05000395/LER-2014-003
| V. C. Summer Nuclear Station, Unit 1 | |
| Event date: | 04-26-2014 |
|---|---|
| Report date: | 06-24-2014 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v), Loss of Safety Function 10 CFR 50.73(a)(2)(i) |
| 3952014003R00 - NRC Website | |
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V. C. SUMMER NUCLEAR STATION, UNIT 1 SEQUENTIAL REV 2.0 EVENT DESCRIPTION On April 26, 2014, during VCS Refueling Outage (RF) 21, surveillance test procedure STP130.005L was performed to verify the Component Cooling (CC) System [CC] emergency makeup valve (XVG09627A-CC) [V] 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 in surveillance test task sheet STTS 1301142-001. An immediate retest resulted in the valve opening outside of the maximum allowed stroke time of ten (10) seconds. The valve was declared inoperable and action taken per Technical Specifications 4.0.5, 3.7.3, and 3.7.4. In the event the valve did not open in an actual event, an Operator would be dispatched to manually adjust the valve off of its seat.
The valve is tested on a ROI (every refueling outage) frequency when the systems are shutdown to prevent cross-contaminating the systems.
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 (SW) emergency makeup supply valve. Excessive CC system leakage could be detected 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.
XVG09627A-CC is a normally closed, fail open, energize to open, four inch air operated valve (AOV) located between the SW supply lines and the CC system. The valve has an air accumulator to maintain the valve closed for approximately three hours on a loss of instrument air. The valve fails open on loss of accumulator air, but fails closed on loss of control signal, if sufficient accumulator air is available. Valve XVG09627A-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.
One side of the XVG09627A-CC valve is located in a raw water (SW) environment. The operator uses air to close which allows the gate to be firmly thrust into the seat so there is no cross contamination between the SW and CC systems. The valve is only cycled during refueling outages to maintain the isolation of the SW system from the CC system to ensure there is no cross-contamination.
In the 18 months between refueling outages, corrosion products and sediment could build-up on the seat and disc face. With this added layer of corrosion product and sediment, there is an increased amount of force required to remove the disc from the seat. The packing used in the valve was an older style of graphite packing that can place a higher frictional force on the stem of the valve. Greater force is required to remove the disc from the seat due to this added frictional force.
Additionally, the valve is a spring open valve. It uses four springs to remove the valve from the seat. Since the valves safety function is to fail open, the springs are normally in a compressed state. When springs are in a fully compressed state for long periods of time they begin to set due to the grain structure realignment. Spring set effectively reduces the overall energy (opening force) stored in the spring.
4.0 SAFETY SIGNIFICANCE When the plant is in Modes 1-4, the train of CC associated with XVG09627A-CC is rendered inoperable when XVG09627A-CC is inoperable. The function of XVG09627A-CC to supply makeup inventory to the CC system is not risk significant and is not monitored in Mitigating Systems Performance Index (MSPI) basis.
During the period of unavailability of XVG09627A-CC, it was discovered that XVG09627B-CC was also inoperable (LER 2014-001-00 documents this failure). The failed stroke test of both XVG09627A-CC and XVG09627B-CC implies that both valves were inoperable during this period of time. A PRA risk evaluation was performed to determine the change in risk with both valves being out of service simultaneously. With both valves out of service simultaneously, the resulting change in risk is as follows:
The delta CDF is: 3.16E-08/yr The delta LERF is: 2.70E-10/yr The values for increased risk are well below the thresholds for safety significance (1E-06/yr and 1E-07/yr, respectively). Therefore, the event of both valves being inoperable is of low safety significance.
Reporting requirement 10CFR50.73(a)(2)(v) was made in LER 2014-001-00 for both XVG09627A-CC and XVG09627B-CC being inoperable at the same time.
5.0 PREVIOUS OCCURRENCE .
- There have been no previous occurrences of XVG09627A-CC failing to stroke open in the last three years.
However, XVG09627B-CC failed to stroke open when surveillance testing was performed during the fall 2012 outage (RF20) and the spring 2014 outage (RF21). LER 2013-002-00 and LER 2014-001-00 were submitted for these events.
6.0 CORRECTIVE ACTIONS The valve was disassembled, inspected and rebuilt using new closure and trip springs. The graphite packing was replaced with Teflon packing. The air piston actuator assembly was rebuilt. All points on the valve linkage were lubricated. The pressure regulator closing force was also reduced.
The valve was stroke tested satisfactorily after being rebuilt.