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In the Chernobyl series it is said that the two nuclear engineers had no alternative except for pressing the "AZ-5" button, couldn't they just insert the Boron rods back in one by one as they renewed the water supply to the core?

Max0999
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    You mean the series has deviations from the facts? – jasxir Jan 08 '21 at 09:23
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    I don't think the science in the show answers why your approach is not viable and was there a "point of no return". However, in the real reactor 4 they almost surely could've done what you describe, but it was unknown to operators that there could be any risk in pressing the button. – Džuris Jan 08 '21 at 21:13
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    @Dzuris We'll probably never know, but I think it's doubtful they had time to reinsert the rods one by one. At the time AZ-5 was pressed, power was already doubling every few seconds. This may be a good question to ask on physics.se. – Ryan_L Jan 08 '21 at 21:57
  • @Ryan_L I was talking about the real life accident where the known sensor reading do not report of such power growth. So there was probably no hurry. – Džuris Jan 09 '21 at 18:32
  • Pressing the AZ-5 button is an emergency way to reinsert the boron control rods. – matt_black Jan 15 '23 at 18:33

3 Answers3

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Through incompetence and ignoring operating guidelines they had maneuvered the reactor into a very unstable state, where small changes would result in large swings in power. They had:

  • Run the reactor longer than expected in the run up to the test, resulting in a large amount of xenon, which "poisoned" the reactor suppressing the power;
  • Disabled various automated safety systems, and cooling systems;
  • Overshot the start conditions for their test, almost shutting down the reactor entirely. As a result they had to remove far more control rods than were recommended for safe operations in order to increase the power to the point where they could run their test.

They had put the reactor in a very low power state, but had removed all controls on positive reactivity. The xenon poisoning is slowly burned off over time, particularly as power slowly increases.

[Please note from this point there are various scenarios hypothesized in reports and articles about the accident. It's not entirely known why the operators used the AZ-5 switch. This answer is based on the scenario dramatized in the TV show that a runaway power excursion forced them into using it as intended as an 'emergency stop' on the reactor. ]

So, as shown in the dramatization, they suddenly find themselves in a situation where the power is starting to rapidly increase. The xenon is all but removed, there are almost no control rods, and to make matters worse, boiling of the cooling water makes the reactor even more powerful.

They see the power double every few seconds, quickly exceeding the maximum operating power of the reactor. They are facing a meltdown of the core. What do they do?

  • Manually restore the full flow of cooling water and lowering the rods they manually removed? It's probably too late for this. The power is continuing to climb, and it's already far above where it should be.

OR

  • Use the AZ-5 (SCRAM) system, which automatically lowers all the control rods, including those which they manually removed. Yes, this takes 16-20 seconds, but it does lower all the rods and it's what they are trained to do when things get out of control.

To me this does feel like they had already gone beyond the point of return when the power spiked. They were already going to experience a meltdown. The AZ-5 system just made it worse, because the graphite tips of the control rods just created localized spots of even higher reactivity, leading to the steam them hydrogen explosion.

Even manually lowering individual rods has the graphite tip problem. The spike in power around the tip as it lowered caused the rods to jam in position. The crew would not have appreciated this, but that route would have probably failed for the same reason as the AZ-5 system did. However in any case, they would have been trained to press the AZ-5 switch in these situations because it is the "emergency stop" of a reactor.

