05000361/LER-2007-007, For San Onfre, Unit 2, Regarding Inoperable SFP Cooling Pumps Results in Loss of Safety Function
| ML100880320 | |
| Person / Time | |
|---|---|
| Site: | San Onofre  |
| Issue date: | 03/25/2010 |
| From: | Bauder D Southern California Edison Co |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| LER 07-007-00 | |
| Download: ML100880320 (4) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat |
| 3612007007R00 - NRC Website | |
text
SOUTHERN CALIFORNIA EDISON An EDISON INTFRNATIONAL Company 10 CFR 50.73(a)(2)(v)(B)
March 25, 2010 U.S. Nuclear Regulatory Commission Document Control Desk Washington, D.C. 20555-0001
Subject:
Docket No. 50-361 Licensee Event Report No. 2007-007 San Onofre Nuclear Generating Station, Unit 2
Dear Sir or Madam:
This submittal provides Licensee Event Report (LER) 2-2007-007 to report a loss of safety function when both spent fuel pool cooling pumps were inoperable. This event was not reported previously because it was not considered a loss of safety function at the time of the event. Based on recent NRC guidance, SCE is reporting this occurrence as a loss of safety function. This event is of low safety significance and did not affect the health and safety of either plant personnel or the public.
If you require any additional information, please contact me.
- Sincer, ouglas R. Bauder Plant Manager Unit 2 LER No. 2007-007 cc:
E. E. Collins, NRC Regional Administrator, Region IV G. G. Warnick, NRC Senior Resident Inspector, San Onofre Units 2 & 3 P.O. Box 128 San Clemente, CA 92672
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Abstract
On February 13, 2007, at 1249 PST, the Train B Spent Fuel Pool (SFP) cooling pump tripped on over current. The Train A SFP cooling pump was out of service for scheduled maintenance at the time.
With both SFP cooling pumps inoperable, the plant entered the loss of SFP cooling Abnormal Operating Instruction. Train B SFP cooling pump was successfully restarted at 1407 PST, restoring SFP cooling. The temperature of the SFP increased from 71.5 to 73.0 degrees F while the pump was off.
SCE determined the cause of the over current trip was a high temperature in the motor control center (MCC) due to malfunctioning of a heater thermostat in the MCC.
This event was not reported previously because it was not considered a loss of safety function at the time of the event. Based on recent NRC guidance, SCE is reporting this occurrence under 10 CFR 50.73(a)(2)(v)(B) as a loss of safety function to remove decay heat.
The safety significance of this event is minimal since the temperature of the SFP remained well below the maximum allowed 140 degrees F, all applicable licensee controlled specifications were met, and alternate methods of cooling were available if required.
5RC FORM 366A U.S. NUCLEAR REGULATORY COMMISSION (7-2001)
LICENSEE EVENT REPORT (LER)
TEXT CONTINUATION
- 6. LER NUMBER PAGE (3)
YEAR SEQUENTIAL REV NO NUMBER San Onofre Nuclear Generating Station (SONGS) Unit 2 05000-361 2007
--007-00 2 of 3 Plant:
San Onofre Nuclear Generating Station (SONGS) Unit 2 Event Date:
February 13, 2007 Reactor Vendor:
Combustion Engineering Mode:
1 Power:
98.5 percent
Background:
The Spent Fuel Pool (SFP) cooling system consists of two SFP pumps [P] and two SFP heat exchangers [HX]. The SFP cooling pump motors [MO] are protected from over current by thermal overload relays (TOLs) in the control circuit which actuate to turn off the power supply to the pump motors. Actuation of the TOL is triggered by high temperature, which typically results from an over current condition.
