Ionizing water, or catalytically decomposing water at cherry red temps would make both H2 and O2 which could also recombine, especially with all that ionizing radiation. Maybe some ionizing radiation from a "hot" isotope of Xe or I2 kicked off the explosion that blew off the roof...or maybe just a motor or a switch that was not rated" explosion-proof".

Nb41

I also read that after Chernobyl, to prevent hydrogen gas explosions, plant safety plans foresee the flooding of the vessel with nitrogen so that there is no oxygen to react with. Hence the explosion only outside after venting.

_{*Lunatic*, n.
One whose delusions are out of fashion.}