iandotkelly
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    Wow ! you were watching that carefully… or you're an admin by day & a secret nuclear scientist by night ;) I understood the explanation as provided in the show, but I couldn't have turned that vague comprehension into a technical answer if you'd paid me. I can't vote +2. – Tetsujin Jan 06 '21 at 19:18
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    Thank you. The show made me end up reading a ton of reports on the accident and watched a number of youtube videos as well. I'm definitely not a secret nuclear scientist. – iandotkelly Jan 06 '21 at 19:33
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    I can see the \S/ glowing from under your shirt… is it a bird, is it a plane, no it's iandotkelley ;)) Nice bit of research. – Tetsujin Jan 06 '21 at 19:41
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    @Tetsujin There were a lot of youtube videos that came out immediately after the series aired explaining what happened in technical detail. My favourite was Scott Manley's explanation: https://www.youtube.com/watch?v=q3d3rzFTrLg but there were many others – slebetman Jan 07 '21 at 04:34
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    Had the operators not chosen the AZ5, and the reactor melted down from its own heat - would there still have been a reactor casing rupture or would the casing hold and the rods melted into slag at the bottom? (not that this had solved any problems, now you have corium at a million degrees eating its way down...) – Stian Jan 07 '21 at 11:27
  • My question was a little bit different in nature, let me rephrase: Had they known ahead of time about the issues with the AZ-5 button could they do anything to try and avert the upcoming meltdown? If they had renewed the water supply the water wouldn't have time to start boiling, if they inserted just 1 rod (or X rods) it would enter graphite first causing a small spike but then hopefully entering with the boron part. – Max0999 Jan 07 '21 at 12:30
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    I mean surely they could not push the reactor to the "point of no return" to begin with, but once they got there could they revert their way back? Hypothetically if I traveled back in time to just before the AZ-5 button was pressed, at what step would there be a 0% chance of reverting it? – Max0999 Jan 07 '21 at 12:38
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    For more information on the "graphite tips" I suggest you watch this video. This was one thing that didn't make too much sense in the show to me. Even if "tip" is the word they used in real life, it doesn't really explain the shape. It's more like a rod that's twice as long as the chamber that is half graphite and half control rod. When the control rod is retracted it is all graphite in the chamber. If they insert the control rod the graphite retracts on the other side. During the trial scene of the show I imagined a tip like a pencil tip, very short. https://youtu.be/hIGtTImeYU4 – Captain Man Jan 07 '21 at 13:55
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    @Max0999 .. it was as power increased after they withdrew more rods than the operating procedures allowed. Yes, they could have immediately put the rods back .... but at some point as the power started climbing the rate of power increase gets to a point where the graphite in the rods will cause a problem re-inserting them. Its not like there was an irreversible 'switch' they threw - but there was a point where the physics of the whole system meant that no action could stop a meltdown of some sort. – iandotkelly Jan 07 '21 at 14:18
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    As described in the show .. they remove almost all the negative reactivity element (such as they remove more rods than the designers of the system intended), the existing negative reactivity, the xenon is reducing over time, the water starting to boil further reduces the negative reactivity. At this point there is nothing surpressing the reactor at all, so power just climbs. At some point on this 'power curve' there is a point of no return. – iandotkelly Jan 07 '21 at 14:30
  • @StianYttervik - that question definitely gets beyond my ability to answer. Without the extra graphite in the rods, the meltdown would have been a little less abrupt. However a steam explosion or a hydrogen explosion (caused by the fuel cladding reacting with water at high temperatures) is still possible. Hydrogen explosions happened in the meltdown in Fukushima and the RBMK reactors were large compared to western reactors and had no real containment. – iandotkelly Jan 07 '21 at 16:25
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    @iandotkelly Yeah. AFAIK in Fukushima the reactors themselves were pretty "unharmed", if that is any way to describe a meltdown reactor - and the hydrogen was from the molten Zr cladding that reacted chemically under the rather uncomfortable temperatures and leaked out into the building and after a while created a lot of unecessary ventilation in that building. The reason I asked was I was thinking that maybe at a high enough steam pressure, the steam could reduce the reactor output . So maybe without the AZ5, no reactor obliteration... – Stian Jan 07 '21 at 18:02
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    @StianYttervik. Steam is so much less dense than water ... they refer to it as "void". I don't think any amount of steam is going to absorb enough neutrons to slow the reaction down. Fukushima was quite different though, they had all the control rods in place, they had successfully SCRAMMED the reactors, it was decay heat without cooling water that melted the core, released hydrogen etc. In Chernobyl they had a power excursion / criticality accident. – iandotkelly Jan 07 '21 at 18:39
  • @iandotkelly I don't think any amount of steam is going to absorb enough neutrons to slow the reaction down. Taken into account your very complete and interesting answer (+1) I think I do not understand that sentence. Neutrons must be slowed down in order to interact (otherwise they scatter). This is done with heavy water (D2O, not in Chernobyl of course where the moderator was not water) and if its temperature rises to the point of boiling, the steam will not slow down neutrons and the reaction slows down. – WoJ Jan 07 '21 at 19:19
  • @WoJ ... light water is a stronger a neutron absorber than moderator. Not a very good absorber like boron, but it does absorb. This is why the 'positive void reactivity' of the RBMK reactor is dangerous. It means replacing the water in the system with voids/steam (i.e. reducing the amount of water) causes the reactivity to go up. We distinguish absorbing a neutron with moderating a neutron (slowing it down, making it more likely to react with another uranium atom in the fuel). – iandotkelly Jan 07 '21 at 19:22
  • @iandotkelly: yes, this is why I mentioned that this does not apply to graphite-moderated reactors such as Chernobyl. But now I understand that you targeted RBMK reactors (which is actually obvious from the question). Thanks. – WoJ Jan 07 '21 at 19:35
  • @iandotkelly "Without the extra graphite in the rods, the meltdown would have been a little less abrupt." -- As I kind of mentioned earlier, it wasn't that they were small tips but big rods. The fact that they were on the control rods was baked into the operation. We can't say it wouldn't have happened faster without this design because the design and operation would have been totally different – Captain Man Jan 07 '21 at 21:56
  • @CaptainMan ... yes, I am aware that 'tip' is a little disingenuous. I'm not entirely sure what function the graphite in the rods served in the design. However it was just a comment I was responding to - I don't think I need to make changes to the answer. – iandotkelly Jan 07 '21 at 22:49
  • @iandotkelly At temperatures and pressures high enough, steam turns into supercritical water. I tried to find any sources on the neutron cross-section of supercritical water but it wasn't very fruitful. In any case, at such a temperature there would not be voids, the fluid would be continuous and have properties similar to that of the liquid. – Stian Jan 07 '21 at 23:58
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    @iandotkelly "I'm definitely not a secret nuclear scientist" - That's precisely what a secret nuclear scientist would say! – aroth Jan 08 '21 at 04:31
  • The reason for the graphite tips: https://www.reddit.com/r/askscience/comments/1lb1lt/what_purpose_did_the_graphite_tips_on_chernobyls/cbxr71w/ – BCdotWEB Jan 08 '21 at 16:44
  • "The power is continuing to climb, and it's already far above where it should be." It must be noted that this answer describes the story of the film not the story of the actual accident where the power was well below normal and the parameters only went out of control after the button was pressed. Here's an example: http://accidont.ru/data25.html – Džuris Jan 08 '21 at 19:19
  • @Džuris ... the show dramatizes one possible scenario. From the World Nuclear Association explanation of the accident "The slower flowrate, together with the entry to the core of slightly warmer feedwater, may have caused boiling (void formation) at the bottom of the core. This, along with xenon burnout, could have resulted in a runaway increase in power. An alternative view is that the power excursion was triggered by the insertion of the control rodse after the scram button was pressed (at 01:23:40)". This indicates the power excursion could have happened in advance of the AZ-5 – iandotkelly Jan 08 '21 at 19:27
  • This was what was dramatized in the show, and this is a question about the show. So I answer in that context. – iandotkelly Jan 08 '21 at 19:27
  • Sure, I was just adding the clarification because people have the tendency to assume that show reflects "the truth" while according to available sensor readings the power was rather stable at 200 MW until the button was pressed, i.e. the reason why the button was pressed IRL is unknown. – Džuris Jan 08 '21 at 20:52
  • I will add that to the answer - because that point it comes up in Nobody's excellent answer too. Thanks @Džuris ! – iandotkelly Jan 08 '21 at 20:54
  • @Max0999 - Yes, had they known about the possibility of AZ-5 speeding up the reaction at the bottom of the reactor, and if they objectively have had minutes (rather than seconds) at their disposal, they would be able to take actions with a much higher chance of success. – Jirka Hanika Jan 12 '21 at 07:22
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The AZ-5 button is the regular, non-emergency off button for that reactor, and they were supposed to turn the reactor off anyway.

