WASHINGTON, July 13 (Kyodo) — The number of incidents of abnormal attrition found in Japanese-made steam generator tubes in two reactors at the San Onofre nuclear power plant in southern California has reached around 15,000, the U.S. Nuclear Regulatory Commission said in a report Thursday.
The attrition, apparently caused by wear or vibration, was found in about 3,400 out of around 39,000 tubes in four generators produced by Mitsubishi Heavy Industries Ltd. and installed at unit 2 and 3 reactors of the plant operated by Southern California Edison Corp.
Some of the tubes are worn in more than one spot, the commission said.
"Mechanical wear was observed at various locations along the tube lengths, similar to what has been observed in comparable steam generators at other plants," the report says.
"The wear observed at the retainer bars (bars that are unique to steam generators fabricated by Mitsubishi) was not expected. The severity of one of the wear indications at a retainer bar was significant enough...to warrant in-situ pressure testing. This pressure test confirmed the structural integrity of this tube."
Earlier, nuclear regulators indicated that design flaws might have resulted in the abnormal attrition.
Antinuclear groups are intensifying their call for the continued shutdown of the plant, which came to a halt following a radiation leak in January.