Second Hydrogen Explosion at Japanese Nuclear Plants
Hydrogen is released when a nuclear core is damaged. The Hydrogen gas was being “vented” as part of the emergency procedures to try to cool these nuclear reactor cores. The building exploding is not the containment, but is the outside building that covers the containment.
I guess the thing to remember is that even after an earthquake of “historic” proportions and the tsunami that followed, these reactors are still preventing a major release of radiation. The releases have, so far, been small and controlled. I think this is a testimate to the robust design of these plants and the efforts of the operators and engineers now working to control further damage.