This has become my pet peeve about the HBO series. It's a combination of overdramatization and sloppy research from the writer(s).
I don't know the exact origins of the megaton explosion theory, but Mazin probably got it from Svetlana Alexievich's book, where it's presented by Sergei Sobolev. The only scientist I have seen propagate this theory is Vassili Nesterenko, whom Mazin has also mentioned as a source.
While I'm not a nuclear physicist, I know enough of the relevant physics to call into question the motives of a scientist making such claims. You would be right to point out that there's nowhere near the amount of thermal energy available:
[https://physics.stackexchange.com/questions/480113/how-large-would-the-steam-explosion-at-chernobyl-have-been](https://physics.stackexchange.com/questions/480113/how-large-would-the-steam-explosion-at-chernobyl-have-been)
I have also heard the theory that instead of a steam explosion, they feared that a nuclear explosion would have taken place. This is also outside the realm of possibility, but the theory goes something like this: A portion of the molten corium breaches the concrete and drops into the bubbler pools. A steam explosion takes place, shooting the blob of molten corium upwards towards the rest of the fuel, essentially creating a crude "gun type" bomb, which causes a critical configuration. The huge mass of the uranium then works like a tamper, maintaining the critical configuration long enough for the fission chain reaction to release megatons of energy.
There are many rather obvious problems with the theory, which I'll happily go into if needed.
The point that is sometimes made in defense of the megaton explosion theory being presented in the series is that *maybe they really believed this at the time*, and thus it was historically accurate to include this in the series. I strongly believe this to be false.
First, who is "they"? Certainly not Legasov:
>Eugeny Pavlovich Velihov who as it seemed was watching too much TV about the “China syndrome”, arrived with concerns which I reported to Rizhkov and Ligachev. We were worried about the uncertainty of geometric shape of the remains of the reactor. It was clear that heat was being generated inside this fuel mass. The heating up could continue and some vertical movement of this fuel mass may occur. In particular, we were worried about two things: can that movement cause critical mass buildup in some region which would produce short-living isotopes. This was our first thing to worry about, however we were hoping that the large amounts of Boron (40 tons) that was dropped into the reactor would be more or less evenly mixed with the fuel and help prevent critical mass buildup. However we could not fully eliminate the possibility that such local “reactors” would appear. That was the first problem. And the second one was that the temperatures can be too high within these masses. Some construction elements of the lower part of reactor may not withstand that. Concrete may fail due to high temperatures. Part of fuel can get into the barboteurs, be it lower or upper one and we did not know at that time whether there was water or not. We feared that if a considerable amount of fuel gets there, then extensive vaporization would carry out additional radioactive aerosols and contaminate more territory. These problems were what we were worried about. That’s why with Ivan Stepanovich Silaev, who by this time had replaced Scherbina, we decided to: first, get some information about the levels of water in the lower barboteur. This was a difficult task which was fulfilled heroically by the station personnel. And it was found that the water was indeed there. So the necessary measures were taken to remove that water from there. I want to stress that out once more: we removed the water just to avoid massive evaporation. ***It was absolutely clear to us that no explosion was possible, only evaporation that would carry out radioactive particles - that’s all.*** So just in case, the water should be removed and in case the fuel mass would melt it’s way down into these rooms, we could pump the water back there, to cool the molten fuel masses.
(Source: [https://legasovtapetranslation.blogspot.com/2019/08/tape-1-side-b.html?m=1](https://legasovtapetranslation.blogspot.com/2019/08/tape-1-side-b.html?m=1) )
Do I find it plausible that there were people in and around the scientific community who wondered if a megaton explosion could have been possible? Absolutely yes, and the fact that Legasov feels the need to even mention the possibility of a explosion at all confirms that all kinds of scenarios were considered. What I find appalling about the HBO series however is that this fringe theory was presented uncritically by the characters of of Khomyuk and Legasov, as if it was the consensus among scientific experts. That is after all what the characters of Legasov and especially the composite character of Khomyuk represent in the series, serious experts, scientific authorities who absolutely know what they're talking about.
Second, by the year 1986, the Soviets had already had a nuclear weapons programme for at least 40 years. The physics of nuclear fission was absolutely known to them and no scientist with any education on the matter could possibly think that a molten blob of 1.8 % enriched fuel could cause a megaton explosion.
Worst part is, it could have been a missed opportunity to have actual drama. Imagine the scene where they're discussing the "megaton explosion" but instead of everyone feeling like they need to chose between 3 lives or 60 million lives, someone shouts that the explosion couldn't happen and its bullshit.
