To be clear, this didn't happen. No one said it did. There was a chance something like this could have happened (the Chernobyl miniseries does a great job of showing this). The issue was that there were large water tanks under the reactor, and that the reactor material would eventually melt into those tanks, superheat the water and cause an enormous steam explosion, further scattering radioactive materials into the atmosphere, and destroying the three other reactors at Chernobyl, scattering their material too.
That said, the idea that a 5MT explosion at Chernobyl would have levelled the city of Kiev is not particularly realistic [0]. (note that the fallout effects from Chernobyl would be much worse than from a nuclear explosion, so given winds, fallout could have made Kiev, or even Moscow, uninhabitable). And in fact, as explained elsewhere, the explosion wouldn't have been 5MT, but much smaller, although that wouldn't have mattered much for the issue of spreading radioactive material.
To be even more clear, a steam explosion cannot by any stretch of the imagination "level" a city fully two hours away. And the phrase "equivalent of a 5 megaton explosion" has no possible interpretation that squares with reality. The idea of a megaton-sized steam explosion is ludicrous, so I assume he means to imply some connection to radioactive fallout - but the amount of fallout isn't really correlated with the strength of the explosion. In fact, early fission bombs were relatively dirty while later thermonuclear bombs were relatively clean. The largest nuclear explosion ever produced was actually remarkably clean due to its unique design. So what is Moxie even talking about? It borders on incoherent, and I think you're being far too charitable.
By the way, the three men who faced "almost certain death" were certainly heroic but weren't in nearly as much danger as he plays up. All three men survived and lived for decades afterward.
See this reddit thread[0] linked elsewhere here. The issue is likely a faulty primary source, some russian official who wildly misspoke or miswrote something.
> but the amount of fallout isn't really correlated with the strength of the explosion.
It absolutely is for a similar burst condition and similar weapon technology, so it's absolutely credible that one would use, say, groundburst yield with a technology then dominant for large weapons as a reference for fallout. (And, IIRC, groundburst yield is both much greater and much less variable by nuclear bomb technology than airburst since most of the fallout is ground material with induced radioactivity while with an airburst most of the fallout is bomb material, so that technology might not even need to be significant variable using groundburst yield as a reference scale.)
While 5 megatons definitely does seem like an overestimation, I think the fear was that it was a much larger mass of molten corium, including not just the fuel itself, but also molten silicon and lead, from the material dumped on the exposed reactor to quench the fires and prevent fallout. There was an estimated 5000 tons of this suppressant material, although I don't know how much could have reasonably been expected to melt (and I think later analysis shows that very little of it actually landed on the exposed core itself.)
Then when the molten corium hits the water, the water could act as a neutron moderator for a runaway fission reaction. You have to think this would be more a nuclear "fizzle" than the prompt criticality that is necessary for a >10kt (let alone megaton) bomb, but it would still be bad.
A 5 megaton explosion that levels Kiev? Probably not, but then again I'm not the nuclear physicist who came up with that number.
The idea that blowing the core material into the atmosphere and causing further meltdowns in the other three reactors would be a major radiological disaster for Northern Europe, though, I can appreciate.
The largest ever fission explosion was, IIRC, something like a few hundred kT. It's just physically impossible to have a bigger yield from fission alone, as the bomb assembly blows apart before it has time to fission. And this was from a carefully designed weapon, not a reactor fueled with LEU melting. So yes, 5 MT is pure fantasy with absolutely no connection to reality.
As for prompt criticality, in a weapon the neutron multiplication time is around a million times faster than in a prompt criticality transient in a thermal neutron reactor. That's what allows a weapon to have any significant yield before it disintegrates.
The worst case scenario is still orders of magnitude less than "5 megaton explosion". Somebody's math was wrong, or lost in translation. Either the math was done wrong, not done at all, or the figures got messed up in translation.
To be clear, this didn't happen. No one said it did. There was a chance something like this could have happened (the Chernobyl miniseries does a great job of showing this). The issue was that there were large water tanks under the reactor, and that the reactor material would eventually melt into those tanks, superheat the water and cause an enormous steam explosion, further scattering radioactive materials into the atmosphere, and destroying the three other reactors at Chernobyl, scattering their material too.
That said, the idea that a 5MT explosion at Chernobyl would have levelled the city of Kiev is not particularly realistic [0]. (note that the fallout effects from Chernobyl would be much worse than from a nuclear explosion, so given winds, fallout could have made Kiev, or even Moscow, uninhabitable). And in fact, as explained elsewhere, the explosion wouldn't have been 5MT, but much smaller, although that wouldn't have mattered much for the issue of spreading radioactive material.
[0]: https://nuclearsecrecy.com/nukemap/?&kt=5000&lat=51.3906031&...