It's actually wrong on several levels. First of all, in the show the characters explicitly state that they are referring to a steam explosion. A steam explosion in the megaton range isn't just implausible, it's patently absurd. The upper bound on the real number is lower by 4-5 orders of magnitude. Don't get me wrong - it would still have been extremely disasterous and could have increased the total radionuclide emissions by up to an order of magnitude, but all the stuff about half of Europe being uninhabitable or everyone in Kiev dying immediately is nonsense. Instead, the only way to get remotely close to a megaton range estimate is to assume the steam explosion would cause a nuclear explosion in one or more of the reactor cores. That possibility was briefly considered and quickly dismissed as improbable in the extreme, which it was.

In the context of a drama about the explosion and aftermath, I'd say it's quite reasonable to use an absurd number from one of the physicists involved.

Perhaps it was indeed wrong, but it sounds like that might be actually what those involved believed at the time.

That is very unlikely, at least at the higher executive commitee level. It's impossible to exclude the possibility entirely, of course, but there is no surviving evidence of it and it certainly doesn't appear in Legasov's original report (or any subsequent study).

>That possibility was briefly considered and quickly dismissed as improbable in the extreme, which it was.

A nuclear explosion would require Uranium enriched to at least 85%, whereas the fuel in RBMK reactors was only enriched to 2%. It's not just unlikely it's actually impossible.

That's not entirely correct. There is no specific level to which uranium (or any other fissile material) must be enriched before supercriticality may occur, the level of enrichment simply determines the critical mass required. For instance, the critical mass for pure U-235 is around 50kg, while for 20% enriched U-235 you'd need about half a ton or so. [1]

In the case of Chernobyl, the concern was that upon contact with water (which acts as a neutron moderator in these circumstances) and given the inevitable sequence of steam explosions that would follow, some regions of superheated corium might be forced into prompt criticality events. These would technically qualify as nuclear explosions, but the sequence of events required to get from this set of increasingly improbable assumptions to anything remotely close to a megaton range explosion is, as I said, improbable to the point of being trivially dismissable.

[1] EDIT: I realized I should probably point out for clarity that I'm referencing the traditional measures for a critical mass - the mass required for a homogenous sphere of a given material to go critical. Determining the critical mass for a highly inhomogenous material of complex geometry, highly varied temperature and potentially surrounded by neutron reflecting substances is not a trivial task.

Prompt critical is a necessary, but not sufficient condition for a nuclear weapon to go boom. In a weapon the neutron multiplication time is a million times faster than in a prompt criticality transient in a thermal reactor. Otherwise it would disintegrate before it could develop any significant yield.

But yes, certainly the melted fuel becoming critical and/or causing more steam explosions could have made the Chernobyl accident worse than it already was.