Summary of Event and Plant Conditions
On January 31, 2012, San Onofre Nuclear Generating Station Unit 3 (Unit 3) experienced a tube leak in one of the two steam generators (SGs). The leak was unexpected, and the unit was shut down in accordance with plant procedures. Operators manually tripped the reactor from 35 percent power as directed by procedure. The affected steam generator was isolated and the unit was brought to cold shutdown conditions.
Prior to the event, Unit 3 was operating at 100 percent rated thermal power with no plant evolutions in progress. On January 31, 2012, Unit 3 control room operators received an alarm that indicated a primary-to-secondary reactor coolant leak from steam generator 3E0-88. The alarm received was from the main condenser air ejector radiation monitors, which continuously samples from a vent line for the purpose of rapidly identifying steam generator tube leaks. Although the leak rate was small, it increased enough in a short period of time for the licensee to perform a rapid shutdown when the estimated leak rate exceeded 75 gallons per day. The facility license allows full power operation with a steady state leak rate of less than 150 gallons per day. Unit 3 reached cold shutdown conditions on February 1, 2012. The licensee reviewed the amount of gaseous radioactivity released during the event and estimated a dose of approximately 0.0000452 mrem (452 pSv) to a member of the public. The annual regulatory limit to a member of the public is 100 mrem (1 mSv) per year. The licensee submitted its written event report to the NRC on March 29, 2012.
During follow-up inspections of the Unit 3 SG tubes, the plant operator, Southern California Edison (SCE), discovered unexpected wear in both SGs, including significant tube-to-tube wear in the free span areas of over 100 tubes. Pre-planned testing of 100 percent of the SONGS Unit 2 (Unit 2) SG tubes was in progress as part of a regularly scheduled refueling outage when the event occurred in Unit 3. Testing results from Unit 2 also revealed unexpected tube wear at the retainer bars. Additional analysis and testing ultimately resulted in identifying two tubes with tube-to-tube wear similar to what was observed in Unit 3.
For both Units 2 and 3, this was the first cycle of operation with new replacement SGs. SCE replaced the Unit 2 SGs in January 2010 and Unit 3 SGs in January 2011. Each replacement SG has 9,727 tubes and was manufactured by Mitsubishi Heavy Industries.
Additional information on the event, the licensee’s response, and the results of the licensee’s steam generator tube inspections are provided below.