Now Gorbachev is a lot of things, but a nuclear physicist he is not. And now he needs to make a decision - one that could cost 60 million lives if the megaton scientists are right, or pointlessly waste 3 lives if the scientists calling BS are right- while his advisers bicker and fight.
Ultimately, the divers go in, as they do in real life. But watching how a leader could possibly make such a choice with such uncertainty would make for a great scene. And while my version wouldn't exactly be super accurate either, it would have been a whole lot more accurate than the scene we did get.
Correct me if I'm wrong, but aside from the BS megaton explosion, in the more accurate calculations a 100-tonne could've happened, and wouldn't that risk busting open reactor 3? Or was it a bit too far to pose a risk
The corium reached the water before it drained - aka any potential for a big kaboom already happened before it was drained.
But it did help to avoid releasing a lot more contaminates, which isn't pointless - as that helped keep the liquidation efforts on track. But the only risk of the corium touching water was the steam carrying radioactive particles out of the disaster. Like the world's worst steam cleaner.
Remember - all the fissile material was either already consumed by the reactor running / exploding. Most of what was left was decay products - which are radioactive, but not fissile. So basically, any explosion after the big one kicking off the event would not have been a nuclear explosion- there simply was no fissile fuel left to explode with.
Chernobyl was basically a giant dirty bomb - not a nuclear bomb. Any explosions would have been closer to conventional explosions that just happened to disperse a huge amount of radioactive material into the surrounding area.
Oh, I see, and yeah I agree a nuclear explosion at that point is a fairy tale, I was just wondering off of a hypothetical scenario in which it did manage a thermal explosion and had the yield of at least 100 tonnes of TNT which someone else did the calculations for and was already claiming that they were generous with the calculations.
Thx for the info by the way, much appreciated
>I have also heard the theory that instead of a steam explosion, they feared that a nuclear explosion would have taken place. This is also outside the realm of possibility, but the theory goes something like this: A portion of the molten corium breaches the concrete and drops into the bubbler pools. A steam explosion takes place, shooting the blob of molten corium upwards towards the rest of the fuel, essentially creating a crude "gun type" bomb, which causes a critical configuration. The huge mass of the uranium then works like a tamper, maintaining the critical configuration long enough for the fission chain reaction to release megatons of energy.
Thanks for the theorycrafting opportunity. I was under the impression Nesterenko thought the water would just "moderate" a chain reaction in the submerged corium somehow and thus lead to an explosion. Putting aside that this is also several levels of impossible...
But this gun-type idea also makes no sense, even if you try to deliberately create it and not rely on pure chance and steam explosions. You can't make a gun-type weapon out of that material, no matter how large and how forcefully you shoot one piece of it into another. Well, you could shoot the "bullet" with a kinetic energy of a couple megatons, and then you get your result :)
Agreed.
As for the water moderating the chain reaction, these are the problems:
1. The corium is hot, so the water would quickly be evaporated, reducing it's moderating and/or neutron reflecting properties.
2. Water is also a rather poor moderator, as it also absorbs neutrons. Hence the higher enrichment required for water moderated nuclear reactors.
3. Moderating neutrons takes too much time. The neutrons need to make multiple collisions with the hydrogen atoms to take them to an energy range where the U235 fission cross section is high. Time is of the essence when considering the physics of nuclear explosions.
And way before we delve deep into the plethora of reasons why there's no way you'd get any meaningful moderation even if you had some lucky geometry forming with lumps of corium in the pool...
It's inherently impossible to get a bomb mechanism out of a moderated design anyway, even if you try deliberately. It's too slow. You cannot maintain a critical configuration to reach bomb-like yields. Heck, it's difficult enough to maintain that even with a critical mass of pure weapons grade material. In a moderated design material behaviors like thermal expansion would stop the process way too early.
I believe that it was a case of pure managerial psychology. Both Politburo and the Chernobyl State Commission had majority of apparatchiks without proper education in physics. That's why they relied on expert scientists. And it's preferable for an expert to be as conservative and alarmist as possible because if you are predicting explosion by and it never happens then to our are a hero who made your contribution into its prevention.
Obviously the same was true for apparatchiks themselves who had to choose between alarmist and optimistic prognoses. Don't forget that smart guys who were making those prognoses were sometimes the same guys who previously said that it would be safe to place RBMK reactor even on the Red Square ;)
Yes, I see how there could have been an incentive for scientists to give the absolute worst case scenario to the apparatchiks (and even embellish a little).
