So far, the only reactor that seems to pose an immediate risk of widespread danger is one of the two shut-down reactors at Fukushima No. 1, also known as Fukushima Daiichi, which was disabled by an explosion overnight that destroyed the building housing the reactor and the backup cooling system.
Photos: Scenes from the earthquake
As part of an automatic safety procedure, control rods were inserted into the cores of the reactors following the earthquake, which stops production of electricity. But the cores continue to generate heat for several days after shutdown and must be continuously cooled by pumping water over them and through cooling towers.
Backup generators powering the pumps at the first five disabled reactors failed almost immediately after the earthquake, apparently inactivated by exposure to seawater from the tsunami that swept through the seaside plants. The facilities had to rely on backup batteries that last up to eight hours until additional batteries and generators could be brought in.
Although the company has released no details about the sixth reactor, it appears the diesel generators there worked for a couple of days before they too finally gave out.
More than 170,000 people have been evacuated from an area within a 12-mile radius of Fukushima No. 1 and within a six-mile radius of Fukushima No. 2, also known as Fukushima Daini, which is about seven miles down the coast from No. 1.