Japan suffered its deadliest nuclear power plant accident Monday when a bursting steam pipe killed at least four workers and injured seven in another blow to the industry in an energy-poor country already worried about nuclear plant safety.

No radiation was released when the boiling water and steam exploded from a cooling pipe at the plant in Mihama (search), a small city about 200 miles west of Tokyo.

But the steam leak followed a string of safety lapses and cover-ups at reactors, and Prime Minister Junichiro Koizumi (search) 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 and perhaps by significant metal erosion in the condenser pipe, said the plant's operator, Kansai Electric Power. The pipe's wall, originally 10 mm thick, had become as thin as 1.5 mm in the 28 years since the reactor was constructed.

Deputy plant manager Akira Kokado on Tuesday said inspectors had notified the company in April 2003 that the cooling pipes were overdue for a safety check. But the pipe wasn't a part of inspections in November.

"We thought we could delay the checks until this month," Kokado told a news conference.

After the accident, Kansai Electric (search) officials found a hole in the pipe that was believed to be the source of the leak. They did not say how big the hole was.

The water flowing through the pipe at the time of the accident was about 300 degrees Fahrenheit, Kokado said Monday.

Four workers died after suffering severe burns. Of the seven injured workers, two were in critical condition, three were in serious condition and the remaining two suffered minor injuries.

"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."

Investigators prepared to inspect the accident site Tuesday, and Japanese newspapers reported the government might be forced to shelve plans to build 11 new plants.

"In Japan, it's virtually impossible to build new nuclear facilities now," the national Asahi daily said in an editorial Tuesday. "But facilities are wearing out, and there are worries about increasing problems with corroding pipes, rupturing valves and the reactor core."

All the workers were employees of Kiuchi Keisoku Co. (search), an Osaka-based subcontractor of Kansai Electric. They were all inside the turbine building to prepare for regular inspections of the plant, which began operating in 1976.

Government officials said there was no need to evacuate the area surrounding Mihama, a city of 11,500.

The plant's No. 3 nuclear reactor automatically shut down when steam began spewing from the leak. Its two other 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."

Kokado told a news conference that the metal erosion in the pipe was more extensive than Kansai Electric had expected. An ultrasound test might have detected the thinning but Kansai Electric never carried out such inspections, Kokado said, adding the company may have to review the way it conducts checkups.

Security guards closed the road leading to the seaside plant after the accident, which the city's residents said caught them off guard.

"I was so shocked. At first, I didn't think it was such a major accident," Naoki Matsubara, a 26-year-old office worker. "I'm so relieved there was no radiation leak."

Resource-poor Japan is dependent on nuclear fuel for nearly 35 percent of its energy supply, and a government blueprint calls for building 11 new plants and raising electricity output from nuclear facilities 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 $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 an 18-inch steel pipe burst and released 30,000 gallons of boiling water and steam, killing four people.

In Japan's 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 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.