Most steam generator tube wear or tube wall thinning at Southern California Edison’s (SCE’s) two-reactor San Onofre Generating Station (SONGS) was less than 20%—far below the 35% wall-thinning limit that would require the tubes to be plugged, data released last week by the Nuclear Regulatory Commission shows. Much of the wear was not "unusual," SCE said in a statement.

The data, which surveys various types of wear on the tubes and the numbers of tubes affected, shows that in Unit 2, 1,595 tubes showed wear of some type. Of these, six tubes showed wear of more than 35%, though 510 tubes were ultimately plugged, most for preventative measures.

At Unit 3, of 1,806 tubes that showed some wear, 807 tubes were plugged. Of these 381, or nearly half, showed wear of more than 35%. The rest were plugged for preventative measures.

"The majority of this wear is related to support structures," SCE said in a July 13 statement. The nature of the support structure wear is not unusual in new steam generators and is part of the equipment settling in."

According to the company, the data covers 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," SCE said.

Both units, which were taken offline this January, remain shut down and are awaiting NRC approval for restart.

Sources: POWERnews, SCE, NRC
—Sonal Patel, Senior Writer (@POWERmagazine)