The incident, which took place on the anniversary of the 1945 atomic bombing of the city of Nagasaki, is certain to increase public distrust of the nuclear industry in Japan, which depends on nuclear power for a third of its energy needs.

"Radioactive materials weren't contained in the steam that leaked out ... We've received a report that there is no impact from radiation on the surrounding environment," an official at the Nuclear and Industrial Safety Agency told a news conference.

Police initially said five workers had died, but later corrected the figure to four.

The accident occurred in a building housing turbines for the Number 3 reactor at the Mihama nuclear plant in Fukui prefecture, 320 km (200 miles) west of Tokyo.

Television showed emergency workers in hard hats clustered around the exit of the facility.

An official at Kansai Electric Power Co. Inc., which runs the plant, said the 826,000 kilowatt nuclear generation unit at the facility shut down automatically when the steam leaked from the turbine, which is in a separate building. The company was unsure when it would restart.

"We are now investigating the cause," the official told a news conference. The temperature of the leaking steam was 142 Celsius (287.60F).

He said the workers involved, who were preparing to shut down the plant for maintenance, were all contractors, and 221 people were in the building at the time.

A trade ministry spokesman briefing reporters said there was no technical problem with the core nuclear reactor at the plant.

Prime Minister Junichiro Koizumi said he had not heard details of the accident. "But I think we must do our best to investigate the cause, to prevent a repeat and to implement safety measures," he told reporters. Chief cabinet secretary Hiroyuki Hosoda told a news conference: "I think the cause will become clear within several days."

WORST PREVIOUS ACCIDENT

The only previous fatal accident at a Japanese nuclear power plant was in 1967, in a fire at a plant in Ibaraki prefecture just north of Tokyo. There was no radiation leak.

The worst previous incident at a nuclear facility was at a uranium processing plant in Tokaimura, a town north of Tokyo.

That took place on September 30, 1999, when an uncontrolled nuclear chain reaction was triggered after three poorly trained workers used buckets to mix nuclear fuel in a tub.

The resulting release of radiation killed two workers and forced the evacuation of thousands of nearby residents.

In a separate incident involving a nuclear facility, Tokyo Electric Power Co. (TEPCO), Japan's biggest power producer, said on Monday it had shut a nuclear power generation unit at its Fukushima-Daini plant due to a water leak.

TEPCO was forced to close all its 17 nuclear power plants temporarily by April 2003 after admitting it had falsified safety documents for more than a decade, revelations that severely undermined public confidence in the nuclear industry.

A number of towns in Japan have held referendums in the past few years and voted against construction of more nuclear plants.

The Mihama plant was the first nuclear plant built by Kansai Electric. The No. 1 reactor began service in November 1970.

A Tokyo-based oil trader said it was unlikely the shutdown of the plant would have an impact on oil demand or prices because Kansai tended to use liquefied natural gas as an alternative source of fuel when its nuclear plants were off line.