Licensee Controlled Specification (LCS) 3.7.117 requires the SFP water level to be maintained greater than 23 feet over the top of the fuel assemblies in the SFP. LCS 3.7.106, "Spent Fuel Pool Operation,"
requires the water in the SFP to be maintained less than 140 degrees F when the core is not fully offloaded. The LCS also requires the following actions when two SFP pumps are inoperable:
B.1 Suspend all operations involving increase in SFP decay heat load within 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br />, B.2 Monitor the temperature of the SFP water and estimate the time before water temperature will increase to the 140 degree F limit within 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> and once per shift thereafter, and B.3.1 Initiate action to restore the components(s) to OPERABLE or operating status more than 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> prior to the estimated time at which water temperature will increase to the limit.
Event Description
On February 13, 2007, SONGS Unit 2 was operating at about 98.5% power with Train A SFP cooling pump removed from service for scheduled maintenance. At about 1249 PST, the Train B SFP cooling pump tripped on over current. Operators initiated actions in accordance with the Abnormal Operating Instruction (AOI) for loss of SFP cooling. The Train B SFP cooling pump was successfully restarted at 1407 PST.
During the period when both SFP cooling pumps were not operating, the temperature in the SFP increased from approximately 71.5 to 73.0 degrees F. With the SFP pumps off, the time to exceed the maximum allowed temperature of 140 degrees F was estimated to be 56 hours6.481481e-4 days <br />0.0156 hours <br />9.259259e-5 weeks <br />2.1308e-5 months <br />.
Initial investigation determined the pump motor had tripped due to actuation of the thermal overload relays; however, the motor was not experiencing an over current condition. Upon further investigation, SCE found the thermal overload relays had actuated because of a high temperature in the Motor Control Center (MCC). The high temperature was a result of a malfunctioning thermostat that failed to turn off a heater strip installed in the MCC to prevent condensation. The heater strip was subsequently disconnected.
Corrective Actions
Strip heaters were permanently disconnected for Units 2 and 3 Class 1 E MCCs, since they are located in areas where normal air conditioning is sufficient to minimize moisture.
5RC FORM 366A U.S. NUCLEAR REGULATORY COMMISSION (7-2001)
LICENSEE EVENT REPORT (LER)
TEXT CONTINUATION
- 6. LER NUMBER PAGE (3)
YEAR SEQUENTIAL REV NO NUMBER San Onofre Nuclear Generating Station (SONGS) Unit 2 05000-361 2007
--007--
00 3 of 3 Basis for Reporting:
This event was not previously reported because it was not considered a loss of safety function at the time of the event. Based on subsequent guidance from the NRC, the temporary loss of the Train B SFP cooling pump while the Train A SFP cooling pump was out of service for scheduled maintenance constituted a loss of safety function as evidenced by the increase in SFP temperature. This event, therefore, is being reported under 10 CFR 50.73(a)(2)(v)(B) as an event that could have prevented the fulfillment of a safety function of systems that are needed to remove decay heat.
Safety Significance
The safety significance of this event was minimal. The maximum SFP temperature during this event was 73.0 degrees F, which is well below the maximum allowed temperature of 140 degrees F per the LCS.
With the SFP pumps off, the time to exceed the maximum allowed temperature was approximately 56 hours6.481481e-4 days <br />0.0156 hours <br />9.259259e-5 weeks <br />2.1308e-5 months <br />. Alternate methods of cooling the SFP were available if required. All LCS requirements for two SFP cooling pumps inoperable were met and the water level in the SFP was maintained greater than 23 feet above the spent fuel.
Previous Occurrences
On February 19, 2010, SONGS submitted a report of a December 23, 2009, event in which both trains of Unit 2 SFP cooling were inoperable resulting in a loss of safety function. One train of the Component Cooling Water (CCW) system was out of service for maintenance when the salt water cooling pump in the other train was taken out of service for about 62 minutes to reverse the salt water cooling flow through the CCW heat exchanger in order to clear debris. The SFP temperature increased by about 4 degrees F during this evolution. Based on guidance from the NRC, Licensee Event Report 2009-004 was submitted to report this event as a loss of safety function.