Because this was a reactor that operated on water that was already at its boiling point, this also meant that the pressure inside the reactor was rising as well.

FORBES: Explainer: What Caused The Incident At Fukushima-Daiichi

In nineteen eighty-one, Iraq was on the point of acquiring nuclear power. 10.17.01 Narrator The Osirak reactor was an experimental plutonium plant.

BBC

Normally, circulation would channel the hydrogen and oxygen to a recombiner where they would be restored back to water, but in the hours after the reactors were shut down, hydrogen was accumulating and separating in the wetwell and reached a point where it was vented into the sparse steel-frame structure at the top of the reactor building.

FORBES: Explainer: What Caused The Incident At Fukushima-Daiichi