Sep. 19--Nuclear regulators took no corrective action at San Onofre Nuclear Generating Station after being briefed on weld defects by Southern California Edison.

The utility met with Nuclear Regulatory Commission project manager James R. Hall early this week to explain why the defects, found in a pair of massive steam generators being manufactured in Japan, are not also present in a set of identical components that have already been delivered to San Onofre.

Edison will soon begin its most extensive upgrade at San Onofre when it installs the generators inside one of the plant's two concrete containment domes. A second, identical operation is scheduled for October 2010.

Hall said the work should start on schedule.

"We're only a few weeks away from the outage at San Onofre," Hall said. "There isn't any deliberation within the NRC to hold up the steam generator replacement as it has been planned."

A steam generator looks like a giant inverted milk bottle and is filled with nearly 10,000 thin metal tubes. Water heated by the plant's nuclear reaction runs through the tubes, which are surrounded by cool water that turns to steam and spins the plant's turbines, creating electricity. The old generators must be replaced, at a cost of $680 million, because their tubes have begun to crack, reducing each reactor's ability to make steam and generate juice.

In the end, the difference between the two sets of monoliths comes down to grinding versus gouging.

Edison, the plant's majority owner, notified regulators in late August that two 640-ton steam generators still being manufactured by Mitsubishi Heavy Industries in Kobe, Japan, had developed cracks in a weld that connects a 5-inch-thick steel plate to each unit's larger steel superstructure. The plates are critical both for supporting the generators' innards and also for separating flows of coolant that circulate through each reactor's core, where highly-radioactive fuel rods sit.

When installing the plate in the first set of generators, workers used grinders to prepare metal surfaces for the welds, which have not cracked under testing, according to Edison's report. However, workers used a special metal gouging apparatus to prepare the second set, and that machinery left behind carbon deposits that eventually caused the weld to become brittle and crack. Workers have already begun repairing the defects and will finish doing so before the units are hauled, via ship, across the Pacific Ocean to San Onofre.

Hall said the NRC will still inspect every aspect of the steam generator replacement project as it goes forward in October. The procedure requires cutting a large hole in the side of one of the plant's two concrete containment domes, because existing access hatches are too small.

An identical procedure will follow in 2010 for the second operating reactor.

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