05000219/LER-2006-003
| Docket Number10 16 2006' 2006 - 003 - 00 2, 13 2006 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 2192006003R00 - NRC Website | |
Description of Event
On Monday, October 16, 2006, with the plant in Cold Shutdown for refueling outage 1R21, the as-found Local Leak Rate Test (LLRT) of Main Steam Isolation Valve (MSIV) NSO3B failed to meet the acceptance criteria of Technical Specification 4.5.D.2. No as-found data could be obtained on this valve due to failure to maintain pressure.
The valve was last refurbished during refueling outage 1R18 in October 2000 and found to be acceptable after maintenance that installed the pilot poppet modification and a new stem. Following maintenance, in 1R18 the valve passed the as-left LLRT. The refurbishment of NSO3B was performed due to the need to install the pilot poppet modification in all four of our MSIVs. During the maintenance performed in 1R18 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 during 1R18 was performed with an acceptable reading of 2.63 scfh at 35 psig, well below the acceptance criteria of 15.957 scfh at 35 psig.
In addition in October of 2002 and 2004 LLRTs were performed with satisfactory results.
On October 16, 2006, a local leak rate test of the 'B' main steam line isolation valves was commenced with the main steam lines drained. As the technicians began to attempt to pressurize the penetration space between the inboard and outboard valves, NSO3B and NSO4B respectively, they found that they were not able to reach a stable pressure level and it appeared that one or both of the valves was leaking by. The test was declared a failure. After flooding the main steam lines another test was attempted on October 17, 2006 to quantify the leakage on the "B" penetration outboard valve. The test could not be performed since water was leaking into the test volume from the main steam lines through the NSO3B valve and the test was terminated as a failure.
Following installation of the main steam line plugs and draining of the steam lines another attempt was made on October 20, 2006 to quantify the leakage on the 'B' main steam line penetration by testing the outboard valve, and this resulted in a successful test. Although the inboard valve was believed to be leaking, an attempt was made to quantify the leakage and this resulted in a successful leak rate value of NSO3B with a value of 2.8 scfh. Note that during the entire sequence of events listed above starting from the initial LLRT on October 16, NSO3B was not stroked and remained in its close/seated position. As part of the process of assessing the condition of the NSO3B strain gauge diagnostic testing was performed on NSO3B to verify the performance of the valve and this test was performed on October 22, 2006 with satisfactory results. Subsequent to Strain Gauge diagnostic testing as-left testing leak rate testing was performed and demonstrated the performance of the valve with a final LLRT; as a result 2.416 scfh at 21.46 psig was obtained between the valves, well below our acceptance criteria of 11.9 scfh at 20 (+3/-0) psig.
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 acceptance criteria, for these valves, is less than or equal to 11.9 scfh at 20 (+3/-0) psig. 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 NSO3B would have been limited by the leak rate of the outboard MSIV (NSO4B) in the same header which met the LLRT acceptance criteria of Technical Specification 4.5.D.2 when tested in 1R21. This leakage provides adequate margin between projected potential offsite dose and 10 CFR 100 guidelines.
An Operational Experience (OE) search was conducted to locate similar events, and only two were found that specifically referred to disc to seat interactions/misalignments. In both of these cases the valves were repaired. However, in these OEs there was no information regarding whether follow-up LLRTs were conducted prior to entering the valve, or what the nature of the repair involved. The assumption is that had a confirmatory LLRT been done prior to entry, with the seriousness of the issues in these cases, those tests would not have been acceptable as was the case at Oyster Creek.
Cause of Event
The apparent cause of the as-found LLRT failure of the Inboard MSIV NSO3B is attributed to an oxide layer build up on the valve poppet seat and in-body seat ring. Oxide layer formation on the seats is a normal occurrence, and will appear again once a valve is returned to service following maintenance. The original design of the valve had a main poppet that did not have a nose guide and these valves had frequent LLRT failures due to the buildup of oxide on the seats. The nose guided poppet modification implemented at Oyster Creek, and throughout the industry, was intended by the Original Equipment Manufacturer (OEM) to minimize the impact of this oxide layer. Even though this modification has been largely successful in the industry it doesn't eliminate completely the formation of the oxide layer. The oxide layer is considered a contributing causal factor that results in the observed variability in MSIV leakage performance. There is no long-term method to permanently remove the oxide layer and prevent it from recurring since it is the result of the materials that are used (stellite) and the environment that they are in (nuclear steam). Historical LLRT data indicates that the existence of the oxide layer is acceptable based on the generally good performance (sometimes passing LLRTs for over 12 years) of the nose-guided poppets that are in service in the industry. Removing the oxide layer provides no long-term solution since it will reform once the valve is back in operation and thereby reintroduce this leakage variability. Placing the MSIV back in service that has acceptable leakage with an established oxide layer is considered an acceptable condition since the existing oxide layer will provide a passive barrier to preclude additional oxide formation.
Corrective Actions:
- Strain Gauge diagnostic testing on NSO3B was performed on October 22, 2006 to confirm total seating force, pilot poppet spring force, backseating force, open and closed packing loads, main seat disc contact profile, pilot poppet disc seat contact profile, main disc guide condition, main disc nose guide bore transition profile and total seating force. This test result was satisfactory.
- Performance of an as-left LLRT following the diagnostic stroke and work on the NSO4B to demonstrate that the leakage on the NSO3B remains acceptable. This test result was satisfactory giving a value of 2.416 scfh at 21.46 psig between the valves, well below our acceptance criteria of 11.9 scfh at 20 (+3/-0) psig.
Additional Information
A. Failed Components:
Main Steam Isolation Valve (MSIV) NSO3B (V-1-008)
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