05000395/LER-2015-001
| V.C. Summer Nuclear Station, Unit 1 | |
| Event date: | 3-17-2015 |
|---|---|
| Report date: | 5-12-2015 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 3952015001R00 - NRC Website | |
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EVENT DESCRIPTION
On March 17, 2015 a past operability evaluation determined that XVG03005A-SP, Outside Reactor Containment (ORC) Reactor Building (RB) Spray Sump Isolation Valve A, was inoperable for failure to fully stroke open.
The failure to stroke open occurred on February 4, 2015 at approximately 0548 during performance of STP-105.016A, Train A Charging Pump & Diesel Generator Slave Relay Testing. When the valve was tested in the open direction, the valve position did not indicate full open on the Main Control Board (MCB). Upon local inspection, the valve appeared to be full open. An at-the-valve Motor Operated Valve Analysis and Test System (MOVATS) was performed. No anomalies were found in the MOVATS data. The MOVATS test data was within Design Bases requirements and XVG03005A-SP operated as required in the open and closed directions.
The plant was operating at 100% power at the time of failure.
EVENT ANALYSIS
XVG03005A-SP is a normally closed Limitorque actuated gate valve that performs an active safety function in the open position.
XVG03005A-SP must open to provide a suction path from the Reactor Building sump to XPP0038A, Reactor Building Spray Pump A, on Lo-Lo Refueling Water Storage Tank (RWST) level. XVG03005A-SP receives an automatic open signal due to recirculation sump auto switchover logic.
XVG03004A-SP and XVG03005A-SP are also logically tied to XVG03001A-SP, RB Spray Pump A RWST Suction Header Valve.
The full-open limit switches on XVG03004A-SP and XVG03005A-SP initiates an automatic closure of XVG03001A-SP.
XVG03004A-SP and XVG03005A-SP automatically open on receipt of RWST Lo-Lo Level coincident with a SI signal.
XVG03001A-SP automatically closes when both of the sump isolation valves are full open. This prevents further drain down of the RWST and completes the suction swap over for XPP0038A to its associated RB sump.
The failure of XVG03005A-SP to stroke open was attributed to spring pack relaxation and associated torque switch setting tolerances which resulted in the torque switch opening prior to the valve reaching the 100% open limit switch. This failure mode would allow XVG03005A-SP to open 90%, but stop before reaching the full open limit switch. Analysis has determined that Limitorque valves stroking open to greater than 90% are essentially fully open. XVG03005A-SP would pass sufficient flow at 90% open to the suction of XPP0038A. However, the closure interlock associated with XVG03001A-SP would not be satisfied.
With XVG03005A-SP not indicating full open on the MCB, Operations would have been procedurally directed to place XPP0038A in pull-to-lock during Cold Leg Recirculation swapover.
During the period of inoperability, the "B" train RB Spray System was removed from service for approximately 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> for surveillance testing. This condition could have prevented the fulfillment of a safety function and is being reported under 10CFR50.73(a)(2)(v).
SAFETY SIGNIFICANCE
Station procedure for swapover of RB Spray from injection to recirculation requires Operations to ensure that XVG03004A-SP and XVG03005A-SP are open. If the valves cannot be placed in their required positions, the procedure directs Operations to place XPP0038A in pull-to-lock. The procedure continues by ensuring Charging Pumps are aligned for Cold Leg Recirculation. The impact of the XVG03005A-SP failure to indicate full open would be the loss of the A train of Spray.
Valve XVG03005A-SP is not modeled in the internal events PRA model for CDF or LERF. XVG03005A-SP is modeled in the fire PRA model, for spuriously open, as a contributor to a possible RWST drain down path. The failure of XVG03005A-SP to fully stroke open was not a spurious open failure.
The XVG03001A-SP interloCk is not modeled in the fire or internal events PRA. Risk associated with XVG03001A-SP being open during the swap to sump recirculation was not identified in the development of the PRA model.
Based on the above considerations there is no impact on core damage frequency (CDF) or large early release frequency (LERF) from the failure of XVG03005A-SP to stroke fully open.
PREVIOUS OCCURRENCE
XVG03005A-SP failed to stroke open on December 11, 2014 during performance of STP-I 12.003, RB Spray System Valve Operability Test. The valve position indicated mid-position and the Engineered Safety Feature (ESF) status light on the Main Control Board (MCB) remained DIM. Operations personnel manually stroked the valve full open and the ESF status light went bright. Local observation verified that the valve stroked open smoothly with no abnormal noise or binding. The torque switch contacts were cleaned and the limit switch contacts were inspected. Motor load data was obtained and found to be consistent with historical values.
XVG03005A-SP was satisfactorily retested and declared operable.
The December 11, 2014 failure of XVG03005A-SP was the first failure of this valve to stroke open. The failure was considered to be a random, one time failure with no firm evidence that the failure would have occurred prior to the December 11, 2014 test. There have been no prior occurrences with XVG03005B-SP or XVG03004A/B-SP.
CORRECTIVE ACTIONS
The torque switch setting was adjusted from 1 to 1.5 to allow higher developed torque before the switch opens to trip the motor.
XVG03005A-SP has a manufacturer supplied torque switch limit plate. The torque switch setting of 1.5 will not increase the developed torque beyond the maximum recommended torque values. Successive performances of STP-112.003 have resulted in satisfactory performance from XVG03005A-SP.
The station is considering an adjustment to the XVG03001A/B-SP interlock to 90% open limit switches instead of 100% open limit switches on XVG03004A/B-SP and XVG03005A/B-SP. This would ensure that XVG03001A/B-SP would automatically close if XVG03004A/B-SP and XVG03005A/B-SP reach 90% and stop due to over torque. The station is also considering changing the ESF lights for XVG03004A/B-SP and XVG03005A/B-SP to the 90% open limit switches to provide indication that XVG03004A/B-SP and XVG03005A/B-SP are open and capable of passing adequate flow.