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New details about problems at San Onofre nuclear power plant

July 14, 2012 |  5:56 am

Data released by the U.S. Nuclear Regulatory Commission and a leaked analysis by Southern California Edison provide some new insights into the situation at the San Onofre nuclear plant.

The plant has been shuttered since Jan. 31, when a small leak in one of the plant's thousands of steam generator tubes drew attention to the fact that the tubes in the newly replaced steam generators were deteriorating much more quickly than expected.

In particular, some of the tubes were showing an unusual type of wear caused by tubes rubbing against adjacent tubes. Since then, the NRC, plant operator Edison, and steam generator manufacturer Mitsubishi Heavy Industries have been probing the cause of the wear.

The data released by the NRC showed that a total of 3401 tubes in the Unit 2 and Unit 3 reactors-- 8.7% of the 38,908 total tubes in the plant's four steam generators -- had shown some signs of wear.

The NRC-released data also gives more extensive information than was previously available about the location of the tubes and whether the wear was caused by rubbing against adjacent tubes, support structures, or in the case of two tubes, by a foreign object that Edison identified as a piece of welding material.

Of the tubes with wear, 387 -- about 1% of the total -- had portions that had worn through by 35% or more. At that level of wear, industry standards require the tubes to be plugged and taken out of service.

In addition to the tubes plugged because of excessive wear, Edison plugged another 930 that were in the same area as the deteriorated tubes, as a precautionary measure.

Generally, once 8% of a plant's tubes are plugged, it must run at a lower power level.

NRC and Edison officials have said that errors in computer modeling apparently led to the problems. Simulations by Mitsubishi underpredicted the velocity of steam and water flowing among the tubes by a factor of three or four, officials said. The high rate of flow caused the tubes to vibrate and knock against each other.

Officials also said that manufacturing differences between the steam generator in the two reactor units meant that anti-vibration bars fit more tightly in Unit 2, which has seen less wear, and particularly less of the unusual tube-to-tube wear. In Unit 2, 1,595 total tubes showed wear, versus 1,806 in Unit 3. The tube-to-tube wear cropped up in 825 spots in Unit 3, but only in 2 in Unit 2.

A May 7 analysis by Edison that was leaked to the advocacy group Friends of the Earth, discussed the issues in more detail. Edison officials confirmed the report was authentic.

According to the analysis, while designing the new steam generators, Mitsubishi input parameters like fluid temperatures, flow rates from the primary side of the reactor system and steam generator dimensions and used a number of computer codes, some developed by Mitsubishi, in various phases of analysis, to predict whether the tubes would experience vibration issues.

The reason that the design codes and assumptions did not accurately predict the issues that arose is not fully understood, the report said.

The report also noted examples of two other plants -- Three Mile Island and Arkansas Nuclear One -- that had experienced problems with tube to tube wear, but noted,“although tube-to-tube wear had occurred in the industry in the past, it was not comparable in magnitude to that experienced at SONGS."

Friends of the Earth and others said the numbers show that the damage at San Onofre is much worse than seen elsewhere in the industry and that both reactor units should remain shut down until the steam generators are replaced or extensively modified or until Edison goes through a license amendment process including public hearings.

Edison officials said the majority of the tube wear, which stems from tubes rubbing against support structures rather than other tubes "is not unusual in new steam generators and is part of the equipment settling in."

NRC spokesman Victor Dricks said, "Other large steam generators have exhibited wear after one cycle of operation which resulted in tube plugging...but not to the extent seen on San Onofre steam generators."

The plant will remain shut down until the NRC gives Edison the go-ahead to restart.


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