{Note: Title updated}
The Associated Press reported on Monday that a design flaw traced to an incorrect computer analysis will keep Southern California Edison’s controversial San Onofre nuclear power plant offline for some time to come. San Onofre, which is Southern California’s only nuclear plant, produces enough power to serve about 1.4 million households.
Back in January, “a leak from a tube at one unit [Unit 3] released a small amount of radiation”, which caused the plant’s operator to shut the reactor down, reported this LA Times story. The story went on to note that two days after the incident, during routine maintenance on the other unit, Unit 2, “nuclear regulation officials found extensive wear on tubes that carry radioactive water in a steam generator. The tubes were installed less than two years ago after they were delivered by the Japanese manufacturer of the generators, Mitsubishi Heavy Industries.”
In 2009 and 2010, the plant’s four steam generators were replaced at a cost of $671 million, and were expected to last until 2022.
According to the LA Times story, Nuclear Regulatory Commission officials found that, “two of the tubes showed more than 30% wall thinning, 69 had 20% thinning and more than 800 had 10% thinning.”
Then in March, the NRC announced (pdf) that the San Onofre nuclear power plant would not be allowed to be restarted until the unusual wear on the steam generator tubes was understood and fixed, a follow on LA Timesstory reported. The NRC stated in a news release that the wear at Unit 3 was caused by the tubes vibrating and rubbing against adjacent tubes and against support structures inside the steam generators, while at Unit 2 the tubes were rubbing against the support structures but not rubbing against adjacent tubes. At the time, the NRC stated that it did not know why this was happening.
The AP story this week now reports that “design flaws” caused by “botched computer analysis” by Mitsubishi when it was designing the replacement steam generators “vastly misjudged how water and steam would flow in the reactors. Also, changes intended to improve manufacturing were never thoroughly reviewed in the context of the generator design, resulting in weaker support around bundles of tubes that contributed to vibration.”
However, while the problem is now understood, it is unclear how to fix it: The NRC says that there are “significant technical issues” to be overcome. Replacing one or more of the steam generators is a possibility.
The AP story states that “the generators were designed to meet a federal test to qualify as ‘in-kind,’ or essentially identical, replacements for the original generators, which would allow them to be installed without prior approval from federal regulators.”
The NRC says it is now going to review its “non-approval" approval process.
The AP story also noted that the replacement steam generators, which weigh 24 tons more than the ones they were replacing, were designed with 400 more tubes and a V-support structure specifically to reduce vibrations and tube wear.
No time estimate for when the power plant will be operating again has been given.
Photo: iStockphoto
Robert N. Charette is a Contributing Editor to IEEE Spectrum and an acknowledged international authority on information technology and systems risk management. A self-described “risk ecologist,” he is interested in the intersections of business, political, technological, and societal risks. Charette is an award-winning author of multiple books and numerous articles on the subjects of risk management, project and program management, innovation, and entrepreneurship. A Life Senior Member of the IEEE, Charette was a recipient of the IEEE Computer Society’s Golden Core Award in 2008.
