05000298/LER-2006-005
| Docket Number | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 2982006005R00 - NRC Website | |
SAFETY SIGNIFICANCE
This condition is not risk significant. The failure of RHR-MOV-25B impacted RHR Loop B ability to provide LPCI and SDC injection to the RPV. The increase in core damage probability resulting from failure of RHR-MOV-25B is 1.5E-07. This increase is characterized as not risk-significant. Qualitatively, the minimal risk increase is justified by the fact that multiple redundant low pressure injection systems and containment heat removal systems were available and not impacted by the RHR-MOV-25B failure.
The failure had no impact to the primary containment isolation function since RHR MOV-25B failed in a closed position. Therefore, the large early release frequency calculated for CNS was not impacted directly by the failure.
CAUSE
The failure of RHR-MOV-25B was a result of a dislocation of the motor pinion gear in the associated Limitorque motor operator. The pinion gear had moved both radially and axially on the motor shaft, resulting in insufficient gear engagement. The set screw was not holding the pinion gear to prevent axial movement. The failure was attributed to improper pinion gear installation.
The improper pinion gear installation was determined to be a result of non-specific procedural guidance governing the pinion gear installation. There was a mismatch between the criticality of the task to install MOV motor pinion gears and the level of detail in the maintenance procedures.
CORRECTIVE ACTION
The following immediate actions were completed.
1. CNS MOV maintenance procedures required for Refueling Outage 23 (RFO-23) governing Limitorque operators prone to pinion gear displacement were revised to include improved directions for securing the motor pinion gear during reassembly. Procedures that have not incorporated the improved motor pinion gear reassembly instructions were placed on administrative hold. Final changes to the procedures within the scope of this action are not yet completed and are being tracked in the CNS corrective action program.
2. The condition associated with the Limitorque operator for RHR-MOV-25B has been corrected.
PLANT STATUS
Cooper Nuclear Station (CNS) was at 92% power in Mode 1 at the time of the event.
BACKGROUND
The safety objective of the Residual Heat Removal (RHR) system [EIIS:BO] is to provide core cooling, in conjunction with other Emergency Core Cooling Systems (ECCS), and to provide containment cooling as required during abnormal operational transients and postulated accidents. The RHR system consists of two heat exchangers [EIIS:HX], four main system pumps [EllS:P] in two divisions, and associated piping, valves, controls and instrumentation. Upon opening the RHR Loop B injection valve (RHR-MOV-25B) [EllS:INV,ISV] a path is provided for low pressure coolant injection (LPCI)[EIIS:B0] and shutdown cooling (SDC) to the reactor pressure vessel (RPV)[EIIS:RPV]. In the normally closed position, this valve provides a primary containment isolation function for the associated line.
EVENT DESCRIPTION
On October 17, 2006 the plant was in Mode 1, in preparation for a refueling outage. At 04:05 CST, during surveillance testing, motor operated valve (MOV) RHR-MOV-25B failed to open when the associated switch [EllS:HS] was placed to open. Immediate action consisted of exiting the surveillance procedure and declaring the subject valve inoperable. The MOV was repaired, tested, and returned to OPERABLE status on October 18, 2006 at 05:18 CST.
BASIS FOR REPORT
This event is reportable under 10CFR50.73(a)(2)(i)(B) as a condition prohibited by Technical Specifications (TS). This is based on determining that injection valve RHR- MOV-25B was incapable of opening for a period of approximately three months. This was a condition prohibited by Technical Specification 3.5.1 where one ECCS train was inoperable for a period greater than seven days. Although other MOV actuators were identified with varying degrees of degradation, only RHR-MOV-25B was determined to be incapable of performing its safety function during the period required by TS. This was not a loss of core cooling or containment safety function because the redundant RHR Loop A remained operable.
3. � MOVs determined to be "high risk" were inspected during RFO-23 for similar problems. Individual corrective actions to ensure continued operability have been completed.
The following actions are being tracked in the CNS corrective action program.
1. Revise CNS procedures associated with Limitorque motor pinion gear inspection and assembly.
2. Perform inspections and repairs (as necessary) of remaining extent-of-condition MOV actuators during Cycle 24 and RFO-24.
3. Establish appropriate Preventive Maintenance Plans to periodically inspect Limitorque motor shaft & pinion gear interface.
PREVIOUS EVENTS
There have been no recent reportable events related to MOV failures related to maintenance.