TL;DR; at the bottom
I have some background in physics and the new HBO show Chernobil became instantly my all time as it was very strong at displaying the emotions as well as very accurate from thy physics standpoint. [SPOILER ALERT] Except the big table scene, when they discuss the possibility of a thermal explosion caused by the fuel melting down into largewater tanks for the ECS. This the physics does not make much sence. The scientists say is will INSTANTLY superheat and vaporize 7000 cubic meters of water, causing a 2-4 megaton explosion (Hiroshima bomb had 15 kilotuns, over 100x smaller).
In physics turning water into steam take tremendous amounts of energy. As a simple example, if you had an eletric kettle that could heat up 1L of water from 0°C to 100°C (both in liquid state) in 5 minutes, it would take you extra 45 minutes to turn it into steam and vaporize it.
I have seen youtube video where they heat up a 50kg anvil until it is glowing light yellow, almost white hot (basically close to its melting point) and throw it into a small pool or a frozen lake. The Chernobil logic says that there should be a thermal explosion as it instantly vaporizes a lot of water, but that is not the case, it will be bubbling and hissing very loudly at the beginning, and over time quietly slowing down, vaporizing an insignificant amount of water. At the beggining, when it was actually turning some water into steam, the steam server as an isolation blanket between the metal and water, further slowing down the heat exchange.
I wont go into calculating thermal equations here, but several tons of melted fuel and sand at 2000°C dont have enough thermal energy to vaporize 7000 cubic meters of water, not even close, not enough to vaporize even 70 cubic meters. Definitely not instantly, the steam itself would work as a great insulator from the rest of the water.
Now the fuel could vaporize the water over a long period of time, because the continued radioactive decay and the nuclear reaction could generate enough heat, as the fuel continues to react and burn for several years, but it would take probably days or even weeks, it would have to be in a closed ennvironment where the steam and pressure can build up into extreme levels. Maybe this way it would be somehow possible, a closed system where the fuel continues to heat up the water, the steam pressure build up into EXTREME levels, and then it would all rapture at the same time, but still it is hard to imagine a 2-4 MEGATON explosion caused by steam.
TL;DR; How can less than 3 tons of 2000°C of melted fuel and sand instantly vaporize 7000 cubic meters of water, the math does not add up. Plus when the melted fuel start rapidly vaporizing some of the water, the steam creates an insulation barrier between the fuel and water, slowing the thermal transfer even more.
In: Physics
Supercritical reactors release an absurd amount of energy as radiation, which can be absorbed by water and causes the water to heat up. Not only this, but in nuclear reactors, water is often pressurized to allow it to become hotter than it would normally boil. When the pressure is released, it all cools down some as some of it vaporizes, but if it is all still over 100C then it all still boils and you get a big explosion. All that said, I don’t know about Chernobyl and have not watched the show.
In addition to what everyone else already said, you’re analyzing a line of dialogue from a work of fiction as if it was real. Stop. Yes, it’s based on real life, but it’s made for entertainment, not to faithfully reproduce the events to 100% accuracy. I mean, do you think in real life all those scientists would give extremely basic explanations of how nuclear reactors work to other scientists? No, that’s expository dialogue for benefit of the audience because the writers want us laypeople to understand what’s going on. Do you think the audience would understand and relate to people sitting a table speaking in technical jargon about steam and thermal energy? No, so the writers introduce the specter of BIG BAD EXPLOSION, so we quickly understand the stakes. It’s a throwaway line of dialogue for the benefit of the audience, not for scientific accuracy.
> I have seen youtube video where they heat up a 50kg anvil until it is glowing light yellow, almost white hot
So around 1200 Celsius. The core was approaching 3000 Celsius.
> it will be bubbling and hissing very loudly at the beginning, and over time quietly slowing down,
Except in this case it isn’t slowing down because it is generating its own heat internally.
> the steam itself would work as a great insulator from the rest of the water.
But it is molten. The violently boiling water might break apart the dripping mass of corium and greatly increase the rate of heat transfer.
> several tons of melted fuel and sand at 2000°C dont have enough thermal energy to vaporize 7000 cubic meters of water, not even close,
If you were just thinking of it as hot material then no, of course it wouldn’t. But this is radioactive material, generating its own heat by the decay of the elements themselves. Yes, it does in concept have more than 100 tonnes worth of TNT in the form of heat bound up inside it.
Now, **megatons**? No, that isn’t realistic. But an immense steam explosion that would impact a huge area? Of course.
Because that was one of a handful of scientific inaccuracies in the show.
In actuality the core melting down and starting a steam explosion would have barely been able to destroy the entire building.
That being said: Chernobyl except the building was blew up makes the Chernobyl we actually had look like Fukushima.
It would have been much much much much worse.
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