The currently accepted answer presents some parts as facts which according to Wikipedia are not known to be true. It is not known whether the two engineers in charge knew about the impeding catastrophe when they pressed the AZ-5 button. There are several possibilities:

  • Maybe they saw a dangerous power spike and tried an emergency shut down using the normal way they were trained to use.
  • Maybe they didn't know about the dangerous power spike, but it did happen already, and they just tried to shut down the reactor for planned maintenance.
  • Maybe no dangerous power spike had happened before they pressed the button and the movement of the control rods is what triggered the catastrophic power spike. This is possible because lowering the control rods actually increases reactivity in the bottom of the reactor first before lowering it many seconds later, plus the movement of the rods could have caused cavitation (spontaneous formation of steam bubbles in liquid water) in the dangerously hot water, further increasing reactivity.

The only power spike that is presented as a fact on Wikipedia is the power spike that happened after the AZ-5 button was pressed.

I mean surely they could not push the reactor to the "point of no return" to begin with, but once they got there could they revert their way back? Hypothetically if I traveled back in time to just before the AZ-5 button was pressed, at what step would there be a 0% chance of reverting it?

Again based on the Wikipedia article: The reactor had various automatic safety measures. All of them were turned off. The reactor was under full manual control. The reactor wouldn't reach a point of no return under normal operation, but it did because they were trying to run a test to determine what would happen under some emergency conditions and so they had to intentionally generate those emergency conditions.

