Electricity wholesaler Japan Atomic Power said on Monday that gas containing a minute amount of radiation had leaked from its Tsuruga nuclear plant in western Japan the previous day, but that the amount was well below the legal limit and there was no impact on the environment.
The amount of radiation was estimated at about one-four hundred thousandth of the annual legal limit, and there were no changes in readings from radiation monitoring devices set up around the plant, the company said.
"The amount that came out was such an extremely small amount, so this is not a problem and it will not have an impact on the environment," said Shinichi Morooka, professor of nuclear energy at Waseda University.
Japan Atomic Power said it now has a policy of disclosing even tiny anomalies at the plant after the crisis at Tokyo Electric Power Co's crippled nuclear power station in northern Japan heightened concerns about nuclear safety.
"Whether or not this incident deserves an official announcement is not an issue. If we don't disclose it and the information comes from somewhere else, in this climate it could look like it's being covered up," company spokesperson Koji Otake said.
However, the spokesperson added that it was the first ever accidental radiation leakage from the Tsuruga plant's No. 2 reactor, which started operations in 1987.
The 1,160-megawatt No 2 reactor, located 350 km (220 miles) west of Tokyo, was shut on Saturday for an unplanned inspection after the company found a technical problem last week.
Japan Atomic said it has stopped the leak and is investigating the cause.
A massive earthquake and tsunami on March 11 triggered cooling system malfunctions at Tokyo Electric's Fukushima Daiichi nuclear power plant and caused radiation to leak into the atmosphere and the ocean. Engineers are still struggling to bring the plant under control.