Update 4:43 PM: Now according to Kyodo, radiation is measured at 1000x normal.
Update 5:33: Both reactors at the plant have been damaged, and officials say they have "lost control" of the pressure, according to Reuters.
Update 7:51: At least 20,000 people are being evacuated from the region says Reuters, which ominously quotes an expert who thinks there may be just a few hours until meltdown.
At 0630 GMT, an explosion was heard at the plant. At 0800 GMT, Fox News Channel reported the explosion and a plume of "white smoke" rising above the plant. At 0803 GMT BBC News reported that Japan's NHK TV reported the explosion as "near" the plant, and that four workers were injured.