The test protocol was worked out by the designers of the reactor and would have probably worked - except that the people carrying out the test went completely off-script, put the reactor into a non-design condition and then went on to do a modified version of the test which wasn't approved by anyone qualified to approve something like that.

Regarding the specific point of no return, that is impossible to know, but it could have been when they intentionally shut off the power supply for the water cooling system. Or maybe the point of no return was only after they pressed the shut down button, and the accident could have still been prevented if they had let the water cooling system work for a moment after turning it back on (it was only supposed to start working at it's normal power level again at the precise moment they hit the shut down button, so couldn't have dissipated the extra heat accumulated while it was not working properly), lowered the rods one by one (thus causing a smaller power spike for each one instead of a single large one) and only used the shutdown button once the reactor was once again in a normal state.

Disclaimer: I didn't watch the show, I'm answering only based on the real facts as presented by Wikipedia.

Nobody
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    Good answer (+1) .... I would somewhat dispute that my answer presents things as facts that are not known to be true. My answer is based on the reports and the TV show, which is what the question is about. Yes, it is plausible that the AZ-5 was pressed to shut down the reactor after the test - however as wikipedia states,"RBMK designers claim that the button had to have been pressed only after the reactor already began to self-destruct" and this is the plausible chain of events that are dramatized and what the question is about. – iandotkelly Jan 08 '21 at 17:03
  • @iandotkelly We can disagree on that point, it's meant to justify why I wanted to add another answer, not attack yours. – Nobody Jan 08 '21 at 17:21
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    Hey, no worries. It's all good. No attack assumed, just explaining my perspective – iandotkelly Jan 08 '21 at 17:22
  • This is a very balanced answer (I haven't seen the show either). Elaborating on possibility 3: If the engineers weren't aware of any substantial power incursion and pressed AZ-5 just to finish their task of the day, then the fastest + lowest risk protocol for shutting down the reactor would have been to initiate rod insertions individually, spaced 3 seconds apart of each other. A rod takes ~18 seconds to fully insert. But only during the first three seconds there's any risk of the particular rod heating up any particular place as a result of its progress. – Jirka Hanika Jan 08 '21 at 22:53
  • At the bottom of the reactor, the control rod columns are kept pretty "cool", such as under 70 °C per the hypothesis of no runaway power incursion prior to pressing AZ-5. The insertion process interferes with cooling of the particular control column, and the first 3 seconds contribute to extra heat near the bottom part of the particular control column. To make these two ill effects feed into each other at full strength you need to be inserting many rods simultaneously, all of them in a similar phase. – Jirka Hanika Jan 08 '21 at 23:00
  • And that's because the average distance a single fission neutron travels from fission to fission in RBMK is measured in meters, not in centimeters. The heat up is local, but not that local. HOWEVER, this logic holds only if the power curve was growing only very slow until AZ-5 was pressed. – Jirka Hanika Jan 08 '21 at 23:04
  • The core point of the show is that there was a known instability in the reactor design and a known flaw in the AZ-5 shutdown mechanism that could tip the reactor into a sudden meltdown. But this knowledge had been suppressed so the engineers didn't know about it. they expected AZ-5 to offer a simple rapid shutdown but at least two other incidents had shown it causing a dangerous power spike (one is described in the show during the investigation). Thus the engineers seeing a power spike wrongly thought AZ-5 would stop it. – matt_black Jan 09 '21 at 13:56
  • @matt_black - That's correct. There were multiple design flaws in the reactor. They were corrigible to the extent, not corrected after their discovery in the Ignalina power plant, not known to Chernobyl operators, and they were corrected after Chernobyl. However, AZ-5 wasn't expected to be used with that many rods in the highest position; that prior configuration was off the safety protocol and the operators knew that. But they probably had no idea that AZ-5 could speed up the reaction briefly (in this configuration in particular), while other people knew that. – Jirka Hanika Jan 09 '21 at 18:54
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RBMK reactors use graphite tipped rods, which exacerbate fission upon installation. Light water functions as a coolant, while moderation is mainly carried out by graphite. Light water reactors in the US use water both as coolant and moderator; it's more expensive.......but safer. This means that the reactor's reactivity (adjustable by appropriate neutron-absorbing rods) has to account for the neutrons absorbed by light water. Water loses all neutron moderation as steam, with no water back systems (having been shutoff) the reactor went into overdrive.

LazyReader
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    Yeah. The real "accident" was the design of the reactor. – Chico the Friendly Monkey Jan 08 '21 at 04:14
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    Just for clarity: this answer explains the design of RBMK reactors as described in the show, not the real life Chernobyl reactor where the graphite displacers ("tips") were in the middle of the reactor all along. – Džuris Jan 08 '21 at 20:56