NRC Issues Augmented Inspection Team Report on San Onofre Nuclear Generating Station Steam Generators
Special NRC Oversight at San Onofre Nuclear Generating Station: Steam Generator Tube Degradation
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.
This page includes links to files in non-HTML format. See Plugins, Viewers, and Other Tools for more information.
| 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. |
