News

WASHINGTON (Kyodo) -- Cracks associated with welding defects were found in a Japan-made steam generator for a nuclear reactor in Southern California in 2009 before its delivery to the plant, U.S. Nuclear Regulatory Commission documents showed Tuesday.

The steam generator, produced and later repaired by Mitsubishi Heavy Industries Ltd., was installed at the unit 3 reactor at the San Onofre nuclear power plant in California, which suffered a minor radiation leak in late January.

It remains unclear whether the cracks and the radiation leak were related.

At the nuclear plant, operated by Southern California Edison, abnormal attrition was also found earlier this month in the tubes of another Mitsubishi-made steam generator installed at the unit 2 reactor.

Mitsubishi Heavy and Southern California Edison both declined to comment, saying they are under investigation.

Edwin Ryman, a nuclear expert at the Union of Concerned Scientists, said the problems could be attributed to either defects in the part or inappropriate operation.

A steam generator, composed of fine tubes, is a critical part of a pressurized water reactor.

According to the NRC documents, Mitsubishi Heavy discovered a 5 inch (12.7 centimeter) long crack in the dissimilar metal weld between the divider plate and the channel head of the steam generator of the unit 3 reactor during its routine visual inspection in March 2009.

A follow-up probe found heat treatment prior to welding caused the problem and the NRC informed the utility and others concerned.