Published: April 11, 2012 Updated: 11:19 p.m.
Odd tube wear seen in both San Onofre reactors
By PAT BRENNAN / ORANGE COUNTY REGISTER
An extremely unusual pattern of steam-generator-tube wear that engineers had previously seen in only one of the San Onofre nuclear plant’s idled reactors has now been seen in the second reactor.
The odd pattern was first seen in the Unit 3 reactor after it was shut down Jan. 31 following a water leak and release of a small amount of radioactive gas.
But it is too soon to draw any conclusions about the implications for the second reactor, Unit 2, which also has been shut down since January, said Jennifer Manfre, spokeswoman for plant operator Southern California Edison.
Both reactors remain offline indefinitely, and will not be restarted until the cause of the wear is determined and the restart is cleared by the Nuclear Regulatory Commission.
“We’ve informed the NRC that we have identified additional minor tube wear in one of the Unit 2 steam generators, and this newly identified wear is similar to the type of wear previously seen in Unit 3,” Manfre said.
The cause of the wear in hundreds of steam generator tubes in both reactors remains a mystery. While the more common type of tube wear seen in Unit 2 is typical throughout the life of steam generators, San Onofre’s four steam generator units — two for reach reactor — are only two years old.
And Unit 3 had shown more extensive wear even though it was in operation for a shorter period than Unit 2: since February of last year for Unit 3 versus February 2010 for Unit 2.
But now the unusual wear pattern — along the length of steam-generator tubes, indicating they were rubbing together — has turned up in two of the tubes in Unit 2, Manfre said.
The wear in both reactors is considered premature. Edison has plugged a total of 321 of the worn tubes in both reactors, and while plugging of worn tubes is common practice and does not affect the generators’ performance, it is unusual so early in the life of the generators.
Edison is continuing testing with NRC inspectors on site, although a special inspection team sent in earlier by the nuclear regulatory agency has now left.
It is too soon to tell whether the latest wear-pattern discovery might help shed light on the cause of the tube wear in the two units, Manfre said. The steam generators were manufactured by Mitsubishi Heavy Industries in Japan.
In a meeting with reporters after touring the troubled plant on Friday, Gregory Jaczko, the chairman of the NRC, said the cause of the premature wear must be determined before the reactors can be restarted.
Water heated by the reactors flows through the tubes, nearly 20,000 for each pair of generators, and converts water in the generators to steam, which turns turbines to create electricity.
But normally, the water that circulates through the tubes is kept separate from the water in the generators.
Plant operators chose to shut down Unit 3 after a water-leak was detected in one of its steam-generator tubes, but the leak of radioactive water from the tube into the generator caused a small release of radioactive gas.
A sensor was tripped in a building next to the reactor, but sensors elsewhere on San Onofre property picked up no changes in radiation. Edison said neither plant employees nor the public were placed in danger.
Unit 2 had been shut down for routine maintenance, but closer inspection found premature wear in its steam-generator tubes.
The indefinite shutdown of the reactors prompted the California Independent Systems Operator, which manages the state’s power grid, to develop contingency plans for electricity generation in case of an extended summer heat wave.
The plans include possible reactivation of two power units at the AES power plant in Huntington Beach that had been shut down.