The Nuclear Regulatory Commission on Monday said it would cite Southern California Edison Co. and Mitsubishi Heavy Industries for alleged design flaws in steam generators that resulted in closure of the San Onofre nuclear power plant.

The NRC said in preliminary findings posted on its website that Mitsubishi employed inadequate computer modeling, which led to deficient design of steam generators at the plant. That created a safety problem of "low to moderate" significance. The regulator said SoCal Edison failed to adequately supervise Mitsubishi.

SoCal Edison, a unit of Rosemead’s Edison International, announced in June that it would close down two units at the electricity generating station, which had been offline since January 2012, when leaks were discovered in the steam tubes of one unit. It later announced that it would sue Mitsubishi to recover damages that are expected to reach into the hundreds of millions of dollars.

SoCal Edison has 10 days to respond to the findings. Mitsubishi has 30 days to respond to its notice of nonconformance. An Edison spokeswoman said the company is studying the findings.

“Mitsubishi designed the system. Mitsubishi built the system. Mitsubishi’s system failed. They are the experts. SCE was the customer,” Pete Dietrich, SCE senior vice president and chief nuclear officer, said in a statement issued late Sunday in advance of the NRC announcement.

Edison International shares closed up $1.03, or 2 percent, to $47.05 on the New York Stock Exchange.

Earlier Business Journal Coverage:

Nuclear Plant Kept in Dark

Edison CEO Says Nuclear Option Still on the Table

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