05000387/LER-2012-006
| Susquehanna Steam Electric Station Unit 1 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 3872012006R01 - NRC Website | |
On April 9, 2012, during local leak rate testing (LLRT) of Main Steam Line Isolation Valve (MSIV) [ENS Code: SB] Penetration Number X-7D, the Unit 1 "D" Main Steam Line Boundary would not pressurize to 25 psig due to excessive leakage. As a result of the testing and subsequent troubleshooting, Susquehanna discovered that the leak rate for the Unit 1 "D" Outboard Isolation Valve exceeded 150 standard liters per minute (SLM). This condition, which was considered to have existed during the previous operating cycle, was a failure to meet Technical Specification 3.6.1.3. As a result, this condition is being reported in accordance with 10 CFR 50.73(a)(2)(i)(B) as a condition prohibited by Technical Specifications.
The sequence of events leading up to the LLRT failure is as follows:
April 9, 2012 at 0200 - the volume between the "D" MSIVs would not pressurize to 25 psig during testing.
April 9, 2012 at 1000 - based on troubleshooting, the leakage on the inboard side of all of the MSIVs, which even though this volume has multiple leakage paths including the four MSIVs, MSL drain valves, HPCI and RCIC steam supplies, etc, was less than 20 SLM.
April 9, 2012 at 1146 - the leakage through the OB MSIV was determined to be greater than 150 SLM which violates the Technical Specification limit of 47.187 SLM.
May 19, 2012 - an apparent cause analysis was completed for the event. This is considered the date of discovery.
CAUSE OF THE EVENT
The direct cause of the component failure was:
Excessive leakage of the "D" OB MSIV due to imperfections in the MSIV seating surfaces caused by less than adequate maintenance done on the in-body seat and the pilot poppet seats.
The cause of the event was determined to be as follows:
The MSIV rework procedure has vague or inadequate guidance on how to properly rework the MSIVs. The procedure has vague steps and human performance traps that, unless the performer is highly experienced, would result in improper machining/boring bar set up and improper valve inspection methodology.
Following initial analysis of the events and submittal of the LER, a root cause analysis was performed to analyze the failure to properly evaluate the testing failures. This analysis identified the following root causes:
Station Engineering management and supervision had not exercised Technical Authority over the Appendix J Program to prevent MSIV LLRT failures.
The Leak Rate Test Program procedure did not establish "maintenance limits" for As-Found and As-Left MSIV LLRT test results for each valve.
The station reportability procedure inappropriately stated that exceeding the MSIV 100 scfh leak rate limit is not reportable.
I I The actual safety consequence of the "D" inboard MSIV LLRT failures is minimal. The MSIVs have a safety function to close to prevent a large release of radiation to the site boundary under accident conditions. As-Found and As-Left LLRTs of "D" Main Steam Line Penetration X-7D demonstrated that "D" inboard MSIV penetration would have performed satisfactorily to prevent the release of radioactive materials through penetration X-7D.
Potential Consequences:
The potential consequence is that, in the event of a LOCA combined with a failure of the "D" Inboard MSIV to isolate, the "D" Outboard MSIV would not have isolated Penetration X-7D sufficiently to prevent fission products from being released. This potential consequence involves multiple safety system failures both to cause the LOCA and then to create the release path.
CORRECTIVE ACTIONS
Key corrective actions:
1. The Conduct of Station Engineering procedure was revised to include the Technical Conscience obligation statement from INPO 10-005, Principles for Maintaining an Effective Technical Conscience.
2. The Leakage Rate Testing Program procedure has been revised to establish administrative maintenance limits for as-found and as-left MSIV LLRT results for each valve. These administrative maintenance limits identify when as-found corrective maintenance needs to be performed as well as what the as-left limit must be achieved prior to returning the valve back to service after maintenance.
3. The station reportability procedure was revised to provide correct guidance associated with reportability of MSIV leakage.
4. Engineering benchmarked the Susquehanna MSIV rework procedure against rework procedures from other plants and developed recommendations for revision of the Susquehanna procedure.
5. The Susquehanna MSIV rework procedure was revised to include the Engineering recommendations.
6. PPL evaluated replacing, instead of reworking, the poppet assembly for MSIVs that have failed their As-Found LLRT and determined that replacement of the poppet assembly is unnecessary on a generic basis.
PREVIOUS SIMILAR EVENTS
A cause of this LER was a less than adequate procedure. The following recent LERs identified similar causes:
instructions as one of the causes.
unclear procedure requirements and less than adequate reinforcement of management expectations for work package content as one of the causes.
test procedures that did not test all aspects of the system that were relied upon for operability.
ANALYSIS/SAFETY SIGNIFICANCE
Actual Consequences: