Data has been released on tube wear in the steam generators at the San Onofre Nuclear Generating Station (SONGS). Plant owner Southern California Edison (SCE) has still to determine when the two reactors there will be restarted.
SCE said that it had identified three major categories of wear affecting the steam generator tubes: from the anti-vibration bars, tube support plates and between tubes. Two minor categories had also been found: from retainer bars and due to a foreign object. It said that wear at the retainer bars - which are unique to steam generators made by Mitsubishi - "was not expected." The utility said that the foreign object wear, "also not unusual in new steam generators," was caused by a small piece of welding material rubbing against two tubes at unit 2.
According to data submitted by SCE to the US Nuclear Regulatory Commission (NRC), most of the wear, or tube wall thinning, was less than 20%. This, it says, "is far below the 35% wall-thinning limit," which would require that the tube be plugged.
The data indicates that some 1595 tubes in unit 2 showed signs of wear and 510 tubes were subsequently plugged. Of these, six showed wear of more than 35%. At unit 3, 1806 tubes showed wear and 807 were plugged. Some 381 tubes were found to have wall thinning of over 35%, with just over one-third of these having thinning of over 50%.
SCE senior vice president and chief nuclear officer Pete Dietrich said, "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. Safety continues to be the guiding principle behind all the work we are doing."
However, the company reiterated that the San Onofre units will remain offline "until SCE and the commission are satisfied that the units are safe to operate."
Steam generators are crucial components of pressurized water reactors (PWRs), transferring the thermal energy generated in the reactor vessel from the primary coolant circuit to a secondary steam circuit that drives the turbine generator.
SCE decided to replace the steam generators of the 1070 MWe unit 2 and 1100 MWe unit 3 PWRs at the SONGS plant after a cost-benefit study showed that the modernization would save customers some $1 billion over about 13 years. The two steam generators of unit 2 were replaced in 2009, while those of unit 3 were replaced in late 2010. Each of the two SONGS units were originally equipped with two Combustion Engineering Model 3340 recirculating steam generators, which were designed for a 40-year service life. The replacement generators - among the world's largest - were manufactured by Mitsubishi Heavy Industries (MHI) at its Kobe Shipyard & Machinery Works in Japan. Each of the generators feature 9727 tubes and are built with surplus tubes so that some can be taken out of service during the life of the plant.
SONGS 3 resumed operation in February 2011, but was shut down on 31 January 2012 after workers detected a leak in one of the unit's steam generator tubes. Meanwhile, SONGS 2 was taken offline on 9 January for a planned maintenance and inspection outage.
Researched and written
by World Nuclear News