April 23rd, 2012, 5:04 pm
Posted by Pat Brennan, Science, Environment Editor

The operators of the idled San Onofre nuclear plant could restart at least one of its troubled reactor units “in the next few months,” the plant’s chief nuclear officer said Monday.

And while the precise cause of premature wear seen in steam generator tubes in both units is not yet known, it appears to be related to vibrations induced by the heat exchange process inside the steam generators, said Southern California Edison Chief Nuclear Officer Pete Dietrich.

Edison must determine the cause or causes of premature wear in both reactor units before either can be started, the Nuclear Regulatory Commission has said, although an extended heat wave could bring power shortages during summer if the plant remains offline.

That prompted the California Independent Systems Operator to develop contingency plans to generate more power.

“It’s still a little premature to be targeting restart dates,” Dietrich said Monday in a phone interview. “We are working quite diligently; we’re particularly mindful of some of the summer conditions that could be created. Here in the next few months, there’s a probability greater than 50 percent that we’ll be able to convince ourselves of that reasonable assurance of safety that we need (in order to restart the reactors).”

Both reactors have been shut down since January. The Unit 2 reactor had been taken offline for routine maintenance, but on Jan. 31, operators chose to shut down Unit 3 after a small leak was detected in one of thousands of tubes in one of its steam generators.

Inspections revealed wear on hundreds of tubes in both units’ steam generators, two for each reactor.

The tubes circulate water heated by the reactor through a separate loop of water inside the steam generators. Normally, the radioactive water makes no contact with the water inside the generators, which is turned to steam by the heat from the tubes. The steam turns turbines that generate electricity.

But when the water leaked from a single tube Jan. 31, it appeared to cause a small release of radioactive gas. Detectors picked it up in an adjoining building, but no other sensors on the San Onofre property registered any changes in radiation, Edison officials said.

Neither employees nor the public were placed in danger, Edison said.

The patterns of wear in the tubes have become a significant issue, Dietrich said. So far, Edison has plugged a total of 509 worn tubes in both reactors, out of nearly 20,000 for each pair of steam generators.

Plugging of some tubes is normal during the life of steam generators, and does not affect their performance.

The more common type of tube wear, seen in both units, occurs when the tubes run against support structures inside the steam generators. Dietrich said he does not consider that type of wear to be premature, even though the two-year-old steam generators had been operating for only a short time — since February for Unit 3, February 2010 for Unit 2.

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