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An explosion blew the roof off an unstable reactor north of Tokyo last night, raising fears of a disastrous meltdown at a nuclear plant damaged in the massive earthquake.
The 8.9-magnitude earthquake - the strongest recorded in Japan - sent a tsunami of up to 10 metres through towns and cities across the northeast coast. Japanese media estimated that at least 1300 people were killed.
Jiji news agency said there had been an explosion at the stricken 40-year-old Daiichi 1 reactor and TV footage showed smoke and vapour rising from the plant, 240km north of Tokyo.
Some of the walls had crumbled and residents were urged to stay indoors and cover their faces with wet towels. Four workers were reported to have been injured at the plant.
The blast came as plant operator Tokyo Electric Power worked desperately to reduce pressures in the core of the reactor that, if not contained, could lead to a release of radiation into the atmosphere.
"An unchecked rise in temperature could cause the core to essentially turn into a molten mass that could burn through the reactor vessel," risk information service Stratfor said. "This may lead to a release of an unchecked amount of radiation into the containment building that surrounds the reactor."
NHK TV and Jiji said the outer structure of the building housing the reactor appeared to have blown off, which could suggest the containment building had been breached.
NHK also said the Government's Nuclear and Industrial Safety Agency had said two radioactive substances, cesium and radioactive iodine, had been detected.
This indicated that some of the metal containers of uranium fuel may have started melting. But University of Tokyo Professor Naoto Sekimura said only a small part of the fuel may have melted and leaked.
A "meltdown" refers to a very serious collapse of a power plant's systems and its ability to manage temperatures.
Yaroslov Shtrombakh, a Russian nuclear expert, said a Chernobyl-style meltdown was unlikely. "It's not a fast reaction like Chernobyl," he said. "I think that everything will be contained within the grounds, and there will be no big catastrophe."
Earlier, the operator released what it said was a tiny amount of radioactive steam to reduce the pressure. The danger was minimal because thousands of people had been evacuated. The evacuation zone had been extended to 10km.
Fukushima Prefecture official Masato Abe said the cause of the rattling and smoke was unclear.