‘The explosion was caused by hydrogen interacting with oxygen outside the reactor. The hydrogen was formed when the superheated fuel rods came in contact with water being poured over it to prevent a meltdown.
“They are working furiously to find a solution to cool the core, and this afternoon in Europe we heard that they have begun to inject sea water into the core,” said Mark Hibbs, a senior associate at the Nuclear Policy Program for the Carnegie Endowment for International Peace. “That is an indication of how serious the problem is and how the Japanese had to resort to unusual and improvised solutions to cool the reactor core.”‘
via Explosion at Japan nuke plant, disaster toll rises – Yahoo! News.