Reuters reports:
Jiji news agency said there had been an explosion at the stricken 40-year-old Daichi 1 reactor and TV footage showed vapor rising from the plant, which lies 240 km (150 miles) north of Tokyo.
The country's nuclear safety agency could not confirm the reported incident, which came as plant operator Tokyo Electric Power Co (Tepco) 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," political risk information service Stratfor said in a report. "This may lead to a release of an unchecked amount of radiation into the containment building that surrounds the reactor."
NHK television said the outer structure of the building that houses the reactor appeared to have blown off, which could suggest the containment building had already been breached.
Most residents in the area, authorities say, have been evacuated to at least six miles away from the reactor. The claim is that anybody that far away is safe.
You can see video of the explosion here (h/t to Kimberley)
Lefty Coaster has a diary discussion here. He's pointed to a backgrounder on potential harm from earthquakes to Japan's nuclear power plants written before yesterday's monster temblor.
h/t to TheMomCat for pointing to this video: