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 acknowledged yesterday.
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.
Tokyo Electric had previously announced that other radioactive materials had escaped from the pipe, but not iodine.
An exhaust fan inside the building may not have been turned off as called for in the operations manual, company spokesman Manabu Takeyama said.
Government inspectors concluded that the iodine leak was too small to harm the environment or public health, Nei said.
The utility also said the leak 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 yesterday.
Chief Cabinet Secretary Yasuhisa Shiozaki urged the operators of Japan's 55 nuclear reactors -- suppliers of one-third of the nation's energy -- to speed up safety checks for earthquake resistance, a top concern in the temblor-prone nation.
"Since there was such a huge earthquake that surpassed our expectations, we need to consider future measures for quake resistance," Shiozaki said. "I asked them to speed up the assessment and checkups wherever possible."
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 Tokyo Electric had been unaware of the nearby offshore fault line in which the quake occurred.
The utility announced yesterday 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 raised the number to 63.