05000395/LER-2014-001
| V. C. Summer Nuclear Station, Unit 1 | |
| Event date: | 04-14-2014 |
|---|---|
| Report date: | 06-12-2014 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| 3952014001R00 - NRC Website | |
Reported lessons learned are incorporated into the licensing process and fed back to industry.
Send comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by intemet e-mail to Infocollects.Resource@nrc.gov, and to the Desk Officer, Office of Information and Regulatory Affairs, NEOB-10202, (3150-0104), Office of Management and Budget, Washington, DC 20503. If a means used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
V. C. SUMMER NUCLEAR STATION, UNIT 1 SEQUENTIAL REV 2.0 EVENT DESCRIPTION On April 14, 2014, during VCS Refueling Outage (RF) 21, surveillance test procedure STP130.005M was performed to verify the Component Cooling (CC) System [CC] emergency makeup valve (XVG09627B-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 1314561-001. 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.
Due to failure of XVG09627B-CC to stroke open during RF20 (LER 2013-002-00), the valve was stroke tested during a mid-cycle outage on March 30, 2013 to increase confidence in the valve's reliability.
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: Excessive CC systeni 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.
XVG09627B-CC is a normally closed, fail open, energize to open, four (4) inch air operated valve (AOV) located between the service water (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 closed 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.
One side of the XVG09627B-CC valve is located in a raw water (SW) environment. The operator is an AOV 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 build up on the disc/seat face and bond the two surfaces together. This corrosion requires more force 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 stern of the valve. Greater force is required to remove the disc from the seat due to this added frictional force.
The valve operator is large in comparison to the valve body. Seismic restraints are used to keep the operator in an acceptable condition during seismic events. If there is any misalignment in the operator, the valve stem and the operator stanchions may be skewed which adds an additional frictional force on the bushings and packing. More 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 valve's 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 XVG09627B-CC is rendered inoperable when XVG09627B-CC is inoperable. The function of XVG09627B-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 XVG09627B-CC, it was discovered that XVG09627A-CC was also inoperable (a separate LER will be submitted documenting 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 thi-esholds for safety significance (1E-06/yr and 1E-07/yr, respectively). Therefore, the event of both valves being inoperable is of low safety significance.
5.0 PREVIOUS OCCURRENCE XVG09627B-CC failed in the same manner on October 31, 2012. While performing surveillance testing during RF20 on
- XVG09627B-CC, the valve failed to stroke open. This event was reported to the NRC by LER 2013-002-00.
6.0 CORRECTIVE ACTIONS The valve was disassembled, inspected and rebuilt using a new closure spring, trip spring, stern bushing, throttle assembly, pilot valve, stern, 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.
The valve was stroke tested satisfactorily after being rebuilt.