05000361/LER-2009-004
| Docket Number | |
| Event date: | 12-23-2009 |
|---|---|
| Report date: | 02-19-2010 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat |
| 3612009004R00 - NRC Website | |
Event Date: ' December 23, 2009 Reactor Vendor: Combustion Engineering Mode: Defueled Power: 0 percent
Background:
The Spent Fuel Pool (SFP) Cooling System is cooled by the Component Cooling Water (CCW) System [CC] through the SFP Heat Exchangers. The Saltwater Cooling (SWC) System [BS] supplies ocean water to cool the CCW System through two redundant CCW Heat Exchangers (Train A and Train B) [HX].
For maximum cooling capacity, the CCW Heat Exchangers are normally configured such that the SWC flow in the tubes and the CCW flow in the shell are in opposite directions. To clear debris which could potentially block the salt water flow through the CCW Heat Exchanger, the system can be realigned so the salt water flow is reversed to flow in the same direction as the CCW flow through the CCW Heat Exchanger. Approved procedures are in place to realign the system for reverse flow conditions.
Technical Specification (TS) 3.7.16 requires the SFP water level to be maintained greater than 23 feet over the top of the fuel assemblies during movement of fuel assemblies in the SFP.
There are no specific applicable TSs for the CCW Heat Exchangers when the plant is in the defueled condition. Licensee Controlled Specification (LCS) 3.7.106 "Spent Fuel Pool Operation," is applicable at all times with irradiated fuel in the SFP. With the plant defueled and the core fully offloaded, the LCS requires the water in the SFP to be maintained less than 160 degrees F. For two SFP cooling pumps and/or two SFP heat exchangers not operable the following actions are required by the LCS:
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 160 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 December 23, 2009, at about 0957 PST, with Unit 2 in the steam generator replacement outage and defueled, Train A Salt Water Cooling (SWC) low flow alarm and CCW Heat Exchanger high differential pressure alarm were received in the control room. Since Train B Component Cooling Water System had been removed from service for scheduled maintenance, Operators initiated actions in accordance with the Abnormal Operating Instructions (A01) for loss of CCW and SWC.
The reduction in SWC flow was suspected to be caused by debris in the CCW heat exchanger. In accordance with the A01 procedure, actions to clear the debris and restore SWC flow were completed.
At approximately 1014 PST, Operators stopped the SWC pump, declared Train A SFP Cooling System INOPERABLE, and completed actions required by LCS 3.7.106B when two SFP heat exchangers are inoperable (listed above). With the SWC pump off, Operators changed the valve line up to reverse SWC flow through the CCW heat exchanger, and at 1116 PST restarted the SWC pump.
Prior to stopping the SWC pump to establish reverse flow conditions, the temperature of the SFP was not affected by the reduction in SWC flow. The cooling capacity of the SWC system significantly exceeds the SFP cooling load when the plant is defueled. While the SWC pump was off, the.SFP temperature increased from about 74 to 78 degrees F. Calculations performed at about 1055 PST estimated the time to exceed the maximum allowed temperature of 160 degrees F was approximately 23 hours2.662037e-4 days <br />0.00639 hours <br />3.80291e-5 weeks <br />8.7515e-6 months <br />.
At 2111 PST, Operators declared Train A SFP cooling OPERABLE after verifying proper SWC flow had
- been restored and the temperature of the SFP was stable.
Train A CCW Heat Exchanger was operated in reverse flow mode until taken out of service for maintenance on January 1, 2010 after Train B was declared OPERABLE. Upon investigation, SCE determined the cause of the SWC low flow alarm was ocean debris (such as kelp) obstructing flow through the CCW heat exchanger. The debris appeared to have entered the system through a damaged SWC pump suction strainer [STR].
Corrective Actions
The SWC pump with the damaged suction strainer was replaced.
Basis for Reporting:
This event was not reported in an eight-hour report under 10 CFR 50.72(b)(3)(v) 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 stopping of Train A SWC flow while Train B was out of service for maintenance constituted a loss of safety function as evidenced by the increase in.
SFP temperature. This event, therefore, is being reported in accordance with 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 residual heat.
Safety Significance:
The safety significance of this event was minimal. The maximum SFP temperature during this event was 78 degrees F, which is well below the maximum allowed temperature of 160 degrees F per the Licensee Controlled Specification. With the SWC pumps off, the time to exceed the maximum allowed temperature was approximately 23 hours2.662037e-4 days <br />0.00639 hours <br />3.80291e-5 weeks <br />8.7515e-6 months <br />. Alternate methods of cooling the SFP were available if required. The approximate 4 degree F increase in SFP temperature was an expected result of the reverse flow evolution, which was performed in accordance with an approved operating procedure. All, LCS requirements for two trains of SFP cooling inoperable were met and the water level in the SFP was maintained at least 23 feet above the spent fuel as required by the TS.
Previous Occurrences
On February 13, 2007, both Unit 2 SFP cooling pumps were inoperable when a thermal overload circuit tripped the Train B,SFP cooling pump while the Train A SFP cooling pump was out of service for scheduled maintenance. The SFP temperature increased by about 2 degrees F while SFP cooling was restored (approximately 70 minutes). SCE did not previously report this event as a loss of safety function. Based on guidance from the NRC, a Licensee Event Report will be submitted to report this event as a loss of safety function.