Chernobyl bubbler tanks explosion

[pavelcherepan]

The other day I was watching HBO's "Chernobyl" which by the way is amazingly shot, beautifully directed and I can't really recommend it enough. There are quite a few factual and authenticity issues with the series, but those are all pretty minor and are well within artistic license boundaries. 

At the same time one part of episode 2 struck me as a gross exaggeration. In this part of episode a Belorussian nuclear physicist Ulana Khomyuk (who authors admitted in end credits was a fictional character, a combination of dozens of scientists helping recovery efforts), discovered that as a result of upcoming meltdown of the reactor, corium lava with temperatures well over 2000 degrees C will penetrate biological shield and then cause the explosion of two 100 m³ bubbler tanks situated underneath the reactor hall. This, as Ulana pointed in a meeting with Gorbachev, would result in an explosion equivalent to 2Mt TNT with complete destruction of all remaining reactors and turning most of Ukraine and Belarus uninhabitable for at least 100 years.

I'm quite confused as how this is at all possible. Assuming both tanks are full with water and assuming the unlikely scenario that the all 100% of water undergoes thermal decomposition and then hydrogen explodes shortly after, I've only been able to come up with an energy output of about 3000 MJ, which is approximately 0.75 kt of TNT equivalent. And if we consider that depending on the temperature of corium lava, only minor percentage of water will undergo thermal decomposition, resulting explosion would be even smaller, possibly as low as 100 tonnes TNT.

Am I missing something here?

EDIT: There was also a possibility of additional large energy release from Zr reacting with water vapour, but from what analysis of Chernobyl corium I'd seen, there was no free Zr in the melt - most of it was in the form of oxides, silicates or complex compounds with uranium.

Edited June 12, 2019 by pavelcherepan

[MigL]

Your scenario doesn't make for exciting TV.

Remember the movie The China Syndrome ?
After Three Mile Island the general public thought a reactor meltdown would penetrate through the Earth and reach the opposite side of the globe.

[pavelcherepan]

1 hour ago, MigL said:

Your scenario doesn't make for exciting TV.

Yeah, you're probably right. You need to keep stakes high to make sure audience doesn't switch off.