Uh-oh, Japan, this is bad

[Tellos Athenaios]

This isn't a steam pressure problem. This is an energy transfer from the reactor into steam which isn't kept moving which means that locally so much energy is “stored” in the steam that the bonds of the atoms from the water molecules in the steam are dissolved and the elements separated. This creates hydrogen and oxygen, which rises to the top of the structure (having a lesser density than steam). Away from this energy source (heat emitted from the core) these elements react into water again, i.e. ignition of the hydrogen gas. The energy required to split the water molecules is released as it were, and since nearly all hydrogen will react with the oxygen within the smallest timeframe, the resulting explosion is very powerful.

Now this danger is fairly unique to BWR reactors, because the temperature and the pressure required to make the steam absorb so much energy and then this energy to make the molecules disintegrate into their component elements is a combination of factors that you don't get much elsewhere. In fact to produce hydrogen you typically need to resort to electrolyse, but this is a much more controlled reaction (and once the power is cut the reaction stops). Since hydrogen is most often undesired (because it is so dangerous in the presence of oxygen) research goes into substitutes for hydrogen producing solutions to avoid it entirely.