Special NRC Oversight at San Onofre Nuclear Generating Station: Steam Generator Tube Degradation

Related Information

In response to an Event Notification Report, dated February 1, 2012, the U.S. Nuclear Regulatory Commission (NRC) modified its regulatory oversight San Onofre Nuclear Generating Station (SONGS), Unit 2 and Unit 3, to address unusual degradation of newly installed steam generator tubes after approximately 1.7 and 1 effective full power years of operation, respectively. The purpose of this special oversight is to review the licensee's response to the initial indications of the tube leak, and verify that the licensee's actions are appropriate to ensure the integrity of the steam generators and to protect the health and safety of the public and the environment. For more information, see the following topics on this page:

Unit 2 Steam Generators

Unit 2 commenced a normally scheduled refueling outage on January 10, 2012. This was the first refueling outage after replacement of both steam generators in January 2010. In accordance with the plant’s Technical Specifications, a 100-percent inspection of all steam generator tubes was conducted. Mechanical wear was observed at various locations along the tube lengths, similar to what has been observed in comparable steam generators at other plants. The wear observed at the retainer bars (bars that are unique to steam generators fabricated by Mitsubishi) was not expected. The severity of one of the wear indications at a retainer bar was significant enough (90 percent thru-wall) to warrant in-situ pressure testing. This pressure test confirmed the structural integrity of this tube (there was no leakage).
Based on the severity of tube-to-tube wear found on Unit 3 steam generators, the licensee completed additional rotating coil eddy current testing in an area of interest consisting of 1,375 tubes in each Unit 2 steam generator. Tube-to-tube wear of 14 percent was identified in two adjacent tubes in each Unit 2 steam generator 2E089.
Based on the initial 100-percent tube inspections, the licensee completed tube plugging and staking (internal cable support of select plugged tubes) of 192 tubes total: 98 in 2E088 and 94 in 2E089. A total of six tubes required plugging because they exceeded the plugging criterion of 35-percent tube wear, and all tubes adjacent to the "retainer bars" were plugged as a precautionary measure due to unusual wear at that location. As a result of information learned from the Unit 3 cause evaluation, a total of 318 additional tubes, located in similar areas of tube-to-tube wear in Unit 3 steam generators, were plugged in the Unit 2 steam generators. The total plugging for Unit 2 was 205 tubes in 2E088, and 305 in 2E089.

Unit 3 Steam Generators

Unit 3 had been operating for approximately 1 year following replacement of the steam generators when control room operators received alarms on January 31, 2012, indicating that reactor coolant was leaking into one of the steam generators (3E088). The leak was unexpected, and the licensee responded in accordance with its procedures by performing a rapid shutdown, since the leak, although small, had increased enough in a short period of time to warrant the precautionary shutdown. The estimated leak rate was 75 gallons per day, which is less than .06 gallons per minute. The Plant Operating License allows full-power operation with a steady leak rate of less than 150 gallons per day.
The first indication of the leak was that the main condenser air ejector radiation monitors reached their alarm setpoint. The radiation monitors continuously sample from a vent stack in order to rapidly identify steam generator tube leaks. This was a direct release of radioactivity to the atmosphere, which is allowed by the plant operating license, up to limits in the regulations. The licensee reviewed the amount released and estimated it was much less than is permitted by the plant operating license. NRC inspectors independently reviewed the release data and verified the licensee's findings. The release posed no threat to the public or the workers onsite.
Unit 3 control room operators performed a controlled shutdown of Unit 3 on January 31, 2012, and reached cold shutdown conditions on February 2, 2012. The operators then prepared the steam generators for tube inspections, which were started on February 12, 2012. The first inspection confirmed the location of the leak in steam generator 3E088. One tube was found with a small leak, and no other tubes were leaking.
The licensee has completed an extensive and expanded eddy current evaluation of 100 percent of the steam generator tubes in both Unit 3 steam generators. Indications of abnormal tube wear, both at support structures and tube-to-tube, were extensive in both Unit 3 steam generators. Following pre-established testing requirements, the licensee identified 129 tubes in Unit 3 (73 in 3E88 and 56 in 3E89) that required in-situ pressure testing.
The in-situ pressure test is performed one tube at a time, by slowly pressurizing the primary side of the tube (the side that normally sees reactor coolant pressure) with water. The first pressure point is approximately 3,200 pounds per square inch gauge (psig), which is the differential pressure the tube would see during a main steamline break. The pressure is determined using normal reactor coolant system (RCS) pressure, and then adding additional pressure due to test conditions (cooler temperature) and gauge uncertainties. This pressure is held for 2 minutes, and then the next pressure of approximately 5,200 psig is attained and held for 2 minutes. The last pressure is approximately three times the normal tube differential pressure, again adding corrections for temperature and gauge uncertainties.
The licensee determined that there was a high probability of several Unit 3 steam generator tubes failing the in-situ pressure test. In-situ testing was conducted from March 13 to March 20, 2012. The licensee accurately ranked each tube according to the probability of failure, and the top eight tubes with the highest probability of failure did fail the pressure test. All other tubes passed, and all of the failures occurred on the 3E088 steam generator. The leaking tube was one of the tubes that failed the pressure test.
The licensee completed extensive plugging and selective staking of 807 tubes in Unit 3 (420 in 3E088 and 387 in 3E089).

Steam Generator Tube Wear Data

The following data is provided to help interested parties understand the observed wear in the u-tubes of all four steam generators. A diagram showing the internal construction of the steam generator has also been provided to help readers understand the components of concern.

Publicly Available Documents

The following table lists the publicly available documents that the NRC has issued in connection with the special oversight of SONGS.
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Date	Title/Description
07/19/2012	Press Release regarding Augmented Inspection Team Report on San Onofre Nuclear Generating Station Steam Generators
07/18/2012	Augmented Inspection Team Report (2012007)
06/26/2012	Meeting Summary for Augmented Inspection Team Exit Meeting
06/08/2012	Press Release regarding Augmented Inspection Team Exit Meeting
06/08/2012	Public Meeting Notice for Augmented Inspection Team Exit
06/04/2012	Part 21-Steam Generator Tube Wear Adjacent to Retaining Bars
05/16/2012	Augmented Inspection Team Charter, Revision 1
05/07/2012	Statement from Chairman on Restart
03/29/2012	Licensee Event Report 2012-001-00, Unit 3 Manual Trip due to Steam Generator Tube Leak
03/27/2012	Press Release regarding Confirmatory Action Letter
03/27/2012	San Onofre Confirmatory Action Letter
03/23/2012	San Onofre Return-to-Service Action Plan
03/19-29/2012	Augmented Inspection Team On-Site Inspection
03/16/2012	Augmented Inspection Team Charter
03/16/2012	Preliminary Notification Update PNO-IV-12-003A, Augmented Inspection Team Onsite at San Onofre Unit 3
03/15/2012	Press Release regarding Augmented Inspection Team Inspection
02/01/2012	Preliminary Notification PNO-IV-12-003, San Onofre Unit 3 Steam Generator Tube Leak and Rapid Shutdown
02/01/2012	Event Notification Report 47628, Manual Trip due to a Primary-to-Secondary Leak Greater than 30 gal/hr.

Saturday, July 21, 2012

NRC Issues Augmented Inspection Team Report on San Onofre Nuclear Generating Station Steam Generators