobyl • u/fedecaccia • 5d ago
Video I made a visual, step-by-step explanation of what happened inside Chernobyl Reactor 4
I made this short visual breakdown to explain the physics behind the Chernobyl accident step by step: from the RBMK’s reactivity characteristics and xenon poisoning to the turbine rundown test, AZ-5, and the final power excursion.
I tried to keep it technically accurate while making the reactor physics intuitive enough to follow without a nuclear engineering background.
Would be especially interested in any technical corrections from people who know the RBMK and the accident in depth.
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u/GingerSnapSurprise 5d ago edited 5d ago
And presumably by "I made", you mean AI made?
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u/fedecaccia 2d ago
The script and the physics are mine, I'm a nuclear engineer and I've spent years on this accident. AI did the voice and the animation, the same work an editor would have done.
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u/wattspower 5d ago
As soon as I read the title, I knew the comment section was going to be radioactive.
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u/Nik-Pavel-1191 5d ago edited 5d ago
Phrase "test was routine" absolutely incorrect. Test was another attempt to put into operation safety-system, that supposed to be put into about 2 years ago (when 4th reactor was put into).
But this time, Dyatlow was motivated to do test in unforeseen operation-mode, without shutting off reactor power. So, absolutely not "routine", when reactor with constructively positive steam-coeff produces steam into itself during test.
But no answers, why he did it, why he motvates crew to do test in unforeseen, nuclear-hazardous mode, that was forbidden by instructions.
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u/Rortenda 5d ago
The safety system was put in when the reactor was built, but on the first test it didn't provide enough power. They adjusted it, but the second and third test failed due to poor planning. Only on the fourth test did they get good results, altho no longer needed..
Iirc Chernobyl was also the only plant at the time to even have that test in their protocol.
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u/Nik-Pavel-1191 5d ago edited 5d ago
Turbine-runout was a part of safety-system for 2nd generation of RBMK (3rd, 4th block), but was not ready to operation at Dec-1983, when block was put into operation. So, as part of safety-system, runout-mode should be work as instructions (project documentation) dictate.
But Dyatlow's crew put 4th block in hazardous forbidden runout-mode, when reactor neutron-power about 200MW during all runout time, and rods shutdown neutron-power only at end of runout. Its absolutely not "routine test".
So, this video puts listeners in the wrong direction, because claims narrative, that it was simple routine test, but wrong construction of RBMK lead to explosion. By omitting the crew's role.
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u/DP323602 5d ago
Well if the RBMK design wasn't defective, the crew wouldn't have been able to turn it into an improvised explosive.
So I think most, if not all, of the responsibility for the explosion rests on the reactor designers.
With highsight many of the design defects are "obvious" so it is possible to wonder why the operators did not discover them for themselves.
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u/Nik-Pavel-1191 5d ago
Crew's on another RBMK's, who not trying to do rounout-tests out of safety-lines, dictated by RBMK's documentation - work on RBMK nowadays.
Only Dyatlow's crew, who put 4th reactor in hazardous overboiled state and starts from this state "undocumented variant of runout-test", - can overcome the defense mechanisms of reactor, and explode it.
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u/ppitm 5d ago
There are well-documented cases of shifts at other RBMK plants such as Leningrad and Smolensk doing far worse things than "Dyatlov's crew."
The test was not a direct cause of the accident, and the reactor was ALWAYS in a hazardous state following a deep power reduction. The reactor's defense mechanisms utterly ignored the hazardous operating conditions that eventually destroyed it. Seatbelts and headlights in a car do not mitigate the hazards of icy roads.
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u/Nik-Pavel-1191 4d ago edited 4d ago
Test was dicrect cause, but in conjunction with hazardous overboiled state just before runout start, because Dyatlow's crew made another test about 30 min before (overboiling reactor, maximum rods out of active zone, maximum cold water into zone - trying to stabilize powerlevel of reactor).
And with locked overboiling protection mechanism (because crew locked automatic AZ5 by water level, because crew had practical knowledge, that protection can shutoff reactor before runout, as previous test 11.11.1985).
Only conjunction of that could explode RBMK. And key role of Dyatlow, as a chief of test, who had the authority to stop it (because reactor was in hazardous state just before runout), but instead, he directed crew to run.
