The Nuclear Regulatory Commission has released further details on the thinning steam generator tubes that have kept both operating reactors at California's San Onofre nuclear plant out of commission since last winter.
Each unit has two steam generators with 9,727 tubes apiece. Testing of unit 3's steam generators detected wear in a total of 10,284 places on 1,806 tubes. It found wear in a total of 4,721 places on 1,595 tubes in unit 2. Southern California Edison pointed out in a release Friday that support structure wear is common in new steam generators, and that the majority of the spots in question had less than 20 percent of the tubes' thickness worn away.
Thirty-five percent is the threshold for plugging thinning tubes, and the utility plugged 807 tubes in unit 3 and 510 tubes in unit 2. With only a handful of exceptions, the tube thinning was caused by anti-vibration wear, tube support plate wear or tube-to-tube wear. Within unit 3, 282 locations showed wear of greater than 50 percent of the tubes' thickness. Two locations showed wear at greater than 50 percent in unit 2.
"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," Southern California Edison Chief Nuclear Officer Pete Dietrich said in the release.
NRC officials said last month they suspect the wear stems from the Mitsubishi Heavy Industries-built equipment's design and by computer modeling that miscalculated the velocity of water flowing through the steam generators.
Download NRC charts of tube wear in each SONGS unit:
-- Unit 2
-- Unit 3