Special NRC Oversight at San Onofre Nuclear Generating Station: Steam Generator Tube Degradation
- SONGS2 Performance Summary
- SONGS3 Performance Summary
- San Onofre (NRC Blog)
- Public Meeting Schedule
- Next Public Meeting:
February 12, 2013
In response to an Event Notification Report, dated January 31, 2012, the U.S. Nuclear Regulatory Commission (NRC) modified its regulatory oversight of San Onofre Nuclear Generating Station (SONGS)- Unit 2 and Unit 3, to address unusual degradation of tubes in the newly installed steam generators after approximately 1.7 and 1 effective full power years of operation, respectively. The purpose of this special oversight is to assess the licensee's evaluation of the cause(s) of unexpected tube degradation, and verify that the licensee's corrective 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 on the event and the status of NRC follow-up and planned activities, see the following topics on this page:
- Summary of Event and Plant Conditions
- Augmented Inspection Team
- Confirmatory Action Letter
- Enhanced Oversight
- Unit 2 Steam Generators
- Unit 3 Steam Generators
- Steam Generator Tube Wear Data
- SONGS Unit 3 Failed In-situ Wear Data
- Next Steps
- Publicly Available Documents
- Public Meetings
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, 2012During 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. Three tubes had wall thinning in excess of 99% through-wall, and many other tubes presented significant wear. 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. Testing results from Unit 2 also revealed unexpected wear at the retainer bars, with only two indications similar to the tube-to-tube wear 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.
Augmented Inspection Team
On March 19, 2012, the NRC initiated an Augmented Inspection Team (AIT) reactive inspection to assess the circumstances surrounding the tube leak and unexpected wear of tubes in the Unit 3 SGs. The AIT charter was subsequently revised on May 16, 2012. The AIT team presented their findings in a public exit meeting near SONGS on June 18, 2012. The AIT report, issued July 18, 2012, identified probable contributing causes of the steam generator tube degradation that ultimately led to the tube leak in Unit 3. An identified common potential cause is related to design control issues, associated with the thermal/hydraulic modeling of the steam generators. The AIT also assessed the licensee’s response to the event and whether the steam generator major design changes were appropriately reviewed in accordance with the 10 CFR 50.59 requirements. The AIT identified ten unresolved items that warranted additional follow-up inspection and review.
A follow-up inspection of the unresolved issues identified by the AIT was conducted from August 20 to September 28, 2012. The follow-up inspection report, dated November 9, 2012, closed 8 of the 10 unresolved items. The two remaining unresolved items are related to the mechanistic cause of the tube-to-tube vibration resulting from fluid-elastic instability. The NRC will conduct subsequent inspections and reviews to disposition the remaining unresolved items.
Confirmatory Action Letter
On March 27, 2012, the NRC issued a Confirmatory Action Letter (CAL) to SCE, to confirm the actions the licensee committed to take prior to returning Units 2 and 3 to power operation. The CAL specifies that prior to restart of either unit, the licensee will identify the cause(s) of the excessive tube wear and take corrective actions to ensure that SG tube integrity can be maintained. The CAL also specifies that the licensee will provide in writing to NRC its protocol of inspections and/or operational limits for the planned operating interval and the basis for SCE’s conclusion that there is reasonable assurance that the units will operate safely. Neither unit will resume power operations until the licensee responds to the items in the CAL, and the NRC has completed a thorough review of those actions and communicates in writing it is satisfied the licensee can operate the plant without undue risk to public health and safety.
On October 3, 2012, SCE submitted its CAL response and return to service report for Unit 2. The licensee is continuing efforts to respond to the remaining CAL items for Unit 3.
Enhanced Oversight
Because of the extended shutdown of SONGS while the licensee has been evaluating the cause of the steam generator tube degradation and developing corrective actions, the NRC transitioned regulatory oversight for SONGS Units 2 and 3 on September 4, 2012, to Inspection Manual Chapter 0351, “Implementation of the Reactor Oversight Process at Reactor Facilities in an Extended Shutdown Condition for Reasons Other Than Significant Performance Problems.” This action was taken to modify the inspection program for SONGS since some inspection procedures cannot be accomplished, and some performance indicators are invalid, for plants that have been shutdown for greater than 6 months.
Additionally, to focus NRC oversight resources on this complicated technical issue, both NRC Region IV, and NRR, created Special Project Branches with senior NRC management and dedicated inspector resources.
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 steam generator 2E088 and 94 in steam generator 2E089. Of the 192 tubes a total of six tubes, four due to retainer bar wear and two due to anti-vibration bar wear, required plugging because they exceeded the plugging criterion of 35-percent tube wear. Of the remaining 186 tubes, two tubes were preventatively plugged due to anti-vibration bar wear (<35%) 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 and expert panel screening criteria, a total of 323 additional tubes, located in similar areas of tube-to-tube wear in Unit 3 steam generators, were plugged in the Unit 2 steam generators including two tubes with indications of tube-to-tube wear of 15%. The total plugging for Unit 2 was 207 tubes in steam generator 2E088, and 308 in steam generator 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.
- SONGS Steam Generator Internal Diagram
- SONGS Unit 2 Steam Generator Tube Wear Data
- SONGS Unit 3 Steam Generator Tube Wear Data
SONGS Unit 3 Failed In-situ Wear Data
In-situ Testing Summary Unit 3 SG 3E088
| Tube Location | Max Pressure Reached | Percent Wear (%) | Length of Wear (in) |
|---|---|---|---|
| * Row 106 Column 78 | 2874 | 100 | 29 |
| Row 102 Column 78 | 3268 | 99 | 23 |
| Row 104 Column 78 | 3180 | 99 | 27 |
| Row 100 Column 80 | 4732 | 81 | 28 |
| Row 107 Column 77 | 5160 | 80 | 34 |
| Row 101 Column 81 | 4889 | 78 | 26 |
| Row 98 Column 80 | 4886 | 72 | 29 |
| Row 99 Column 81 | 5026 | 72 | 27 |
Next Steps
The NRC will continue its independent oversight of the issues at SONGS. The NRC will not allow the plant to restart until we are satisfied that the licensee can operate the plant without undue risk to public health and safety. The NRC will take the time needed to determine the appropriate actions to ensure adequate protection of the public, and will continue efforts to communicate the status of its actions and the determination regarding the restart of SONGS, Units 2 and 3.
The NRC is evaluating the licensee’s CAL response for Unit 2, and will fully assess the actions the licensee has taken to evaluate and correct the SG problems prior to making a restart determination for Unit 2. This assessment includes inspection of the adequacy of the actions the licensee has taken in response to the CAL, and an evaluation of whether the licensee’s corrective actions to ensure that SG tube integrity is maintained, and protocol of inspections and/or operational limits for the planned operating interval, provide reasonable assurance that Unit 2 can be operated safely. Currently, the licensee has not completed the CAL actions for Unit 3, and has stated publicly that it will be “some time” before Unit 3 is ready for restart.
The NRC is committed to ensuring that the public is informed about our oversight activities related to SONGS and our work to assure adequate protection of the public. Four public meetings have been held, and others are planned to be held during NRC’s enhanced oversight of SONGS. On June 18, 2012, NRC presented the results of the Augmented Inspection Team inspection. On October 9, 2012, the NRC conducted an open panel discussion to receive and understand the views of the public and other interested local stakeholders. On November 30, 2012, Southern California Edison presented their response to the CAL for Unit 2 to the NRC in a public meeting. On December 18, 2012, the NRC staff held a public meeting with Southern California Edison representatives at NRC headquarters to discuss additional information needed to complete the ongoing technical review. Links to video archives of these meetings can be found in the Public Meeting Videos section below.
The next public meeting is scheduled for February 12 in Dana Point to discuss the status of the NRC's ongoing review. Related documents explaining the details and format of the meeting are available in the Public Meetings section of this website. NRC public meetings are announced on NRC's website.
