Also to add on, not in the hostile way that I was before, nuclear reactors don't explode like nukes. They either do pressure explosions or gas explosions. Since there is never enough material and even then the fuel is not arranged in such a way that it would cause a chain reaction that causes it to explode. But nuclear reactors can explode if there is too much pressure or if hydrogen builds up and is ignited, this is because the cladding used for fuel rods has a reaction with steam which produces hydrogen gas (steam bubbles can form when stuff is not going right in the pressure vessel). Also, a bomb dropped into the reactor would not cause it to blow up like a nuke either, I feel like no one should have to say that...
It would have been cool and logical though if instead the reactor melted down and the molten corium hit the water below (given that it is in the sewers) and instead creates a steam explosion throughout the tunnels (would not really work because not all the water is hit at the same time but it makes a lot more sense then doing nuke=reactor).
Basically think of it as this. Your car does not just up and explode right? So think of a nuclear reactor as that, sure the reactor and a car can break and things can still kill you, but it won't just explode like a bomb. Now think of someone sealing up gas in a tank and throwing a match in, like a nuke it requires it to be arranged in such a way that it does a
sudden release of energy (nuclear bombs need to be very fast in order to
turn the area around it into plasma so it goes what is called prompt critical, this can also happen in reactors but the fuel is not arranged right in order to explode, were basically most of the neutrons if not all of them are hitting each atom of bomb-grade material inside which itself is specially arranged so little to no neutrons escape).
(Can you see that this is a field I have looked at and want to go into)?
This Quora post is a pretty good summery, still should look up stuff on your own to get a full grasp and fact check though.
