TOKYO A fresh explosion rocked Japan's quake-stricken nuclear power complex on Tuesday, around its overheating No.2 reactor, but there was no immediate word of any damage to the reactor itself, the country's nuclear safety agency said.
Jiji news agency quoted authorities as saying radiation levels around the complex immediately after the blast, the third at the site, were rising but still relatively low.
It added, however, that some workers had been told to leave the plant -- a development that one expert had warned beforehand could signal a worsening stage for the crisis.
Authorities at the Fukushima Daiichi complex, damaged in Friday's massive earthquake and tsunami, are trying to prevent meltdowns in all three of the plant's nuclear reactors, trying to flood the chambers with sea water to cool them down.
"It was a hydrogen explosion. We are still assessing the cause and unsure whether the explosion was caused by damage to the suppression chamber," an official at the safety agency told Reuters. He did not have any more details.
Twice before, there have been hydrogen explosions which have ripped off some roofing from the plant, but these had not damaged the reactor vessels, authorities have said. There was no immediate word on any damage from this third blast.
The plant's owner, Tokyo Electric Power Co, was not immediately able to gather data from the complex's No.2 reactor, Jiji said, adding that surrounding radiation levels had risen to 1,941 microsieverts an hour, up from about 1,000 moments earlier.
Japanese authorities say levels would need to reach 1 million or so before causing large-scale radiation sickness.
(Reporting by Shinichi Saoshiro; Editing by Mark Bendeich)