05000395/LER-2013-004
| Virgil C. Summer Nuclear Station Unit 1 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(i)(C), 50.54(x) TS Deviation |
| 3952013004R00 - NRC Website | |
I. Event Description
In November 2012, during RF-20, XVR03026-SP was removed from the system for surveillance testing. XVR03026 is a three quarter inch by one inch Model JMB Type E relief valve by Crosby Valve and Gage Company. This valve conforms to the ASME B&PV Code,Section III, Summer 1974 Addendum, Code Class 2. Testing XVR03026-SP was performed in accordance with Surveillance Test Procedure (STP) STP-401.003, ASME Code Class II and III Relief Valve Testing. The relief valve did not lift within the required setpoint of 75 pounds per square inch gauge (psig) +1- 3 percent or prior to the maximum allowable test pressure of 82.5 psig. Testing was continued to a maximum pressure of 113 psig, as described in STP-401.003, but the valve failed to lift at this elevated pressure.
The relief valve is connected to the body of XVG03004B-SP in order to prevent pressure locking due to fluid that might become trapped in the bonnet after the valve is closed. Therefore, XVR03026-SP serves as a thermal relief valve for the bonnet/body of XVG03004B-SP. Failure of XVR03026-SP to lift and relieve pressure could potentially prevent XVG03004B-SP from opening under accident scenarios.
The apparent cause of this event was bonding of the valve disc and nozzle seats due to normal aging and slight internal misalignment. VCSNS did find some corrosion and wear on the valve seats, which required lapping to restore the required finish. The valve seats indicated that the most likely cause of the high lift was seat bonding due to the 6 year service interval.
The valve was tested every fourth refueling outage. For a corrective action, the PM frequency was revised from every fourth refueling outage to every other refueling outage.
II. Event Analysis
Post accident Reactor Building heat removal is provided by two systems acting in concert as discussed within the FSAR section 6.2.2. One system is the Reactor Building Spray System and the other is the Reactor Building Cooling System. The Reactor Building Spray System removes iodine, which may be released from the core following a loss of coolant accident (LOCA). The Reactor Building Cooling System removes heat from the Reactor Building during normal operation. During normal plant operation, the Spray System is in standby condition with the suction aligned to receive water from the Refueling Water Storage Tank (RWST). Operation of the system is initiated automatically following a LOCA or main steam line break by signals from the Engineered Safety Features Actuation System when the Reactor Building pressure increases to the actuation set point. When the RWST reaches the low-low level setpoint, the reactor building spray pump sump suction valves automatically open to initiate the swap over from the injection mode to the recirculation mode.
XVG03004B-SP is one of the sump isolation valves for the "B" train SP system.
The failure of XVR03026-SP to relieve pressure could potentially prevent XVG03004B-SP from opening and require the "B" train SP system to be manually secured during the recirculation phase.
III. Safety Significance
This condition had no notable safety consequences that were determined to impact the public or the plant. Taking the "B" train spray system out of service at the same time as "B" train Reactor Building Cooling Unit (RBCU) results in a very small change in risk as calculated by the Equipment Out of Service program. The delta Core Damage Frequency is 3.9E-07. The delta Large Early Release Frequency (LERF) is essentially zero (0.7% increase). The "B" train spray system and "B" train RBCU could both remain out of service for the entire year with only a very small increase in LERF.
IV. Corrective Actions
The valve was refurbished in RF-20. This activity required internal inspections, cleaning, and adjustment. Subsequently the valve was successfully tested three times prior to the relief valve being returned-to-service. As a result of this failed test, VCSNS revised the valve test frequency from every fourth refueling outage to every other refueling outage.
V. Previous Occurrences
No previous occurrences to this event have been identified.