05000219/LER-2004-006

LER-2004-006,

Docket Number11 05 ' 2004 2004 - , : 006 -'1, 00 0 04 2005
Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications
2192004006R00 - NRC Website

Description of Event

On Friday, November 5, 2004, with the plant in Cold Shutdown for refueling outage 1R20, the as-found Local Leak Rate Test (LLRT) of Main Steam Isolation Valve (MSIV) NSO4A (EIIS SB-ISV) failed to meet the acceptance criteria of Technical Specification 4.5.D.2. The acceptance criteria is less than or equal to 11.9 SCFH at 20 (+3/-0) psig. The actual measured leakage was 24.3 SCFH at 20.6 psig.

The valve was last refurbished during forced outage 1F07 in September 2004 and had an acceptable LLRT after maintenance that included replacing the valve poppet. The refurbishment of the valve was performed due to the valve failing to close during surveillance testing. The failure to close was due to guide rib wear. The valve was refurbished by weld repair of the guide rib. A new poppet was installed at that time. The poppet and seat were blue checked to assure 360° contact between the two components with the blue check indicating complete seat contact. The as-left LLRT was performed with an acceptable reading of 15.965 SCFH at 35 psig.

In the 1R20 refueling outage, dimensions of the valve internals were taken and no changes were noted from the as left conditions identified in 1F07. In 1R20, the main seating surface was lapped eliminating any potential minor seating surface issues that existed. The valve was blue checked and a successful LLRT was performed.

This event is reportable per 10 CFR 50.73(a)(2)(i)(B), any operation or condition which was prohibited by Technical Specifications.

Analysis of Event

The MSIVs are containment isolation valves designed to minimize coolant loss from the vessel, and the resultant offsite dose, in the event of a main steam line break accident. The design basis loss of coolant accident was evaluated at the primary containment maximum allowable accident leak rate of 1.0% per day at an initial pressure of 35 psig that decays to 1.0 psig after 2.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br />. The 1.0 psig is assumed to remain for the next 21.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br />. The exceeding of 10CFR50 Appendix J leakage limits could result in unacceptable dose rates downstream of the MSIVs during an accident.

The safety significance of this event is considered minimal. The leakage past NSO4A would have been limited by the leak rate of the inboard MSIV (NSO3A) in the same header which met the LLRT acceptance criteria of Technical Specification 4.5.D.2 when tested in 1R19. NSO4A leak rate would therefore have been limited to the leak rate past MSIV NSO3A. This leakage provides adequate margin between projected potential offsite dose and 10 CFR 100 guidelines.

The acceptance criteria for MSIV LLRT had been changed between 1F07 and 1R20 by Amendment 250 to the Facility Operating License for Oyster Creek. This change allowed LLRT to be done at 20 (+3/-0) psig versus previous testing which was performed at 35 psig. The new criteria of 11.9 SCFH @ 20 psig is more conservative than the previous criteria adjusted per ASME code calculation.

Analysis of Event (cont'd) A review of industry Operating Experience (OE) for MSIV LLRT revealed numerous cases where there were leak rates significantly in excess of their limits that were unexplained and when the valve seat was lapped successful LLRTs were performed. One reported incident at another BWR plant during 1987 had an original LLRT failure on July 5th and then failed another LLRT on September 6th. The cause of the second failure was determined to be deformation of the lower seating surface that had not been removed by lapping during the prior maintenance outage in July.

Cause of Event

The apparent cause of the as-found LLRT failure was the irregularity of the mating surfaces between the poppet seating surface and the valve body seating surfaces. These minor irregularities may have caused the valve to seat differently when the valve was stroked entering the 1R20 refueling outage causing the LLRT failure. The decision to proceed with valve re-assembly in 1F07 without lapping was made based on an acceptable blue check indicating a complete seat contact.

Corrective Actions:

• The valve was disassembled in 1R20 and the valve body seat was lapped. A satisfactory blue check was achieved and the valve was reassembled.

• The MSIV Overhaul Procedure will be revised to include a documented management review prior to eliminating seat lapping even if a successful blue check has been obtained.

A. Failed Components:

Main Steam Isolation Valve (MSIV) NSO4A (V-1-009)

B. Previous similar events:

C. Identification of components referred to in this Licensee Event Report:

Components IEEE 805 System ID�IEEE 803A Function MSIV SB� ISV