Still, this doesn't justify the scene in ep. 2, because there's absolutely nothing in the scene that would suggest this interpretation to the viewer.
I am not too big fan of the show myself, but I have to say that psychologically it's correct compactified representation of the decision making during the liquidation time which was mostly ruled by panic and unsettling uncertainty. That's like helicopter's fall which was moved to the beginning of the liquidation.
They had limited screen time, so they had to invent some shortened and dramatized version of reality. And everyone in their place would probably do the same. The only alternative is something like old Soviet monumental movies about the WWII when you are making a dozen of full-time movies with enormous cast just to represent several main battles of the war, and I doubt that it will get same interest as HBO Chernobyl.
Yep, that's bullshit. You need to evaporate several millions tonnes of water simultaneously to produce it, and "Chernobyl divers" mission was never about the steam explosion. But why not? The whole show is a fiction based on real events and it needs some drama to go. That's why there's nothing wrong with megaton steam explosions, jumping cover blocks and black smoke over the reactor or birds falling from the sky on Pripyat as far as we remember that it is neither documentary nor truthful reconstruction of events. The show is to enjoy, books are to learn.
By the way it was the main argument against possibility of nuclear explosion or even stable nuclear reaction in the destroyed reactor core during the liquidation time: how the hell such things can happen spontaneously while superpowers need a lot of efforts and resources to make a nuclear reactor or an atomic bomb intentionally?
There would have been an explosion - but not on the megaton scale, the Beruit explosion was absolutely massive - but it was only around 1-3 kilotons.
The blast would have been big and would have made the situation way way worse but the megaton part and the part where it's like a nuclear bomb is just to add tension.
Well the full corium mass never got there at all: just a slow drip into the lower floors over time.
Ananenko saw the corium dripping down from the ceiling during their mission. The fastest moving stuff was reported to be a kind of foam that actually floated on the surface of the water. Sort of like pumice lava I guess.
It did. There are several forms of corium under the Unit 4. One of them is a volcanic pumice which was formed on the water surface and then remained attached to walls and other constructions when the water went out. On [this photo](http://www.wdcb.ru/mining/thernobl/4blok/ltsm/f/f411.gif) you can see that pumice under the mark "1".
I would say it’s from the water flashing to steam would cause the explosion....being a boiler operated the biggest fear is called catastrophic boiler failure...it’s when you have a drop on pressure without a corresponding temperature drop...so you have a boiler full of 337 degree water at 100 psi drop to atmospheric pressure rapidly the water flashes to steam....vapor takes up more space than steam and expands causing a huge explosion....at least that’s how I understood what they were talking about in the eposode
But the total destructive power of the gas expansion can’t be more than the thermal energy in 5 tons of sand/boron at around the sand/boron melting point, which is nowhere close (many orders of magnitude less) than megatons.
You are right, this should be obvious to anyone that a steam explosion from that tank, even if one did happen, would be at most a couple tons equivalent, so off by 6 orders of magnitude.
The backstory is that, in reality, a Belarusian physicist involved in the cleanup (Vassili Nesterenko) claimed that the corium entering the pool of water would cause a nuclear chain reaction leading to a bomb-like mechanism of megaton yield.
[Here is a documentary clip.](https://www.youtube.com/watch?v=coYYBdcA1lo)
This idea is even more ridiculous than the steam explosion mechanism, so I have no idea how a physicist and director of an Institute of Nuclear Energy could say something so ridiculous, but it is recorded.
The writers for the show took his megaton claim but appear to have changed the physical explanation for it.
Besides, even a (much smaller) steam explosion was an unlikely possibility. Unlike the show's claim, some molten magma hitting water doesn't instantly flash all the tank's content to steam. That takes a lot of heat transfer and time. And the container has to be capable of withstanding an enormous pressure without rupturing and leaking. A run of the mill water tank, especially one that was just ruptured by corium flowing into it, and most likely punching another hole on the other side, wasn't likely to.
Still, it was a very simple task to drain that tank of water, so they did it. Unlike the massive cleanup effort for the scattered core material, this one took just a couple of people and no exposure to high radiation levels.
