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."
Officials have said that radiation levels were elevated before the blast: At one point, the plant was releasing each hour the amount of radiation a person normally absorbs from the environment each year.
The explosion was preceded by puff of white smoke that gathered intensity until it became a huge cloud enveloping the entire facility, located in Fukushima, 20 miles (30 kilometers) from Iwaki. After the explosion, the walls of the building crumbled, leaving only a skeletal metal frame.
Tokyo Power Electric Co., the utility that runs the Fukushima Dai-ichi plant, said four workers suffered fractures and bruises and were being treated at a hospital.
The trouble began at the plant's Unit 1 after the massive 8.9-magnitude earthquake and the tsunami it spawned knocked out power there, depriving it of its cooling system.
