05000395/LER-2013-002
| V. C. Summer Nuclear Station, Unit 1 | |
| Event date: | 10-31-2012 |
|---|---|
| Report date: | 08-28-2014 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(i) |
| 3952013002R01 - NRC Website | |
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V. C. SUMMER NUCLEAR STATION, UNIT 1 SEQUENTIAL REV 2.0 EVENT DESCRIPTION On October 31, 2012, during VCS Refueling Outage (RF) 20, surveillance test procedure (STP130.005M) was performed to verify the Component Cooling (CC) System emergency makeup valve (XVG09627B-CC) would stroke open on demand. The valve was actuated to the open position from the Main Control Board (MCB) but the valve remained closed as documented under surveillance test task sheet STTS 1113262-001. The valve was procedurally declared inoperable and action taken per Technical Specifications 4.0.5, 3.7.3, and 3.7.4. Operations dispatched personnel to investigate and manipulate the valve. The valve was retested successfully under STTS 1113262-002. The valve is tested on a RO) (every refueling outage) frequency when the systems are shutdown to prevent contaminating the systems.
Due to the failure of XVG09627B-CC to stroke open on October 31, 2012, VCSNS performed STP130.005M during a mid-cycle outage on March 30, 2013 to increase confidence in the valve's reliability. The valve tested satisfactory during the stroke test.
On April 14, 2014, during the performance of surveillance test procedure STP130.005M in VCS RF 21, XVG09627B-CC failed to stroke open. This failure is documented in LER 2014-001-00.
3.0 EVENT ANALYSIS In the event of a large CC system leak or a loss of the normal makeup capability of the Demineralized Water System, each CC System train has a service water emergency makeup supply valve. Detection of excessive CC system leakage is monitored by pressure changes, flow rate changes, increase in the frequency and/or duration of surge tank water makeup cycles, or visual inspection of the system. The emergency makeup supply valves are designed to open automatically at one foot below the Low-Low surge tank level alarm setpoint to supply makeup water to the affected loop.
XVG09627B-CC is a normally closed, fail open, energize to open, four (4) inch air operated gate valve located between the SW supply lines and the CC system. The valve has an air accumulator to maintain the valve closed for approximately three (3) hours on a loss of instrument air. The valve fails open on loss of accumulator air, but fails close on loss of control signal, if sufficient accumulator air is available. Valve XVG09627B-CC must open in 10 seconds or less to maintain required CC Pump Net Positive Suction Head (NPSH) and to provide makeup flow for a system break or through wall crack.
4.0 SAFETY SIGNIFICANCE The significance of the XVG09627B-CC failure was determined to be low. Small leaks in the CC system are addressed by normal system makeup. Large leaks are low probability occurrences. Conservatively, the Individual Plant Examination (IPE) Initiating Event failure rate for large leaks in the CC system is 9.9E-04 failures per year.
The Conditional Core Damage Probability (CCDP) is conservatively approximated by solving the CAFTA model with one train of CCW out of service (A-train Heat Exchanger failure rate set to 1). The resulting value is 2.26E-05. This is a conservative approximation, and is used as a simplified, bounding condition for the XVG09627B-CC failure.
The change in Core Damage Frequency (CDF) is determined by multiplying the Initiating Event Failure rate by the CCDP and subtracting zero (which conservatively assumes the baseline risk is zero). Using this method, the Delta-CDF for the failure configuration was determined to be 2.24E-08/yr. Since this is less than1E-06/yr, the failure of XVG09627B-CC is considered low significance. This result is consistent with the IPE decision to screen the valve from the model.
During RF 21, while XVG09627B-CC was unavailable, it was discovered that XVG09627A-CC was also inoperable. The failures of XVG09627B-CC and XVG09627A-CC are documented in LER 2014-001-00 and LER 2014-003-00 respectively.
5.0 PREVIOUS OCCURRENCE There had been no previous occurrences in the three years prior to October 31, 2012.
6.0 CORRECTIVE ACTIONS The initial corrective action, due to the October 31, 2012 failure, was to manually adjust the valve then retest in accordance with the surveillance test procedure. The valve passed the surveillance requirements and was declared operable. The valve was subsequently stroke tested three additional times during the outage to flush the seat in support of troubleshooting efforts to determine if the valve was the source of in-leakage to the CC system. The redundant train component was also successfully tested during the outage.
With the successive failure of XVG09627B-CC on April 14, 2014, the valve was disassembled, inspected and rebuilt using a new closure spring, trip spring, stem bushing, throttle assembly, pilot valve, stem, and body to bonnet gasket. The graphite packing was replaced with Teflon packing. All points on the valve linkage were lubricated. Valve supports were adjusted to enhance the alignment of the operator. The pressure regulator closing force was also reduced.