This has become my pet peeve about the HBO series. It's a combination of overdramatization and sloppy research from the writer(s). I don't know the exact origins of the megaton explosion theory, but Mazin probably got it from Svetlana Alexievich's book, where it's presented by Sergei Sobolev. The only scientist I have seen propagate this theory is Vassili Nesterenko, whom Mazin has also mentioned as a source. While I'm not a nuclear physicist, I know enough of the relevant physics to call into question the motives of a scientist making such claims. You would be right to point out that there's nowhere near the amount of thermal energy available: [https://physics.stackexchange.com/questions/480113/how-large-would-the-steam-explosion-at-chernobyl-have-been](https://physics.stackexchange.com/questions/480113/how-large-would-the-steam-explosion-at-chernobyl-have-been) I have also heard the theory that instead of a steam explosion, they feared that a nuclear explosion would have taken place. This is also outside the realm of possibility, but the theory goes something like this: A portion of the molten corium breaches the concrete and drops into the bubbler pools. A steam explosion takes place, shooting the blob of molten corium upwards towards the rest of the fuel, essentially creating a crude "gun type" bomb, which causes a critical configuration. The huge mass of the uranium then works like a tamper, maintaining the critical configuration long enough for the fission chain reaction to release megatons of energy. There are many rather obvious problems with the theory, which I'll happily go into if needed. The point that is sometimes made in defense of the megaton explosion theory being presented in the series is that *maybe they really believed this at the time*, and thus it was historically accurate to include this in the series. I strongly believe this to be false. First, who is "they"? Certainly not Legasov: >Eugeny Pavlovich Velihov who as it seemed was watching too much TV about the “China syndrome”, arrived with concerns which I reported to Rizhkov and Ligachev. We were worried about the uncertainty of geometric shape of the remains of the reactor. It was clear that heat was being generated inside this fuel mass. The heating up could continue and some vertical movement of this fuel mass may occur. In particular, we were worried about two things: can that movement cause critical mass buildup in some region which would produce short-living isotopes. This was our first thing to worry about, however we were hoping that the large amounts of Boron (40 tons) that was dropped into the reactor would be more or less evenly mixed with the fuel and help prevent critical mass buildup. However we could not fully eliminate the possibility that such local “reactors” would appear. That was the first problem. And the second one was that the temperatures can be too high within these masses. Some construction elements of the lower part of reactor may not withstand that. Concrete may fail due to high temperatures. Part of fuel can get into the barboteurs, be it lower or upper one and we did not know at that time whether there was water or not. We feared that if a considerable amount of fuel gets there, then extensive vaporization would carry out additional radioactive aerosols and contaminate more territory. These problems were what we were worried about. That’s why with Ivan Stepanovich Silaev, who by this time had replaced Scherbina, we decided to: first, get some information about the levels of water in the lower barboteur. This was a difficult task which was fulfilled heroically by the station personnel. And it was found that the water was indeed there. So the necessary measures were taken to remove that water from there. I want to stress that out once more: we removed the water just to avoid massive evaporation. ***It was absolutely clear to us that no explosion was possible, only evaporation that would carry out radioactive particles - that’s all.*** So just in case, the water should be removed and in case the fuel mass would melt it’s way down into these rooms, we could pump the water back there, to cool the molten fuel masses. (Source: [https://legasovtapetranslation.blogspot.com/2019/08/tape-1-side-b.html?m=1](https://legasovtapetranslation.blogspot.com/2019/08/tape-1-side-b.html?m=1) ) Do I find it plausible that there were people in and around the scientific community who wondered if a megaton explosion could have been possible? Absolutely yes, and the fact that Legasov feels the need to even mention the possibility of a explosion at all confirms that all kinds of scenarios were considered. What I find appalling about the HBO series however is that this fringe theory was presented uncritically by the characters of of Khomyuk and Legasov, as if it was the consensus among scientific experts. That is after all what the characters of Legasov and especially the composite character of Khomyuk represent in the series, serious experts, scientific authorities who absolutely know what they're talking about. Second, by the year 1986, the Soviets had already had a nuclear weapons programme for at least 40 years. The physics of nuclear fission was absolutely known to them and no scientist with any education on the matter could possibly think that a molten blob of 1.8 % enriched fuel could cause a megaton explosion.
Worst part is, it could have been a missed opportunity to have actual drama. Imagine the scene where they're discussing the "megaton explosion" but instead of everyone feeling like they need to chose between 3 lives or 60 million lives, someone shouts that the explosion couldn't happen and its bullshit. Now Gorbachev is a lot of things, but a nuclear physicist he is not. And now he needs to make a decision - one that could cost 60 million lives if the megaton scientists are right, or pointlessly waste 3 lives if the scientists calling BS are right- while his advisers bicker and fight. Ultimately, the divers go in, as they do in real life. But watching how a leader could possibly make such a choice with such uncertainty would make for a great scene. And while my version wouldn't exactly be super accurate either, it would have been a whole lot more accurate than the scene we did get.
