Four Japanese electricity workers were killed on Monday and seven others injured when turbine steam escaped at a nuclear power plant in Mihama, on the Sea of Japan.
The reactor automatically shut down while rescue workers tried to help the victims.
The Nuclear and Industrial Safety Agency said there were no radioactive materials in the steam and no radiation impact on the environment.
The incident involved the highest number of fatalities at a Japanese nuclear power plant, and it has heightened concern about the nuclear industry. The dead, who have not been identified, were severely burned and suffered heart and lung failure.
Officials of Kansai Electric Power, which runs the plant, said its two other reactors were operating normally.
They said they had yet to determine the cause of the accident, but the local media quoted a company official as saying that the leak might have been caused by a lack of cooling water in the pipe.
The managing director, Hiroshi Matsumura, apologised for the accident at a news conference.
”This is very regrettable, he said. ”I feel sorry for the victims and their families. We are determined to find out what caused the accident and to report our findings as soon as possible.”
The Prime Minister, Junichiro Koizumi, promised that the government would ”do our best to investigate the cause, to prevent a repeat and to implement safety measures”.
Nobutake Masaki, a Mihama city official, said no radioactive material was involved and that the steam had leaked only inside the building.
About 200 people are believed to have been inside the building when the leak occurred. All 11 of those affected were on the second floor of the three-storey building, although it is not clear what work they were doing at the time.
Anti-nuclear campaigners said the incident would strengthen opposition to nuclear power.
Since an incident at a uranium reprocessing plant at Tokaimura in 1999 which killed two people and exposed 400 to radiation, a number of communities have voted against the construction of nuclear power plants.
”There is already widespread mistrust beneath the surface,” said Aileen Mioko Smith, director of Green Action.
”But when something like this happens, those feelings will come to the fore.”
She said that the continuing deregulation of the Japanese nuclear power industry would encourage power companies to cut costs when they should be investing more in safety.
”There have been fewer inspections and a reduction in the number of items that are checked.
”If this continues, there are going to be more accidents.”
Japan has more than 50 nuclear power plants, providing 34% of its energy needs, but the industry has been dogged by a series of accidents and attempted cover-ups.
Less than six months after the Tokaimura incident there was a fire at the Onagawara nuclear power plant.
Two years ago the biggest power company, Tokyo Electric, was forced to close its 17 nuclear power plants temporarily after admitting that it had hidden dozens of cracks over 15 years.
In February eight nuclear workers were exposed to low-level radiation when they were accidentally sprayed with contaminated water.
The Mihama plant leaked 55 tons of radioactive water from its number two reactor in 1991. – Guardian Unlimited Â