The explosions at the Fukushima nuclear reactors were caused by a build of hydrogen. I was wondering where this hydrogen came from.
This question Fukushima - Isn't the hydrogen presence a proof of meltdown? suggests that the hydrogen is caused by thermolysis of water.
But surely this can't be the case? If it were, wouldn't it require the hydrogen and the oxygen to be separated whilst they cooled - else they would recombine into water in a more gradual fashion as the gases cool once they are clear of the scorching hot fuel rods?