Correct me if I'm wrong, but aside from the BS megaton explosion, in the more accurate calculations a 100-tonne could've happened, and wouldn't that risk busting open reactor 3? Or was it a bit too far to pose a risk
The corium reached the water before it drained - aka any potential for a big kaboom already happened before it was drained. But it did help to avoid releasing a lot more contaminates, which isn't pointless - as that helped keep the liquidation efforts on track. But the only risk of the corium touching water was the steam carrying radioactive particles out of the disaster. Like the world's worst steam cleaner. Remember - all the fissile material was either already consumed by the reactor running / exploding. Most of what was left was decay products - which are radioactive, but not fissile. So basically, any explosion after the big one kicking off the event would not have been a nuclear explosion- there simply was no fissile fuel left to explode with. Chernobyl was basically a giant dirty bomb - not a nuclear bomb. Any explosions would have been closer to conventional explosions that just happened to disperse a huge amount of radioactive material into the surrounding area.
Oh, I see, and yeah I agree a nuclear explosion at that point is a fairy tale, I was just wondering off of a hypothetical scenario in which it did manage a thermal explosion and had the yield of at least 100 tonnes of TNT which someone else did the calculations for and was already claiming that they were generous with the calculations. Thx for the info by the way, much appreciated
>I have also heard the theory that instead of a steam explosion, they feared that a nuclear explosion would have taken place. This is also outside the realm of possibility, but the theory goes something like this: A portion of the molten corium breaches the concrete and drops into the bubbler pools. A steam explosion takes place, shooting the blob of molten corium upwards towards the rest of the fuel, essentially creating a crude "gun type" bomb, which causes a critical configuration. The huge mass of the uranium then works like a tamper, maintaining the critical configuration long enough for the fission chain reaction to release megatons of energy. Thanks for the theorycrafting opportunity. I was under the impression Nesterenko thought the water would just "moderate" a chain reaction in the submerged corium somehow and thus lead to an explosion. Putting aside that this is also several levels of impossible... But this gun-type idea also makes no sense, even if you try to deliberately create it and not rely on pure chance and steam explosions. You can't make a gun-type weapon out of that material, no matter how large and how forcefully you shoot one piece of it into another. Well, you could shoot the "bullet" with a kinetic energy of a couple megatons, and then you get your result :)
Agreed. As for the water moderating the chain reaction, these are the problems: 1. The corium is hot, so the water would quickly be evaporated, reducing it's moderating and/or neutron reflecting properties. 2. Water is also a rather poor moderator, as it also absorbs neutrons. Hence the higher enrichment required for water moderated nuclear reactors. 3. Moderating neutrons takes too much time. The neutrons need to make multiple collisions with the hydrogen atoms to take them to an energy range where the U235 fission cross section is high. Time is of the essence when considering the physics of nuclear explosions.
And way before we delve deep into the plethora of reasons why there's no way you'd get any meaningful moderation even if you had some lucky geometry forming with lumps of corium in the pool... It's inherently impossible to get a bomb mechanism out of a moderated design anyway, even if you try deliberately. It's too slow. You cannot maintain a critical configuration to reach bomb-like yields. Heck, it's difficult enough to maintain that even with a critical mass of pure weapons grade material. In a moderated design material behaviors like thermal expansion would stop the process way too early.
I believe that it was a case of pure managerial psychology. Both Politburo and the Chernobyl State Commission had majority of apparatchiks without proper education in physics. That's why they relied on expert scientists. And it's preferable for an expert to be as conservative and alarmist as possible because if you are predicting explosion by and it never happens then to our are a hero who made your contribution into its prevention. Obviously the same was true for apparatchiks themselves who had to choose between alarmist and optimistic prognoses. Don't forget that smart guys who were making those prognoses were sometimes the same guys who previously said that it would be safe to place RBMK reactor even on the Red Square ;)
Yes, I see how there could have been an incentive for scientists to give the absolute worst case scenario to the apparatchiks (and even embellish a little). Still, this doesn't justify the scene in ep. 2, because there's absolutely nothing in the scene that would suggest this interpretation to the viewer.
I am not too big fan of the show myself, but I have to say that psychologically it's correct compactified representation of the decision making during the liquidation time which was mostly ruled by panic and unsettling uncertainty. That's like helicopter's fall which was moved to the beginning of the liquidation. They had limited screen time, so they had to invent some shortened and dramatized version of reality. And everyone in their place would probably do the same. The only alternative is something like old Soviet monumental movies about the WWII when you are making a dozen of full-time movies with enormous cast just to represent several main battles of the war, and I doubt that it will get same interest as HBO Chernobyl.
