One of the three nuclear reactors at the plant in Mihama, 350 kilometres (220 miles) west of Tokyo, shut down automatically when an alarm sounded just before super-heated steam leaked from a turbine and scalded workers.

Prime Minister Junichiro Koizumi said an investigation was needed into the accident, which is Japan's worst at a nuclear facility in terms of the number of deaths.

But Kansai Electric Power Company, the operators of the Mihama plant which went into service in 1976, stressed there was no danger of a radiation leak and no need to evacuate the area.

"This incident will have no radiation effect on the surrounding environment," the company said in statement.

A police spokesman in Fukui prefecture confirmed four people were killed and seven injured in the accident, which happened in the turbine room of a pressurised water reactor at the plant.

The dead men were aged from 29 to 46, according to local police.

A police spokesman said two others were in critical condition.

An official from Japan's Nuclear and Industrial Safety Agency said the leaked steam would not have contained radiation as the turbines in the water reactors do not come into contact with the nuclear reactors.

Residents and visitors to the beaches along the plant on the Japan Sea Coast seemed unruffled by the accident.

"Many activists against nuclear power come from other cities like Osaka (in western Japan) in busloads to occasionally demonstrate but there are no militant supporters and opponents among locals at the moment," said Toshie Okuda.

The entrance of the bridge to the power plant was guarded by security officials.

Several hundred meters (yards) away from the plant about 100 young men and women were camped and partied on the beach.

Most of them were Japanese-Brazilian families visiting from industrial cities some 100 to 200 kilometers to the south.

"I'm a bit scared but we came all the way, so we are going to stay for a few more days," said Sachie Omura, 30, as her three-year-old child played on the beach.

Meanwhile, a minor fire broke out Monday evening at another nuclear facility in Shimane prefecture, 600 kilometers (375 miles) southwest of Tokyo, but no one was injured and it posed no threat of radiation leaks.

"A fire broke out at a waste processing site, where we also have a laundry facility. But it was quickly extinguished," said a spokesman for Chugoku Electric Power Co., which operates the site.

The incidents are likely to further erode confidence in Japan's nuclear industry which has been shaken by a series of accidents and scandals in recent years.

By an uncomfortable coincidence, Monday's deadly accident also happened on the 59th anniversary of the atomic bombing of Nagasaki.

Koizumi expressed regret at the loss of life at Mihama and stressed the need for high safety standards in an industry that provides a quarter of Japan's energy.

"The cause of the accident must be clarified. Prevention efforts and safety measures have to be fully enforced," Koizumi said.

Hiroshi Matsumura, managing director of Kansai Electric, apologised. "It is extremely regrettable. To those who were injured and to the public, we apologise," he told a press conference at the company headquarters in Osaka.

Following the automatic shutdown of the reactor, steam -- reportedly at over 200 degrees Centigrade (390 degrees Fahrenheit) -- filled up the turbine room causing severe injuries to workers trapped inside, Kansai Electric said in the statement.

At least one hole was found in a pipe in the turbine room, but officials were still investigating what caused it and what sounded the alarm, he said.

It was the first fatal incident at a nuclear-related plant since September 1999, when two workers were killed at the Tokaimura uranium plant northeast of Tokyo.

More than 600 people were also exposed to radiation after the workers set off a critical reaction by using steel buckets to pour uranium solution into a precipitation tank.

About 320,000 people were evacuated in the incident, regarded as the world's worst nuclear disaster since Chernobyl in 1986.

Japan's nuclear power industry had only just been recovering from the crisis of confidence caused when Tokyo Electric Power Company, the world's largest energy utility, admitted in 2002 it had systematically covered up inspection data showing there were cracks in its reactors.

Japan is the third largest nuclear power producer after the United States and France. Nuclear power accounts for over 25 percent of its electricity supply according to the Paris-based Nuclear Energy Agency.

The area of Fukui Prefecture around Mihama hosts 13 of the 52 active nuclear power reactors in Japan, which are run by 10 private firms.

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