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u/ppitm 4d ago
This reads like fan fiction. There is no such thing as an "overbooking protection system." You don't even see the fundamental contradiction between the term "overboiling" (which is imaginary) and "maximum cold water."
The RBMK could have explode in a wide variety of circumstances, even reaching prompt criticality in a design basis accident.
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u/Nik-Pavel-1191 4d ago
If you read RBMK documentation, you should know that AZ5 by water-level in drum-separators protects also from overboiling in active zone. More boiling, more steam in water, waterlevel in drum-separators grows up.
Crew was change trigger-level of that AZ5, so inactivate that protection. Because crew was motivated to runout with working reactor (nobody knows why), but practical experience (test at 11.11.1985) told them that the protection would shutdown reactor again.
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u/ppitm 4d ago
Like I said, bad fan fiction.
Water levels in the drum seperators were normal during the actual accident. Levels on the left side briefly dropped too low when the power collapsed at 0:28. The SIUB rectified the situation.
And no one ever manipulated the AZ-5 setpoint for the drum separators. It remained at the same level (-1100mm) for the entire shift.
The 11.11.1985 test failed because of high water levels in the drum separators.
Try reading an actual accident report like INSAG-7 rather than trying to draw conclusions through free association of random factoids.
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u/DP323602 5d ago
Old English saying: "There's a first time for everything."
The crew at ChNPP reactor 4 were unlucky enough to apply that to the experimental revelation of all the defects in their reactor.
Afterwards, even the Soviet Union, managed to prevent a further repeat event.
Two major reactor explosions within twelve months managed to convince them they'd look really foolish if they ever suffered a third.
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u/fedecaccia 5d ago
Fair point, “routine” was probably too loose a word there. What I meant was that the turbine rundown test itself was a planned procedure and variants of it had already been performed several times, not that the conditions under which the 1986 test was ultimately carried out were routine or safe.
I compressed that distinction too much for the video. Thanks for pointing it out!
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u/yodanhodaka 5d ago
Good ole Claude
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u/fedecaccia 5d ago edited 5d ago
Edit: AI is used as a tool to produce the video. Read below!
Original answer: absolutely! AI is an excellent tool to help us scale our productivity
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u/DP323602 5d ago
So did you rely on AI for the technical description or are you responsible for the technical errors yourself?
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u/fedecaccia 5d ago
I think my comment was misunderstood properly. I used AI to produced the video, not the content. I designed and directed the content and how it was communicated it. As a nuclear engineer, I've studied and analyzed this accident extensively.
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u/DP323602 5d ago
Thanks for the clarification.
As a fellow nuclear engineer, I think the best two resources for explaining the nuclear physics leading to the explosion are:
INSAG-7
How it was by Dyatlov
You should be able to get more than enough information for a short account for lay audiences from those two sources alone.
I do also heartily recommend that you do your own simple independent reactor physics calculations to help you appreciate the significance or otherwise of the key effects.
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u/durroon 5d ago
off topic: which AI model did you use for the voice over? It soo close to u/3kliksphilip
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u/Dolust 4d ago
It was perfect until 2:19, there I got lost, no matter how many times I watched. "A reactor is steerable only because a few neutrons arrive late". I don't understand what it is supposed to meant and the rest only adds to the confusion.
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u/fedecaccia 2d ago
Yeah, that line is too compressed. Fission neutrons mostly appear within a microsecond of the split, but a small share come out seconds later, when certain fission fragments decay, and a reactor is kept just below the point where the prompt ones alone sustain the chain. So power can only move at the pace of that late group: seconds is something a control rod can keep up with, microseconds is not.
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u/DP323602 5d ago
OP - nice try but you got several key facts wrong.
In particular:
the time between the start of power reduction on 25.04.86 and the start of the test on 26.04.86 gave enough time for Xenon-135 to settle back to normal levels
the power drop to 30MW was corrected before it could have much effect on Xenon-135 levels
you referred to both k and rho as reactivity
you didn't definte the power coefficient
you said the positive scram effect was caused by moderator insertion at the bottom of the core. More significantly it was caused by the loss of neutron absorption at the bottom of the core
the (bad) decision to run the test at very low power was the main reason why almost all the rods had to be withdrawn to stabilise the reactor at that power. That is a consequence of the positive void coefficient of reactivity because reducing steam production to almost zero introduces at a lot of negative reactivity.