05000282/LER-2006-002
| Docket Number | |
| Event date: | |
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| Report date: | |
| 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 |
| 2822006002R00 - NRC Website | |
EVENT DESCRIPTION
Prairie Island Nuclear Generating Plant (PINGP) Unit 1 conducted Mode changes during plant heatup following refueling outage 1R24 with 11 Turbine-Driven' Auxiliary Feedwater2 (AFW) Pump3 inoperable.
On June 1, 2006 at 0430 CDT PINGP Unit 1 transitioned to Mode 3. PINGP Unit 1 subsequently transitioned to Mode 2 on June 5, 2006 at 1414 CDT and Mode 1 on June 6, 2006 at 0913 CDT. At approximately 1811 CDT on June 6, 2006, the 11 Turbine-Driven AFW Pump was shut down during performance of a flow test due to turbine outboard bearing4 temperature exceeding the limit in a surveillance procedure.
Initial investigation via oil samples taken from both bearing sumps found bearing material in the oil reservoir for the inboard bearing indicating bearing damage. Some bearing material was found in the outboard bearing sump sample also indicating bearing wear or damage. Upon disassembly, the inboard bearing was found to be damaged. The outboard bearing had minor indications of wear. The 11 Turbine-Driven AFW Pump was reassembled with new inboard and outboard bearings, tested, and declared operable on June 8, 2006 at approximately 1940 CDT. Additional clearance checks during installation found minimum clearances were not adequate on the inboard bearing requiring manual machining of the bearing to obtain adequate clearance.
Based on the ongoing engineering evaluation, the 11 Turbine-Driven AFW Pump has currently been declared inoperable from the time the inboard bearing was installed without the manual machining (prior to Unit 1 entering Mode 3). Upon completion, the engineering evaluation will validate this determination.
EVENT ANALYSIS
Technical Specification Limiting Condition for Operation (LCO) 3.7.5, AFW System, is applicable to the 11 Turbine-Driven AFW pump when the plant is in Mode 3, Mode 2, or Mode 1. Because the plant entered Mode 3, Mode 2, and Mode 1 with 11 Turbine-Driven AFW Pump inoperable and Technical Specification 3.0.4.b is not applicable to LCO 3.7.5, the plant conducted mode changes prohibited by Technical Specifications and is reportable per 10 CFR 50.73.(a)(2)(i)(B).
1 EIIS Component Identifier: TRB 2 EIIS System Code: BA 3 EIIS Component Identifier: P 4 EIIS Component Identifier: 38 FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) 06 02 Impact on Safety System Functional Failure Performance Indicator The redundant motor-driven 12 AFW pump was operable during the time the 11 AFW pump was inoperable. Therefore, this event does not represent a loss of safety function. Consequently, this event is not reportable per 10CFR 50.73(a)(2)(v).
SAFETY SIGNIFICANCE
This event did not result in a loss of the AFW function, since the 12 AFW pump remained operable.
This event did not affect the health and safety of the public. Therefore, the safety significance of this event is considered minimal.
CAUSE
The equipment root cause for the damaged inboard bearing was determined to be failure to manually machine the bearing to compensate for an apparent internal misalignment of the turbine bearings.
Additional clearance checks performed during bearing replacement efforts revealed minimum clearances were not adequate. The effect of the insufficient clearances was overheating of the bearing which led to the bearing being damaged.
CORRECTIVE ACTION
The 11 AFW pump was reassembled with new inboard and outboard bearings, tested, and declared operable.
The bearing installation activity will be strengthened by revising the Preventative Maintenance (PM) procedure to include instructional detail and acceptance criteria. This action will be completed prior to the next scheduled pump/turbine PM or included in the work instructions until the PM update is completed. There are also actions to correct contributing causes involving equipment deficiencies or enhancements. This includes calibrating or replacing temperature elements and upgrading turbine insulation by the end of 2006, and investigation of internal turbine misalignment at the next opportunity.
PREVIOUS SIMILAR EVENTS
Review of Licensee Event Reports for Unit 1 and Unit 2 since 2003 found no previous similar events.