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?

  • 6
    You mean the series has deviations from the facts?
    – jasxir
    Jan 8, 2021 at 9:23
  • 2
    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 8, 2021 at 21:13
  • 1
    @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 8, 2021 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 9, 2021 at 18:32
  • Pressing the AZ-5 button is an emergency way to reinsert the boron control rods.
    – matt_black
    Jan 15, 2023 at 18:33

3 Answers 3


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.


  • 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.

  • 8
    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 6, 2021 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 6, 2021 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 6, 2021 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: youtube.com/watch?v=q3d3rzFTrLg but there were many others
    – slebetman
    Jan 7, 2021 at 4:34
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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 7, 2021 at 12:38

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.

  • 3
    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 8, 2021 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 8, 2021 at 17:21
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    Hey, no worries. It's all good. No attack assumed, just explaining my perspective
    – iandotkelly
    Jan 8, 2021 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. Jan 8, 2021 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. Jan 8, 2021 at 23:00

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.

  • 5
    Yeah. The real "accident" was the design of the reactor. Jan 8, 2021 at 4:14
  • 2
    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 8, 2021 at 20:56

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