The following milestones are tentative and offered to promote understanding of the key events that will occur as the NRC conducts its independent review. Specific dates listed are subject to change based upon conduct of the inspection and discovery of new information. The NRC staff will take the time that is required to complete the inspection and technical evaluation of the licensee’s CAL response without regard to specific schedules. Updated information on these activities will be provided as it becomes available.
| Milestone | Expected Date |
|---|---|
| • CAL inspection (Unit 2) | November 2012 – March 2013 |
| • Technical Evaluation (Unit 2) | November 2012 – March 2013 |
| • Category 3 Public meeting in Southern California on status of CAL inspection & technical evaluation | February 12 |
| • Public meeting in Southern California with licensee on CAL inspection & technical evaluation | Mid-April |
| • Technical evaluation and Unit 2 CAL inspection report complete and publicly available | Late-April |
| • Notify ASLB and parties of intent to issue decision regarding restart for Unit 2 | Late-April |
| • Issue decision regarding restart for Unit 2 | 5-30 days after notification to ASLB and parties |
Publicly Available Documents
The following table lists the publicly available documents that the NRC has issued in connection with the special oversight of SONGS.
This page includes links to files in non-HTML format. See Plugins, Viewers, and Other Tools for more information.
| Date | Title/Description |
|---|---|
| 01/28/2013 | SCE Responses to NRC Requests for Additional Information as received through January 31, 2013 |
| 01/17/2013 | SONGS Oversight Panel Charter |
| 12/26/2012 | Request for Additional Information Regarding Response to Confirmatory Action Letter |
| 11/30/2012 | Report of NRC Vendor Inspection at Mitsubishi Heavy Industries, Ltd. |
| 11/09/2012 | Augmented Inspection Team Follow-up Inspection (2012010) |
| 11/08/2012 | Commission Memorandum and Order regarding Petition to Intervene and Request for Hearing by Friends of the Earth |
| 10/04/2012 | Press Release: NRC Receives Response to Confirmatory Action Letter, Action Plan for San Onofre Unit Proposed NRC Review to Take Months |
| 10/03/2012 | Southern California Edison Response to NRC’s Confirmatory Action Letter and Return to Service Plan for Unit 2 |
| 09/04/2012 | Mid-Cycle Assessment Letter for San Onofre Nuclear Gernerating Station, Units 2 and 3 |
| 09/04/2012 | NRC Oversight of San Onofre Nuclear Generating Station (SONGS) Units 2 and 3 during Extended Shutdown |
| 07/18/2012 | Augmented Inspection Team Report (2012007) |
| 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 |
| 03/16/2012 | 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 |
| 01/31/2012 | Event Notification Report 47628, Manual Trip due to a Primary-to-Secondary Leak Greater than 30 gal/hr. |
Public Meetings
The following table lists the public meetings in connection with the special oversight of SONGS, along with related documents and videos. For these and other videos, please see NRC Webcast Portal.
This page includes links to files in non-HTML format. See Plugins, Viewers, and Other Tools for more information.
| Date | Title/Description |
|---|---|
| 02/12/2013 | Public Meeting to Discuss the Status of NRC Inspection and Technical Evaluation of SCE's Response to Confirmatory Action Letter for Unit 2 |
| 01/16/2013 | Public Meeting with Petitioner Requesting Action Against Southern California Edison |
| 12/18/2012 | Public Meeting to Discuss NRC's review of Southern California Edison's response to the Confirmatory Action Letter for San Onofre Nuclear Generating Station, Unit 2 |
| 11/30/2012 | NRC Meeting with Southern California Edison (SCE) to Discuss SCE's Response to the NRC Confirmatory Action Letter |
| 10/09/2012 | Update: NRC Names Panelists for Public Meeting to Discuss San Onofre Nuclear Generating Station Issues |
| 09/13/2012 | Roundtable Meeting to Discuss San Onofre Nuclear Generating Station Issues |
| 06/18/2012 | Public Meeting for NRC Augmented Inspection Team to provide a status of their inspection following the Unit 3 steam generator leak on January 31, 2012
|
