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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

The Conversation (0)
This photograph shows a car with the words “We Drive Solar” on the door, connected to a charging station. A windmill can be seen in the background.

The Dutch city of Utrecht is embracing vehicle-to-grid technology, an example of which is shown here—an EV connected to a bidirectional charger. The historic Rijn en Zon windmill provides a fitting background for this scene.

We Drive Solar

Hundreds of charging stations for electric vehicles dot Utrecht’s urban landscape in the Netherlands like little electric mushrooms. Unlike those you may have grown accustomed to seeing, many of these stations don’t just charge electric cars—they can also send power from vehicle batteries to the local utility grid for use by homes and businesses.

Debates over the feasibility and value of such vehicle-to-grid technology go back decades. Those arguments are not yet settled. But big automakers like Volkswagen, Nissan, and Hyundai have moved to produce the kinds of cars that can use such bidirectional chargers—alongside similar vehicle-to-home technology, whereby your car can power your house, say, during a blackout, as promoted by Ford with its new F-150 Lightning. Given the rapid uptake of electric vehicles, many people are thinking hard about how to make the best use of all that rolling battery power.

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