05000277/LER-2006-001
| Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber No. 05000 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(ii)(A), Seriously Degraded 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability |
| 2772006001R00 - NRC Website | |
Unit Conditions at the Time of Discovery Unit 2 was shutdown and in Mode 5 when the event was discovered. Unit 2 had been recently shutdown on 9/15/06 for the P2R16 Refueling Outage. Prior to the shutdown, Unit 2 had been operating for the previous 367 days. There were no other structures, systems or components out of service that contributed to this event.
Description of the Event
On 9/22/06, Engineering Department personnel determined that there were multiple leak rate test failures involving the Main Steam Isolation Valves (MSIVs)(EIIS:ISV). This determination was based on local leak rate testing that had been performed over the time period of 9/18/06 to 9/21/06 during the P2R16 Refueling Outage.
Unit 2 has four Main Steam Lines with each line having one inboard and one outboard MSIV. Four of eight MSIVs were found to be leaking in excess of their allowable leakage limits including both the inboard and the outboard MSIVs for the D Main Steam (EIIS: SB) Line. The following MSIVs did not pass their local leak rate tests:
MSIV Inboard / Steam Line Technical Specification Leak As-found Leak Rate Outboard Rate Limit 86A Outboard A 86C Outboard C 80D Inboard D 86D Outboard D * This value does not include high as-found leakage through the valve packing.
At the time of discovery, the MSIVs were not required to be operable. The MSIVs were repaired during the P2R16 refueling outage and restored to within allowable leak rate values.
� The valves were returned to an operable status in support of the Unit startup on 10/6/06 for Cycle 17 operations.
This report is being submitted pursuant to the following reporting requirements:
10CFR 50.73(a)(2)(i)(B) — Condition Prohibited by Technical Specifications — Because there were mulitple leak test failures identified with the MSIVs, there is an indication that the discrepancies occurred over a period of time.� Therefore, in accordance with NUREG-1022, this event is considered as reportable as a condition prohibited by Technical Specifications.
IOCFR 50.73(a)(2)(ii)(A) — Principal Safety Barrier Degraded — Because the 80D and 86D MSIVs both had leakage that exceeded Technical Specification allowable leakage limits, the D MSIV line had a minimum pathway leakage that exceeded 11.5 scfh. Therefore, the D Main Steam Line isolation barrier was degraded.
Description of the Event, continued IOCFR 50.73(a)(2)(vii) — Single Cause of Independent Trains Being Inoperable - Because there were mulitple leak test failures identified with the MSIVs, there is a condition where a single cause resulted in multiple independent trains of a safety system to be inoperable.
Analysis of the Event
Unit 2 has four Main Steam Lines with each line having one inboard and one outboard MSIV. The safety objective of the MSIVs is to close automatically to:
1. Prevent damage to the fuel barrier by limiting the loss of reactor coolant in case of a major leak from the steam piping outside the primary containment.
2. Limit the release of radioactive materials by closing the nuclear system process barrier in case of gross release of radioactive materials from the reactor fuel to the reactor cooling water and steam.
3. Limit the release of radioactive materials by closing the primary containment barrier in case of a major leak from the nuclear system inside the primary containment.
The redundant MSIVs for the A, and C Main Steam Lines and both MSIVs for the B Main Steam Line met their as-found leakage limits and were therefore, capable of isolating the associated Main Steam Lines to limit the release of radioactive materials. Since both MSIVs associated with the D Main Steam Line exceeded leakage limits, the Technical Specification limit of Steam Line during a design basis event. The minimum pathway leakage rate for the D Main Steam Line would be the leak rate through the 80D MSIV (17.2 scfh).
The safety basis for the assumed MSIV leak rate is discussed in the bases for Technical Specification 3.6.1.3, Primary Containment Isolation Valves (PCIV s). Technical Specification 3.6.1.3 requires that leakage through each MSIV must be Bases, the Plant Safety Analyses (UFSAR Section 14) assume that all four Main Steam Lines leak at the Technical Specification limit of result in a total assumed leakage of 46.0 scfh. As a result of stroke testing of the MSIVs during P2R16, the MSIVs would have closed during a design basis event. If the minimum pathway leakage for each Main Steam Line is added together, then the total leakage through the MSIVs would not have exceeded the total of 46.0 scfh assumed in the design basis event analyses. Therefore, the overall safety objective of isolation of the main steam lines would have been met.
Analysis of the Event, continued The leakage deficiencies of the MSIVs did not affect their ability to close within their required closure time.
Other design basis events assumptions involving closure times were not affected.
In accordance with NUREG-1765, Basis Document for large early Release Frequency (LERF) Significance Determination Process (SDP), excessive leakage of MSIVs that would be significant for LERF consideration is defined as leak rates greater than 10,000 scfh. The total as-found minimum pathway leakage was substantially less than 10,000 scfh. Therefore, this event was determined to be not significant from a risk perspective.
Cause of the Event
The proximate cause of the leakage through the MSIVs was due to the MSIV main poppets not concentrically seating on the in-body MSIV main seating surfaces. In addition, the 86A MSIV had as-found packing leakage of sufficient magnitude that prevented pressurizing the test volume to the target value.
A root cause investigation is being performed for this occurrence. This investigation has identified contributing factors that, in combination, resulted in the multiple MSIV leakage deficiencies. The contributing factors include the outboard MSIVs not being modified with the enhanced 'nose-guided' poppets and not maintaining precise in-body design tolerances. The root cause investigation will be finalized in accordance with the Corrective Action Program.
Corrective Actions
The MSIVs were repaired and returned to an operable status. The as-left leakage rates were well below the Technical Specification allowable limits.
Additional corrective actions to resolve the causes identified by the root cause investigation including extent of condition evaluations are being evaluated in accordance with the Corrective Action Program.
Previous Similar Occurrences There were no previous similar events identified involving multiple, concurrent MSIV leakages that exceeded Technical Specification allowable values.