Faulty computer analysis and manufacturing changes appear to have caused excessive wear in steam generator tubes at Edison International’s San Onofre nuclear plant in California, according to federal regulators.
Edison altered the way tubes were assembled at its Unit 3 reactor, where support structures failed to keep some pipes from rubbing together and degrading prematurely, U.S. Nuclear Regulatory Commission officials said at a public meeting. Inspectors found much more wear on Unit 3 compared with the Unit 2 reactor, which didn’t use the same assembly design, the regulators said. The reactors have been shut since January.
“This is a significant and serious safety issue,” Elmo Collins, regional administrator for the NRC, said at the meeting yesterday in San Juan Capistrano, California.
The computer design simulation software used by Edison failed to predict accurately the velocity of steam and water flowing through the tubes, underestimating it by a factor of three to four, Greg Werner, NRC branch chief, said at the meeting. This contributed to the excess corrosion, Collins said.
Eight tubes failed during earlier pressure tests at Unit 3, which is “unprecedented” in the industry, Collins said.
The NRC hasn’t determined whether Edison violated any rules and doesn’t have a timeline for restarting the plant. Federal regulators are also still evaluating whether Edison needed to amend its license for changes made to its generators.
Edison agreed with the facts of NRC’s presentation, the company’s Chief Nuclear Officer Peter Dietrich said at the meeting. The utility will keep its plant offline until the NRC decides it’s safe to restart, Dietrich said.
The closure increases the risk of blackouts in Southern California, where San Onofre provided power to about 1.4 million homes, federal regulators said last month. State grid operators are encouraging conservation and have restarted two retired natural-gas-fired plants to help replace the output from Edison’s crippled reactors.
San Onofre will be shut through August as Edison develops a restart plan, Chief Executive Officer Ted Craver said in a statement on June 8.
The NRC, which sent extra inspectors to the plant after a small leak shut its Unit 3 reactor in January, found unusual wear on some steam generator tubes that carry radioactive water at both reactors. San Onofre’s Unit 2 was shut in January for planned maintenance and refueling.
Some tubes were rubbing, causing them to degrade, Craver said on May 30. Edison has plugged and repaired about 3 percent of the tubes in the generators, Craver said.
The plan to repair and restart its reactors will probably be submitted by the end of July, the company has said.
The steam generators, built by Mitsubishi Heavy Industries Ltd., were installed in 2009 and 2010 at a cost of $671 million. Mitsubishi Heavy also provided the computer simulation software used at the plant, though Collins said Edison was “ultimately responsible” for it.
Edison estimated its share of inspection and repair costs to be in the range of $55 million to $65 million, according to a filing in May. The utility paid $30 million for replacement power through March 31 and those costs are probably going to increase in the summer, Edison said in the filing.
Edison’s Southern California Edison utility and Sempra Energy’s San Diego Gas & Electric Co. will pay $2.5 million a month for power from two AES Corp. generating units to help make up for lost capacity from San Onofre, the state’s grid operator, California ISO, said on May 11.
Edison owns 78.21 percent of San Onofre, while Sempra owns 20 percent and the city of Riverside owns 1.8 percent, according to Edison’s website.
San Onofre, located about 60 miles (97 kilometers) south of Los Angeles, can produce enough power for 1.76 million average homes, based on Energy Department statistics.