05000461/LER-2007-001
| Clinton Power Station | |
| Event date: | 02-07-2007 |
|---|---|
| Report date: | 08-20-0716 |
| 4612007001R01 - NRC Website | |
DESCRIPTION OF EVENT
During an NRC inspection conducted in November 2005, inspectors reviewed calculation IP-M-0384, "Evaluation of Vortex in the RCIC (Water) Storage Tank", Revision 0 (performed in 1994) and Revision 1 (performed in 1998). The original purpose of calculation IP-M-0384 was to determine the analytical level (i.e., elevation of water) where vortexing would occur above the High Pressure Core Spray (HPCS) and Reactor Core Isolation Cooling (RCIC) pumps suction lines. The analytical level was then used to calculate the automatic RCIC water storage tank to suppression pool low level suction transfer setpoint for the HPCS pump. The calculations did not adequately account for where air entrainment into the HPCS suction line could potentially occur. An operability evaluation documented the low margin condition, and on December 1, 2005, Clinton Power Station (CPS) shifted the HPCS suction source from the RCIC storage tank to the suppression pool, as allowed by Technical Specifications. In December 2005, CPS issued another revision to calculation IP-M-0384 to demonstrate that the HPCS system was operable using a different methodology that had been accepted at another utility to address vortexing concerns. In addition, an analysis was completed to evaluate the potential introduction and transport of air in the HPCS suction piping. On August 12, 2006, the HPCS suction piping in the RCIC water storage tank was modified by installing downward-turned elbows to increase submergence height to avoid vortexing.
On February 7, 2007, the NRC issued a White Finding and Notice of Violation, related to the failure to select an appropriate method for calculating the minimum elevation (i.e., the analytical level) of water above the HPCS pump suction line in the RCIC storage tank to preclude vortex formation and subsequent air entrainment in the pump's suction. Since the NRC had concerns with the methodologies used in the determination of vortex formation and air entrainment, the NRC concluded that CPS had not adequately demonstrated that the HPCS system would be capable of performing its safety function during the suction swap over from the RCIC water storage tank to the suppression pool.
This event is reportable under 10CFR50.73 (a)(2)(v)(D) as a condition that could have prevented fulfillment of the HPCS safety function.
SAFETY ANALYSIS
There were no actual safety consequences. However, since this was an original design issue and until the HPCS system was aligned to the suppression pool in December 2005 and modified in August 2006 to eliminate the concern regarding vortexing and air entrainment, the HPCS system may not have been capable of performing its safety function. Based on a Phase 3 Significance Determination Process evaluation, this condition was determined to have a low to moderate safety consequence.
CAUSE OF EVENT
A root cause evaluation determined that the cause of the event was the failure to adequately evaluate the uncertainties and associated margins in the calculation used to determine the suction transfer point.
CORRECTIVE ACTION
In December 2005, when it was determined that the margin was low to preclude vortexing with the configuration that existed with the suction source for the HPCS system aligned to the RCIC storage tank, the suction was transferred from the RCIC storage tank to the suppression pool. A plant modification was installed in August 2006 to increase the submergence of the suction piping in the RCIC water storage tank to preclude possible vortex formation and air entrainment.
The following additional corrective actions were identified:
1. Design basis documents for the HPCS system have been updated to include the need to analyze vortexing for the tank low-level setpoint; 2. The calculation developed for the plant modification to increase the submergence of the HPCS piping has been revised to document the uncertainty with respect to margin with regard to vortexing and air entrainment; 3. A sampling of design basis calculations will be reviewed for extent of condition; 4. Other safety-related tank vortex calculations have been reviewed, evaluated and revised as necessary; and, 5. Configuration Change (CC) procedures (CC-AA-309, "Control of Design Analyses" and CC-AA-309- 101, "Engineering Standard Use and Adherence") have been revised to ensure that the calculated available margin is adequate to bound the combination of uncertainties associated with both the analysis methodology and the actual calculation.
PREVIOUS SIMILAR EVENTS
None.
COMPONENT FAILURE DATA
None.