nuclear-news

New, and worrying, data about safety status of San Onofre nuclear power plant

 the numbers show that the damage at San Onofre is much worse than seen elsewhere in the industry 

New details about problems at San Onofre nuclear power plant LA Times, July 14, 2012 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….. 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….. The plant will remain shut down until the NRC gives
Edison the go-ahead to restart.
http://latimesblogs.latimes.com/lanow/2012/07/san-onofre-new-information.html

July 16, 2012 - Posted by Christina Macpherson | safety, USA