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.

The three workers wading through water to open the sluice gates did indeed happen, but it's worth noting that all of them survived just fine and at least two were still alive as recently as a few years ago.

I agree, though, I love how many tiny things that are actually true they managed to squeeze in there. As another example, the part where the firefighter's wife bribes her way into the closed hospital in Moscow - that really happened too.