But Edano told reporters there was no "direct health damage" so far from the crisis. "The accident itself is really serious, but we have set our priority so as not to cause health damage."
Hironobu Unesaki, a nuclear physicist at Kyoto University Research Reactor Institute, said the revision was not a cause for worry, that it had to do with the overall release of radiation and was not directly linked to health dangers. He said most of the radiation was released early in the crisis and that the reactors still have mostly intact containment vessels surrounding their nuclear cores.
The change was "not directly connected to the environmental and health effects," Unesaki said. "Judging from all the measurement data, it is quite under control. It doesn't mean that a significant amount of release is now continuing."
Prime Minister Naoto Kan, in a national television address, urged the public not to panic and to focus on recovering from the disaster.
"Right now, the situation of the nuclear reactors at the Fukushima plant has been stabilizing step by step. The amount of radiation leaks is on the decline," he said. "But we are not at the stage yet where we can let our guards down."
Continued aftershocks following the 9.0-magnitude megaquake on March 11 are impeding work on stabilizing the Fukushima plant — the latest a 6.3-magnitude one Tuesday that prompted plant operator Tokyo Electric Power Co., or TEPCO, to temporarily pull back workers.
Officials from Japan's Nuclear and Industrial Safety Agency said that the cumulative amount of radioactive particles released into the atmosphere since the incident had reached levels that apply to a Level 7 incident. Other factors included damage to the plant's buildings and accumulated radiation levels for its workers.