Can nuclear power plants explode

Answer 1

It depends. Most, if not all, nuclear power plants are incapable of exploding in a nuclear fashion. Yes, it is possible to have super prompt criticality, where reactivity, or what we call KEffective is much greater than 1 and the reaction geometry degrades to the point where it can be sustained with only thermal neutrons.

Problem is, that physics intervenes. Fissionable material wants to expand when it is super prompt critical, and the pressures involved are astronomical. The core, in this case, then tends to go sub critical due to prompt dispersal, which is where the core gets suddenly, somewhat explosively, "larger", and the reaction stops.

In order to have a true nuclear detonation, you need to hold the core together long enough for a sizeable portion of the fuel to convert to energy, and that is just not possible with a power plant - a bomb, yes - but not a power plant.

Chernobyl went super prompt critical, and it did explode, but that explosion was not a complete detonation and, again, it just as suddenly went sub-critical due to prompt dispersal.

There are other things involved, of course. You can have a hydrogen explosion, caused by a buildup of hydrogen gas (no, I'm not talking about a hydrogen bomb) due to a high temperature reaction with the zirconium cladding on the fuel pins, such hydrogen then being detonated perhaps by mixing with water. This is what happened recently in Japan.

Even Chernobyl had no nuclear explosion, it did have a strong nuclear energy excursion, but the resulting explosion was still entirely a steam explosion as water in the cooling system and steam generator tanks flashed into high pressure vapor.

Also nuclear explosives operate on fast neutrons, not thermal neutrons, as they have no moderator to slow the neutrons. LLNL tried nuclear explosives with a moderator (Uranium Hydride fuel/moderator) in several tests in the 1950s, they all fizzled. This strongly suggests that any thermal neutron (moderated) reactor is incapable of a nuclear explosion (however fast neutron reactors like breeders might be a different issue).