Two workers were in critical condition late Monday after the steam leak in Mihama, a small city about 200 miles west of Tokyo. The accident was the deadliest ever at a Japanese nuclear power plant and follows a string of safety lapses and cover-ups at reactors.
Prime Minister Junichiro Koizumi vowed to launch a thorough investigation into the accident. Fears about the safety of the country's 52 nuclear power plants soared in 1999, when a radiation leak northeast of Tokyo killed two workers and exposed hundreds to radiation.
Monday's leak was caused by a lack of cooling water in the reactor's turbine, said Kimihito Kawabata, a spokesman for plant operator Kansai Electric Power. The steam was believed to be about 270 degrees Celsius (518 Fahrenheit).
After the accident, Kansai Electric officials found a hole on the condenser pipe, believed to be the source of the leak. Officials couldn't elaborate on its size.
Four workers died after suffering severe burns. Two others were critically injured, a police official said on condition of anonymity.
"The ones who died had stark white faces," said Yoshihiro Sugiura, the doctor who treated them at the Tsuruga City Hospital. "This shows they had rapidly been exposed to heat."
Three others were seriously injured while the remaining two suffered minor injuries. All the workers were employees of Kiuchi Keisoku Co., an Osaka-based subcontractor of Kansai Electric. They were all inside the turbine building to prepare for regular inspections of the plant.
Government officials said the accident did not result in a radioactive leak and that there was no need to evacuate the area surrounding Mihama, a city of 11,500. The No. 3 nuclear reactor in Mihama, which started operations in 1976, automatically shut down when steam began spewing from the leak, but the plant's other two reactors were operating normally.
Yosaku Fuji, president of Kansai Electric, apologized for the accident as he bowed deeply before reporters at a televised news conference.
"We are deeply sorry to have caused so much concern," Fuji said. "There is nothing we can say to the four who lost their lives. We pray for their souls from the bottom of our hearts and offer our condolences to their families. We are truly sorry."
Despite the lack of radioactive leakage, the accident was yet another blow to Japan's nuclear power industry.
Resource-poor Japan is dependent on nuclear fuel for nearly 35 percent of its energy supply, and a government blueprint calls for building 11 plants and raising electricity output to nearly 40 percent of the national supply by 2010.
The deaths in Mihama also come as Japan is bidding to host the world's first large-scale nuclear fusion plant, the US$12 billion International Thermonuclear Experimental Reactor, or ITER. But the project's sponsors — the European Union, the United States, Russia, Japan, South Korea and China — remain deadlocked over whether to build the plant in Japan or France.
The government vowed to quickly find out what happened on Monday.
"We must put all our effort into determining the cause of the accident and to ensuring safety," Koizumi said. He added that the government would respond "resolutely, after confirming the facts."
The United States had a similar accident at the Surry nuclear power plant in southern Virginia almost two decades ago when a 46-centimeter (18-inch) steel pipe burst and released 114,000 liters (30,000 gallons) of boiling water and steam, killing four people.
In Japan's fatal 1999 accident, a radiation leak at a fuel-reprocessing plant in Tokaimura, northeast of Tokyo, killed two workers and caused the evacuation of thousands of local residents. That accident was caused by two workers who tried to save time by mixing excessive amounts of uranium in buckets instead of using special mechanized tanks.
Several major power-generation companies have since been hit with alleged safety violations at their reactors, undermining public faith in nuclear energy and leaving Japan's nuclear program in limbo.
A 2002 investigation revealed that Tokyo Electric Power, the world's largest private utility, systematically lied about the appearance of cracks in its reactors during the 1980s and 1990s. The company later temporarily shut down all 17 of its reactors for inspections to reassure the public they were safe.
In February, eight workers were exposed to low-level radiation at another power plant when they were accidentally sprayed with contaminated water. The doses were not considered dangerous.