SONGS steam generator tube leakage under investigation

A small number of steam generator tubes have failed pressure tests at unit 3 of Southern California Edison's San Onofre Nuclear Generating Station (SONGS) in California as the company investigates the cause of a leak in the recently installed components.

A small number of steam generator tubes have failed pressure tests at unit 3 of Southern California Edison's (SCE) San Onofre Nuclear Generating Station (SONGS) in California as the company investigates the cause of a leak in the recently installed components.

One of the steam generators being delivered to SONGS 3 (Image: MHI)

Steam generators are crucial components of plants based on pressurized water reactors (PWRs), transferring the thermal energy generated in the reactor vessel from the primary coolant circuit to a scondary 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 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.

SCE is now in the process of conducting in-situ pressure tests on 129 of SONG 3's 19,454 tubes, which show higher than normal wear. The tests involve the tubes being slowly pressurized with non-radioactive water in stages, using up to three times the normal operating condition pressure. Tubes undergoing the tests are removed from service by plugging them, whether or not they pass the test. The generators are built with an allowance of extra tubes so that some can be taken out of service during the life of the plant.

So far, SCE reported, three steam generator tubes failed during the tests, which are being conducted in the presence of inspectors from the US Nuclear Regulatory Commission (NRC). SCE anticipates additional NRC inspections.

Meanwhile, SONGS 2 was taken offline on 9 January for a planned maintenance and inspection outage. This work is nearly completed. SCE has plugged those steam generator tubes at unit 2 showing signs of wear or susceptibility to wear.

"Tests at a nuclear plant are designed to detect potential safety issues, and these tests serve that purpose," said SCE president Ron Litzinger. "Our tube testing plan, in accordance with industry standards, is designed to help us understand the safety issues and significance of this situation."

Due to the two-loop design of the SONGS reactors, their replacement steam generators are among the world's largest, each measuring approximately 20 metres in length, seven metres in diameter and weighing some 580 tonnes. In a recent article in Nuclear Engineering International magazine, SCE's Boguslaw Olech and MHI's Tomoyuki Inoue said, "The major shortcoming of such large steam generators, as seen during their operating history, was tube wear, particularly in the U-bend region." SCE and MHI worked together to improve the design of the replacement steam generators for SONGS. These "were intended to include all possible improvements introduced by the industry into the steam generator design and fabrication processes based on the US industry operating experience with all PWR plants, inside and outside of the USA."

Researched and written
by World Nuclear News