05000458/LER-2014-005
| River Bend Station - Unit 1 | |
| Event date: | 08-01-2014 |
|---|---|
| Report date: | 09-26-2014 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 4582014005R00 - NRC Website | |
REPORTED CONDITION
On August 1, 2014, at approximately 9:42 p.m. CDT, with the plant operating at 100 percent power, the high pressure core spray system (HPCS) (BG) was declared to be inoperable as a result of an engineering evaluation of an apparent leakage path through a part of the system. The evaluation determined that, should the I-IPCS system be initiated in response to a design basis event, the leakage path could potentially cause the suppression pool inventory to be depleted to the extent that the pool would not support the 30-day mission time assumed in the station's accident analysis. Operators closed the HPCS pump suction valve at the suppression pool, resulting in the inoperability of the system. This event is being reported in accordance with 10 CFR 50.73(a)(2)(v) as a condition that defeated the safety function of the the I-IPCS system.
A subsequent evaluation has quantified the adverse effects of the test return line leakage on the ability of the suppression pool to perform its safety function. Had the HPCS system actuated as assumed in the station's accident analysis, the leakage would have depleted the pool water inventory before the completion of its 30-day mission time. (That mission time is addressed in the RBS Updated Safety Analysis Report in terms of the radiological consequences of a loss-of-coolant accident over the course of 30 days.) Given the prevailing conditions, the pool water level would 1.ave decreased to the minimum allowed by the plant design and licensing basis within approximately 18 days, and to the minimum required submergence of the emergency core cooling system pump suction strainers within approximately 21 days. With respect to the suppression pool, this event constituted operations prohibited by Technical Specifications (10 CFR 50.73 (a)(2)(b)), as well as a condition that defeated the safety function of the suppression pool (10 CFR 50.73 (a)(2)(v)).
BACKGROUND
The HPCS system suction is normally aligned to the condensate storage tank (CST). A full-flow test path can be aligned from the pump discharge back to the CST, facilitating the performance testing of the pump. The test return line contains two motor-operated isolation valves (**ISV**).
The CST is not rated for the design basis seismic event, and so is not assumed to be available under accident conditions. Safety- related instrumentation that monitors the CST for low water level automatically initiates a realignment of the HPCS pump suction from the CST to the suppression pool in the primary containment.
INVESTIGATION and IMMEDIATE ACTIONS On July 12, 2014, operators noted an unexpected decrease in suppression pool water level of approximately 2 inches over the previous 24 hour period. Action was taken to restore the pool to normal level. The cause of the trend was not specifically known, but thought to be associated with the installation of a danger tag-out to drain the common suction line from the CST to HPCS and the reactor core isolation cooling system (RCIC) for planned work. An immediate operability evaluation determined that the suppression pool was capable of performing its function because the water level was being maintained in the required range. Between July 14 and 18, actions were taken to identify the source of the leak.
On July 21, the potential implications of leakage past the test return line isolation valves were recognized, and action was initiated to investigate sources of valve seat leakage. Due to the complexity of the system and its interrelation with the CST and the suppression pool, an evaluation was also made of the potential adverse effects on the operability of the HPCS system.
00 2014 - 005 On August 1, it was determined that the leakage through the test return line was a previously unrecognized condition that potentially compromised the 30-day mission time of the suppression pool.
The following are highlights of the troubleshooting effort:
1. The test return valves are included in the In-Service Testing (IST) Program. They have a safety function to close in order to maintain the safety-related flow path boundary. The valves are leak-tested every two years, with a criterion of equal or less than 2 gpm. This is not required to satisfy any specific surveillance requirement in LCO 3.5.1, "ECCS Operating.
2. The IST program requires that the HPCS pump deliver 5010 gpm at a differential pressure of > 415 psid. The design capacity of the I-IPCS line fill pump is 50 gpm at 101 feet of head. During the troubleshooting, these parameters were not recorded, but the most recent test results indicated that both pumps are performing within specifications.
3. The periodic valve leak-rate test has an acceptance criterion of pool. The test return line was found to be leaking 4.47 gpm. This amount had a negligible effect on the ability of HPCS pump to perform its safety function.
With HPCS in the standby configuration and only the line fill pump running. the leakage stabilized at 1.88 gpm. This amount had a negligible effect on the ability of the pump to provide the required flow and pressure to maintain the HPCS system filled and available for injection.
4. The CST is the normal suction source for the HPCS pump. The CST suction isolation valve has a safety-related function to close when the CST inventory is depleted. This function is tested quarterly to ensure that the HPCS system can fulfill its injection requirements. The CST suction valve was suspected to be leaking due to its inability to be drained during hanging of a danger tag-out.
However, this valve does not have a leakage limit and is not tested for seat leakage. The suspected leakage would not impact its ability to perform its safety functions.
IMMEDIATE ACTIONS
On August 5, engineering evaluations concluded that the 1-1PCS system was operable with compensatory measures. The compensatory measures deemed necessary were:
1. Perform a leak rate test of the test return line after any cycling of the isolation valves in order to maintain operability margins for the I-IPCS system. If the leak rate doubled to 9.0 gpm, then a new operability evaluation would be required.
2. Isolate HPCS from the suppression pool by closing and disabling the pool suction valve until action to ensure the suppression pool inventory could be taken; and 3. Install a temporary modification to replace the restricting orifice in the CST test return line with a blind flange to preserve pool inventory.
The blind flange was installed in the CST test return line on August 3, and the HPCS pump was restored to an operable status on August 5.
CORRECTIVE ACTION TO PREVENT RECURRENCE
The failure mechanism of the test return line isolation valves has not been determined. Work orders have been scheduled to disassemble and repair the valves.
AM,
SAFETY SIGNIFICANCE
The leakage rate through the test return line isolation valves has been quantified at 4.47 gpm. This diverted flow is not significant with respect to the ability of the HPCS pump to provide its design rated flow to the reactor vessel in accident conditions.
The challenge to suppression pool inventory due to the valve leakage over a 30-day period would also have negligible effect on risk, using core damage frequency as a metric. The probablistic risk assessment (PRA) mission time is 24 hours, or until the reactor and containment have reached a safe, stable condition. PRA methodology assumes that the emergency response organization provides effective mitigation of component malfunctions. PRA also credits non-safety systems and, on a probabilistic basis using industry data, off-site power recovery. With these realistic considerations, the capability to provide makeup to the suppression pool prior to 30 days could clearly be credited with high confidence.
During the time that the HPCS suppression pool suction valve was closed, the system remained available, but not technically operable, while it was aligned to the CST.
(NOTE: Energy Industry Component Identification codes are annotated as (**XX**).)