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Southern California Edison (SCE) has released steam generator tube wear data associated with the San Onofre Nuclear Generating Station (SONGS) to the Nuclear Regulatory Commission.

According to a release, the data show that most of the wear, or tube wall thinning, was less than 20 percent. This is far below the 35 percent wall-thinning limit, which would require that the tube be plugged.

The majority of this wear is related to support structures. The nature of the support structure wear is not unusual in new steam generators and is part of the equipment settling in.

"We're using this information and additional detailed data collected through testing to develop our repair plans according to best practices and industry standards, particularly the data on the unexpected tube-to-tube wear," said Pete Dietrich, senior vice president and chief nuclear officer.

The data include the various types of wear on the tubes and the number of tubes affected.

There were three major categories of wear: anti-vibration bar wear, tube support plate wear and tube-to-tube wear. Two minor categories of wear were also included: retainer bar wear and wear due to a foreign object.

The foreign object wear, also not unusual in new steam generators, was only found in Unit 2 and was caused by a piece of welding material about the size of a quarter rubbing against two tubes.

In Unit 2, 1,595 tubes showed wear of some type and 510 tubes were ultimately plugged -- six tubes for showing wear of more than 35 percent and the rest for preventative measures.

In Unit 3, 1,806 tubes showed wear of some type and 807 tubes were ultimately plugged -- 381 tubes for wear of more than 35 percent and the rest for preventative measures.

The complete data for both units is available on the commission website for SONGS information: www.nrc.gov/info-finder/reactor/songs/tube-degradation.html.

Both units of the plant are currently safely shut down for inspections, analysis and tests.

Unit 2 was taken out of service Jan. 9 for a planned outage. Unit 3 was safely taken offline Jan. 31 after station operators detected a leak in a steam generator tube.

The units will remain shut down until SCE and the commission are satisfied that the units are safe to operate.