IBM Thomas J. Watson Research Center｜Learning From Fukushima Dai-ichi [pdf]

Although emergency injection of seawater was improvised to remove the decay heat from the reactors, it was too late to avoid boiling off of much of the water in the reactor pressure vessels and the reaction with steam of the zirconium alloy “clad” of the fuel rods in the reactor with the evolution of hydrogen, which in turn overpressurized the massive concrete containment of the reactor and compelled venting of the hydrogen and some of the radioactive material from the reactor.