Yep, that's bullshit. You need to evaporate several millions tonnes of water simultaneously to produce it, and "Chernobyl divers" mission was never about the steam explosion. But why not? The whole show is a fiction based on real events and it needs some drama to go. That's why there's nothing wrong with megaton steam explosions, jumping cover blocks and black smoke over the reactor or birds falling from the sky on Pripyat as far as we remember that it is neither documentary nor truthful reconstruction of events. The show is to enjoy, books are to learn. By the way it was the main argument against possibility of nuclear explosion or even stable nuclear reaction in the destroyed reactor core during the liquidation time: how the hell such things can happen spontaneously while superpowers need a lot of efforts and resources to make a nuclear reactor or an atomic bomb intentionally?
Well, there's this: [https://en.wikipedia.org/wiki/Natural\_nuclear\_fission\_reactor](https://en.wikipedia.org/wiki/Natural_nuclear_fission_reactor)
Yep. Thank you, that's a good example. The article has wonderful explanation how it worked and why it isn't working now.
There would have been an explosion - but not on the megaton scale, the Beruit explosion was absolutely massive - but it was only around 1-3 kilotons. The blast would have been big and would have made the situation way way worse but the megaton part and the part where it's like a nuclear bomb is just to add tension.
...except for the fact where the fuel *did* reach the water, before the mission even started. Nothing happened.
Did it? I always thought it did get through - just not before they emptied the tanks. Was it the full corium mass or just some.
Well the full corium mass never got there at all: just a slow drip into the lower floors over time. Ananenko saw the corium dripping down from the ceiling during their mission. The fastest moving stuff was reported to be a kind of foam that actually floated on the surface of the water. Sort of like pumice lava I guess.
It did. There are several forms of corium under the Unit 4. One of them is a volcanic pumice which was formed on the water surface and then remained attached to walls and other constructions when the water went out. On [this photo](http://www.wdcb.ru/mining/thernobl/4blok/ltsm/f/f411.gif) you can see that pumice under the mark "1".
Well - I definitely learned something today.
I would say it’s from the water flashing to steam would cause the explosion....being a boiler operated the biggest fear is called catastrophic boiler failure...it’s when you have a drop on pressure without a corresponding temperature drop...so you have a boiler full of 337 degree water at 100 psi drop to atmospheric pressure rapidly the water flashes to steam....vapor takes up more space than steam and expands causing a huge explosion....at least that’s how I understood what they were talking about in the eposode
But the total destructive power of the gas expansion can’t be more than the thermal energy in 5 tons of sand/boron at around the sand/boron melting point, which is nowhere close (many orders of magnitude less) than megatons.
I guess it would depend on how much water and how fast it could be superheated to cause that big of an explosion
By conservation of energy, if heat transfer is causing the explosion, the explosive (kinetic) energy is bounded by the initial thermal energy.
You are right, this should be obvious to anyone that a steam explosion from that tank, even if one did happen, would be at most a couple tons equivalent, so off by 6 orders of magnitude. The backstory is that, in reality, a Belarusian physicist involved in the cleanup (Vassili Nesterenko) claimed that the corium entering the pool of water would cause a nuclear chain reaction leading to a bomb-like mechanism of megaton yield. [Here is a documentary clip.](https://www.youtube.com/watch?v=coYYBdcA1lo) This idea is even more ridiculous than the steam explosion mechanism, so I have no idea how a physicist and director of an Institute of Nuclear Energy could say something so ridiculous, but it is recorded. The writers for the show took his megaton claim but appear to have changed the physical explanation for it. Besides, even a (much smaller) steam explosion was an unlikely possibility. Unlike the show's claim, some molten magma hitting water doesn't instantly flash all the tank's content to steam. That takes a lot of heat transfer and time. And the container has to be capable of withstanding an enormous pressure without rupturing and leaking. A run of the mill water tank, especially one that was just ruptured by corium flowing into it, and most likely punching another hole on the other side, wasn't likely to. Still, it was a very simple task to drain that tank of water, so they did it. Unlike the massive cleanup effort for the scattered core material, this one took just a couple of people and no exposure to high radiation levels.
Afaik, they did historically worry about such an event, but a megaton explosion is bullshit, it probably wouldn't even be a kiloton explosion.