Nuclear Regulatory Commission (NRC) inspectors in a report released July 19 said faulty computer modeling that inadequately predicted conditions in steam generators at the two-unit, 2,200 MW San Onofre Nuclear Generating Station (SONGS) and manufacturing issues contributed to excessive wear of the components. Inspectors also determined that Southern California Edison (SCE), operator of the SONGS plant, provided the NRC with all the information required under existing regulations about proposed design changes to its steam generators prior to replacing them in 2010 and 2011.
The NRC said Unit 3 at the plant near San Clemente, Calif., operated for about one year following replacement of its steam generators when control room operators received alarms Jan. 31 indicating reactor coolant was leaking into a steam generator.
These and other findings of the Augmented Inspection Team sent to the plant were contained in the report. The inspection was conducted by a team from the NRC’s Region IV and Region II offices, the resident inspector from SONGS, one engineer from the NRC Office of New Reactors, two engineers from the NRC Office of Nuclear Reactor Regulation, and one engineer from the NRC Office of Research.
Regulators said the inspection team was directed to identify the circumstances surrounding the tube degradation; review the licensee’s actions following discovery of the conditions; evaluate the licensee’s review of potential causes of the unusual steam generator tube wear; review the computer modeling used in the design of the steam generators; and assess the differences in wear between the Unit 2 and Unit 3 steam generators, manufactured by Mitsubishi Heavy Industries. Tube degradation occurred in both units, but it was greater in Unit 3 than Unit 2.
The NRC said SCE brought in a large number of outside industry experts, consultants, and steam generator manufacturers, including Westinghouse and Areva, to perform thermal-hydraulic and flow induced vibration modeling and analysis. The licensee identified the most probable causes of the tube-to-tube wear as a combination of higher than predicted thermal/hydraulic conditions and changes in the manufacturing of the Unit 3 steam generators, a conclusion with which the NRC team agreed. The changes in the manufacturing resulted in less contact forces between anti-vibration bars and the tubes. The combination of these causes allowed excessive vibration to occur.
Operators shut down Unit 3 on Jan. 31, after the tube leak in one steam generator was identified; Unit 2 had been shut down for a scheduled maintenance outage. Both reactors have remained shut down.
The NRC said SONGS will not be permitted to restart until SCE has developed a plan to prevent further steam generator tube degradation and the NRC independently verifies that it can be operated safely.