Federal regulators found flaws in the process followed by Mitsubishi Heavy Industries in building and testing mock-up replacement parts for the troubled San Onofre nuclear plant.

The plant has been out of service for more than 10 months because of excessive wear on tubes carrying radioactive water, one of which leaked a small amount of radioactive steam in January, prompting the plant's shutdown.

The tubes are part of the four massive replacement steam generator assemblies manufactured by Mitsubishi for plant operator Southern California Edison.

The replacement project cost Edison and San Diego Gas & Electric, which co-owns the plant, a combined $771 million, a cost passed on to ratepayers. A warranty on the equipment is capped at $137 million.

Commission staff found flaws in Mitsubishi's process of mock-up and testing of redesigned anti-vibration bars that could be part of a long-term repair plan for the plant, according to a recently released U.S. Nuclear Regulatory Commission report. Inadequate support structures in the steam generators have been blamed in part for allowing the tubes to vibrate excessively, leading to the unusual wear.

The NRC report said Mitsubishi failed to verify that the tubes used in the mock-up conformed to requirements for outside diameter straightness, tube bending radius and total tube length that were outlined in the purchase order and purchase specifications for the tubes.

It also faulted Mitsubishi for failing to properly vet calibration services performed by another company in connection with measuring strain on the tubes.

The report, completed Nov. 30, asked Mitsubishi to respond within 30 days.

A spokesman for Mitsubishi said he was awaiting information from Japan.

NRC officials will meet with Edison representatives Tuesday in Maryland to discuss the utility's restart plan for one of the reactor units. The proposal involves running the unit at 70% power for five months to see if that alleviates the conditions leading to the excessive vibration. A final decision by the agency is probably months away.

No restart plan has been proposed for the other unit, which was more heavily damaged.

However, the NRC report on the mock-up anti-vibration bars said the redesigned support structures "may be used as a long-term repair of both Unit 2 and Unit 3 San Onofre ... steam generators."

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