Six people were injured Monday after an explosion at a nuclear reactor in north-east Japan following an earthquake, news reports said, dpa reported.
The steel hull of the reactor in Fukushima, 240 kilometres north of Tokyo, was not damaged, a government spokesman said, citing information from the plant operator, the Tokyo Electric Power Co (TEPCO).
The city is home to 10 reactors at two power plants, and explosions have occurred at two of the reactors since Friday's magnitude-9 earthquake and tsunami. One happened Saturday and the second Monday after a magnitude-6.2 aftershock.
A TEPCO spokesman said radiation was measured at the plant at 20 microsievert per hour 25 minutes after Monday's blast. Japan allows an hourly exposure of 500 microsievert, which is a measure of the biological effects of radiation.
Despite the measurement, the government ordered anyone within a 20-kilometre radius of the reactor to remain indoors with the windows and doors closed. Residents in the zone had already been evacuated.
The wind Monday in Fukushima was blowing in an easterly direction, so any radioactivity would be blown toward the Pacific and not into Japan's interior, news reports said, but the governor of Tokyo, Shintaro Ishihara, said that he expected radioactivity to eventually be measured in the capital.
Before Monday's blast, radiation levels had risen at the Fukushima plant, the Kyodo News agency reported.
TEPCO has been scrambling since Saturday to prevent a meltdown at the plant, even pumping seawater into the reactor where Saturday's explosion occurred after Friday's quake caused the reactor cooling systems to break down. The disruption allowed nuclear fuel rods to become dangerously overheated.
On Sunday, radiation at one of the Fukushima reactors was at 882 microsievert per hour and briefly topped 1,204.
Despite elevated radiation levels, government officials said Sunday that there was no immediate danger outside the plant.