Investigations continue into tube wear at SONGS

More than 1300 steam generator tubes have now been plugged at Southern California Edison's San Onofre Nuclear Generating Station (SONGS) in California as the utility continues to investigate the cause of excessive wear in some of the tubes. It is not yet known when the two-unit plant will resume operation.

More than 1300 steam generator tubes have now been plugged at Southern California Edison's (SCE's) San Onofre Nuclear Generating Station (SONGS) in California as the utility continues to investigate the cause of excessive wear in some of the tubes. It is not yet known when the two-unit plant will resume operation.

SONGS (Image: SCE)

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 - are among the world's largest - were manufactured by Mitsubishi Heavy Industries (MHI) at its Kobe Shipyard & Machinery Works in Japan.

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.

SCE has been conducting in-situ pressure tests on tubes showing higher than normal wear in the steam generators of both units 2 and 3. The tests involve the tubes being slowly pressurized with water in stages, using up to three times the normal operating condition pressure. Tubes undergoing the tests are removed from ongoing service by plugging them, whether or not they pass the test. 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.

SCE has now conducted in-situ pressure testing on 129 tubes in unit 3 after initial inspections identified tube-to-tube wear. A total of eight tubes failed the pressure tests. The company also said that it had identified additional tube wear in two of the 19,454 tubes in unit 2 that was similar to the type of wear seen in unit 3. SCE has been plugging all tubes showing excessive wear, as well as some additional tubes as a precautionary measure. A total of 510 tubes have been plugged in unit 2's steam generators, while 807 have been plugged at unit 3.

The company said that it had identified a wear mechanism caused by tubes that were vibrating and rubbing against adjacent tubes. Wear was also found where tubes had rubbed against certain supports in both units. SCE continues to analyze inspection and data, and is conducting additional inspections to determine why these wear mechanisms are occurring.

Recent media reports have suggested that SCE plans to restart both SONGS units in June. However, the company said that the California Independent Systems Operator Corporation (ISI) - which operates the state's wholesale transmission grid - was provided with these estimated restart dates in March purely for long-range planning purposes and that they are subject to change. SCE said that the units would only be returned to service when it and the US Nuclear Regulatory Commission (NRC) are satisfied it is safe to do so. It added that it has yet to request permission from the NRC to restart the reactors.

SCE president Ron Litzinger commented, "We want to clarify the use of planning dates and make sure it is clear that there is no timeline on nuclear safety."

Researched and written
by World Nuclear News