Timeline for Why was there a "point of no return" in the Chernobyl series that ended in the meltdown?
Current License: CC BY-SA 4.0
34 events
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| Jan 12, 2021 at 7:22 | comment | added | Jirka Hanika | @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. | |
| Jan 9, 2021 at 16:13 | history | edited | iandotkelly♦ | CC BY-SA 4.0 |
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| Jan 8, 2021 at 20:54 | comment | added | iandotkelly♦ | I will add that to the answer - because that point it comes up in Nobody's excellent answer too. Thanks @Džuris ! | |
| Jan 8, 2021 at 20:52 | comment | added | Džuris | 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. | |
| Jan 8, 2021 at 19:27 | comment | added | iandotkelly♦ | This was what was dramatized in the show, and this is a question about the show. So I answer in that context. | |
| Jan 8, 2021 at 19:27 | comment | added | iandotkelly♦ | @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 | |
| Jan 8, 2021 at 19:19 | comment | added | Džuris | "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: accidont.ru/data25.html | |
| Jan 8, 2021 at 16:44 | comment | added | BCdotWEB | The reason for the graphite tips: reddit.com/r/askscience/comments/1lb1lt/… | |
| Jan 8, 2021 at 4:31 | comment | added | aroth | @iandotkelly "I'm definitely not a secret nuclear scientist" - That's precisely what a secret nuclear scientist would say! | |
| Jan 7, 2021 at 23:58 | comment | added | Stian | @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. | |
| Jan 7, 2021 at 22:49 | comment | added | iandotkelly♦ | @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. | |
| Jan 7, 2021 at 21:56 | comment | added | Captain Man | @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 | |
| Jan 7, 2021 at 21:13 | vote | accept | Max0999 | ||
| Jan 7, 2021 at 19:35 | comment | added | WoJ | @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. | |
| Jan 7, 2021 at 19:22 | comment | added | iandotkelly♦ | @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). | |
| Jan 7, 2021 at 19:19 | comment | added | WoJ | @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. | |
| Jan 7, 2021 at 18:39 | comment | added | iandotkelly♦ | @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. | |
| Jan 7, 2021 at 18:02 | comment | added | Stian | @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... | |
| Jan 7, 2021 at 16:25 | comment | added | iandotkelly♦ | @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. | |
| Jan 7, 2021 at 14:30 | comment | added | iandotkelly♦ | 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. | |
| Jan 7, 2021 at 14:18 | comment | added | iandotkelly♦ | @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. | |
| Jan 7, 2021 at 13:55 | comment | added | Captain Man | 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. youtu.be/hIGtTImeYU4 | |
| Jan 7, 2021 at 12:38 | comment | added | Max0999 | 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? | |
| Jan 7, 2021 at 12:30 | comment | added | Max0999 | 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. | |
| Jan 7, 2021 at 11:27 | comment | added | Stian | 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...) | |
| S Jan 7, 2021 at 9:00 | history | suggested | John Kugelman | CC BY-SA 4.0 |
Minor grammatical fixes
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| Jan 7, 2021 at 4:34 | comment | added | slebetman | @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 | |
| Jan 7, 2021 at 2:35 | review | Suggested edits | |||
| S Jan 7, 2021 at 9:00 | |||||
| Jan 6, 2021 at 19:41 | comment | added | Tetsujin | 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. | |
| Jan 6, 2021 at 19:33 | comment | added | iandotkelly♦ | 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. | |
| Jan 6, 2021 at 19:18 | comment | added | Tetsujin | 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. | |
| Jan 6, 2021 at 15:17 | history | edited | iandotkelly♦ | CC BY-SA 4.0 |
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| Jan 6, 2021 at 15:06 | history | edited | iandotkelly♦ | CC BY-SA 4.0 |
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| Jan 6, 2021 at 14:56 | history | answered | iandotkelly♦ | CC BY-SA 4.0 |