05000482/LER-2010-010
| Wolf Creek Generating Station | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| 4822010000R01 - NRC Website | |
PLANT CONDITIONS PRIOR TO EVENT
Mode 1 100% power
EVENT DESCRIPTION
The Component Cooling Water (CCW) [EIIS Code: CC] system consists of two trains with two 100 percent capacity pumps per train, each supplying cooling water flow to the independent safety loads and a common service loop. An independent surge tank is connected to the suction of each train via a 4-inch line. The common service loop provides flow to the non-essential or non-safety related loads such as Reactor Coolant Pump [EIIS code: AB-P] motor coolers and is constructed of ASME Section III Class 3 piping except for the portion that supplies cooling water to Radwaste. The Radwaste section of piping is automatically isolated via valves EGHVO069A and EGHVO070A (supply to Radwaste); and valves EGHVO069B and EGHVO070B (return from Radwaste) upon the receipt of a Safety Injection Signal (SIS), high flow indication in the non-safety related piping, or a low CCW surge tank level. The automatic isolation occurs within 10 seconds of receiving the signal. The Radwaste section is also isolated if the service loop is isolated. During normal operation only one train of CCW is connected to the common service loop. This train has one pump in operation supplying cooling flow to the common service loop and supplying Radwaste. The other three CCW pumps are normally secured.
An issue was brought up by an NRC inspector during the Component Design Basis Inspection (CDBI) at Wolf Creek Nuclear Operating Corporation (WCNOC) postulating a pipe break initiated by an external or seismic event in the non-safety related section downstream of automatic isolation valves mentioned above, Radwaste. The original design architect of WCGS analyzed a break in the supply side pipe in 1985 with a determination that the break would be isolated in sufficient time to ensure adequate net positive suction head (NPSH) would be available for the CCW pumps. Upon further review, several nonconservative assumptions were identified in the calculation. The determination at this point is that the CCW surge tanks may not keep up with the break flow rate and water column separation could occur that will jeopardize the required CCW pump NPSH and could result in the system ingesting large volumes of air that could challenge pump operability.
As a result of this issue, the CCW train connected to the service loop portion that supplies cooling water to Radwaste was considered to be inoperable. Technical Specification (TS) 3.7.7 was entered on September 22, 2010 at 1317. The TS was exited on September 23, 2010 at 0241 by isolating the Radwaste from the CCW train using temporary procedure TMP 10-028, "Isolating CCW to Radwaste.
Periodically the common service loop was transferred to the opposite CCW train. During the short period of time for this transfer both CCW trains are aligned to the service loop. If the Radwaste portion of the service loop were inservice, then both CCW trains would be inoperable.
TS 3.7.7 requires two trains of CCW to be operable in Modes 1 through 4. Since plant startup, one train of CCW has been aligned to the service loop with Radwaste making that train inoperable. The amount of time that the service loop Radwaste portion is aligned to the CCW train exceeds the allowed outage time in TS 3.7.7. Therefore this issue is reportable per 10 CFR 50.73(a)(2)(i)(B) as a condition prohibited by Technical Specifications.
When the service loop, with the Radwaste portion inservice, is transferred to the opposite CCW train, both trains are aligned to the service loop for a short period of time. During this short period of time, both trains of CCW are inoperable. Additionally, when one train of CCW is out of service and the other train is aligned to the service loop, with the Radwaste portion inservice, both trains are inoperable. This condition is reportable per 10 CFR 50.73(a)(2)(v)(B) and (D) as a safety system functional failure.
This condition is considered an unanalyzed condition that significantly degraded plant safety and is reportable per 10 CFR 50.73(a)(2)(ii)(B).
ROOT CAUSE
This is a legacy issue that has existed since plant startup. The original calculation performed to support the Component Cooling Water System Pipe Break had nonconservative assumptions.
CORRECTIVE ACTIONS
A temporary procedure, TMP 10-028, "Isolating CCW to Radwaste," is in place to provide instructions for isolating and restoring CCW flow to Radwaste. The temporary procedure is an interim action to ensure technical specification compliance.
Long-term resolution of this issue is the installation of a design modification. The design modification separates the non-seismic portion of the common service loop to Radwaste and safety related portions of the CCW system. This separation eliminates the effect of a pipe break in the non-seismic portion of the system on the safety related portion. With no effect on the safety related portion during a pipe break, the CCW system remains operable. The design modification is scheduled for Refueling Outage 19 (Fall 2012).
SAFETY SIGNIFICANCE
The CCW System provides a heat sink for the removal of process and operating heat from safety related components during a Design Basis Accident (DBA) or transient. During normal operation, the CCW System also provides this function for various nonessential components, as well as the spent fuel storage pool. The CCW System serves as a barrier to the release of radioactive byproducts between potentially radioactive systems and the Essential Service Water System, and thus to the environment.
The safety significance for this issue is low. When one train of CCW is aligned to Radwaste, the likelihood is low for an event that would cause a break in Radwaste that would make the CCW train inoperable. In this case, a second train of CCW is available and operable.
Also, during the short period of time when both trains of CCW are aligned to Radwaste, the likelihood is low for an event that would cause a break in Radwaste.
OPERATING EXPERIENCE/PREVIOUS EVENTS
None