KASHIWAZAKI, Japan – Radioactive material leaked undetected for days at an earthquake-battered nuclear power plant even as the utility was assuring the public that the damage posed no danger to those outside the site, company executives admitted Thursday.

The revelation cast more doubt on the plant’s emergency measures and the response by Japan’s largest power company, while the indefinite shutdown of the world’s most powerful electricity generating facility raised serious fears of a summer power shortage.

Tokyo Electric Power Co. confirmed reports that radioactive material was leaking as late as Wednesday night, nearly three days after the plant suffered a near-direct hit from a quake that killed 10 people and injured more than 1,000 in Kashiwazaki on Japan’s northern coast.

It was government inspectors who found radioactive iodine venting from an exhaust pipe at the plant’s No. 7 nuclear reactor, said Hisanori Nei, an official with the Nuclear and Industrial Safety Agency. It escaped between Tuesday and Wednesday night, Nei said.

An inspection Thursday night confirmed the leak had stopped, the utility said on Friday.

Tokyo Electric previously announced other radioactive materials had escaped from the pipe, but not iodine. An exhaust fan inside the building may not have been turned off as instructed in the operations manual, company spokesman Manabu Takeyama said.

Chief Cabinet Secretary Yasuhisa Shiozaki said the government would issue a “stern warning” to Tokyo Electric. “This is an error of not implementing the manual,” Shiozaki said.

Government inspectors concluded the iodine leak was too small to harm the environment or public health, Nei said. The utility also stressed the amount was extremely low and said it posed no threat to the environment or local people.

But the revelation reinforced concerns about the plant’s safety, coming a day after Tokyo Electric issued a list of previously unreported damage from the quake _ including a fire, burst pipes and waste spillage.

The seven reactors at the Kashiwazaki-Kariwa plant shut down automatically when the quake hit, and authorities have ordered the plant closed indefinitely while inspections and repairs are carried out to assure it can be restarted safely.

Tokyo Electric has warned that the closure could cause a power shortage in Japan as demand rises from summer use of air conditioners.

Six other power companies have said they will cooperate in providing emergency electricity and Tokyo Electric is considering restarting generating plants fueled by oil and natural gas, the utility said late Thursday.

The government has urged the operators of Japan’s 55 nuclear reactors _ which supply one-third of Japan’s energy _ to speed up safety checks for earthquake resistance, a top concern in the temblor-prone nation.

Officials at the plant conceded earlier that they had not foreseen the possibility of an earthquake as powerful as the magnitude-6.8 temblor that hit Monday. They also said the utility hadn’t known about the nearby offshore fault line in which the quake occurred.

The utility announced Thursday that the force of the quake exceeded its resistance guidelines at all seven reactors, sometimes by more than double. Public broadcaster NHK said the reading at the No. 1 reactor was the strongest quake ever measured at a Japanese reactor.

Tokyo Electric has repeatedly underreported the quake’s impact. After initially saying it had caused a fire in an electrical transformer and the spill of radioactive water into the Sea of Japan, the company reported 50 incidents of damage or leaks. Then it upped the number to 63.

Its stock tumbled again Thursday, sliding 5.6 percent to 3,400 yen a share, or $27.88, bringing its losses since the quake to 10.3 percent.

Members of the Nuclear Safety Commission toured the sprawling plant Thursday and criticized Tokyo Electric for missteps in its response to the earthquake.

Even so, they concluded none of the errors had threatened public health.

The safety of the “plant was fundamentally maintained and we avoided the serious consequences of a nuclear accident,” commission Chairman Atsuyuki Suzuki said in a statement. “The list of problems announced by TEPCO have no serious effect on the safety of the reactor.”

Tokyo Electric has been punished for failing to accurately inform the public of problems in the past.

Four years ago, the utility was forced to halt all of its 17 nuclear reactors after admitting it misreported safety problems in the late 1980s and early 1990s. The halt caused a power shortage in the summer of 2003, and other utilities stepped in with emergency electricity production.

In that scandal, a trade ministry report revealed 29 cases of cracks or minor structural damage in eight of Tokyo Electric’s reactors, including two reactors at Kashiwazaki-Kariwa. The company’s top three executives resigned, but the utility insisted the cracks never posed a serious danger. The last of the shuttered reactors wasn’t cleared to reopen until July 2005.

The impact of Monday’s quake has spread far beyond the region. Japan’s auto companies had to suspend production because a key parts maker sustained damage during the temblor. Officials at the damaged factory said they expected to restart production early next week.

People in the Kashiwazaki region struggled to put their lives back together but basic services such as water had not been restored to some areas.

“We’re just getting by day by day,” said Masatoshi Ogawa, sitting in front of his closed pinball parlor. “Our houses were OK so we didn’t have to go to evacuation centers, but life without water is really inconvenient.”

Associated Press writers Kozo Mizoguchi and Chisaki Watanabe in Tokyo contributed to this report.

A service of the Associated Press(AP)