[pfdietz]

Chernobyl had two explosions, a few seconds apart. The second, larger one was after most of the water had left the core. The reactor had a positive void coefficient, so losing water increased reactivity.

Of course these were prompt supercritical reactions in a moderated system, so they are not explosions in the sense of bombs, which are fast systems with neutron doubling times measured in fractions of a microsecond.