05000324/LER-2007-003
| Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber No 05000 | |
| Event date: | 06-26-2007 |
|---|---|
| Report date: | 10-18-2007 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 3242007003R01 - NRC Website | |
Energy Industry Identification System (EIIS) codes are identified in the text as [XX].
Introduction On June 26, 2007, the Brunswick Steam Electric Plant received the results of as-found testing of 11 safety/relief valves (SRVs) [SB/RV]; which had been removed from Unit 2 during the spring 2007, refueling outage (i.e., B218R1). These results indicated that four of the 11 valves actuated at pressures outside of the 3 percent tolerance allowed by Technical Specification 3.4.3, "Safety/Relief Valves.
Event Description
Initial Conditions At the time the condition was identified, Unit 2 was,in Mode 1, at approximately 100 percent of rated thermal power.
� - Discussion -
- During the spring 2007, Unit 2 refueling outagethe II Model 7567F Target Rock Two-Stage pilot valve assemblies were replaced with certified spares. Me. removed SRVs were sent to Wyle Laboratories for set pressure testing. On June 26, 2007, Engineering personnel received the results of the as-found testing which indicated that four of the 11 valves actuated at pressures outside of the 3 percent tolerance allowed by Technical Specification 3.4.3. The test data is provided in the following table.
Technical As-Found SpecificationValve Identification Percent Difference(psig) Setpoint (psig) 2-B21-F013B 1206 1150 ± 34.5 +4.9% 2-B21-F013D 1200 1140 ± 34.2 +5.3% 2-B21-F013F 1171 1130 ± 33.2 +3.6% 2-B21-F013K 1247 1140 ± 34.2 +9.4% Since Technical Specification 3.4.3 requires 10 of the 11 installed SRVs to be operable, this condition is being reported in accordance with 10 CFR 50.73(a)(2)(i)(B) as operation prohibited by the plant's Technical Specifications.
Event Cause The root cause of each SRV is discussed separately due to the individual nature of the findings.
SRVs 2-B21-F013B and 2-B21-F013D SRV 2-B21-F013B Inspections of the 2-B21-F013B pilot valve assembly identified steam cutting on the pilot disc and seat.
This exposed the stellite base material and locally reduced the diameter of the pilot disc which, in turn, allowed the disc to jam into the seat creating a pinch. Also, the exposed stellite base material allowed the formation of a weak oxide bond between the disc and seat. The steam cutting was caused by improper lapping of the pilot disc to the seat that allowed the pilot to leak.
SRV 2-B21-F013D Inspections of the 2-B21-F013D pilot valve assembly identified multiple indications of misalignment between the pilot disc, the pilot rod, and the set spring. When the valve was lapped, a non-uniform seat band was formed as evidenced by ditched seating area-predominately on one side of the disc. This resulted in improper seating of the pilot disc; causing the misalignment in the pilot rod and spring retainer which ultimately caused the pilot disc to develop a pinch in the pilot seat. Additionally, removal of a portion of the disc's platinum coating may have allowed a weak oxide bond to be formed.
Starting in 2000, BSEP personnel have performed the SRV maintenance without vendor support. A group of three mechanics were trained and qualified to perform the maintenance. The cause of the setpoint drift for both valve 2-B21-F013B and valve 2-B21-F013D was an incomplete understanding of proper lapping techniques when the BSEP SRV rebuild program began. This led to improper lapping of the pilot disc to the seat. This is the same cause as reported in LER 1-2006-004, Supplement 1, dated November 17, 2006 for valves 1-B21-F013H and 1-B21-F013K.
SRV 2-B21-F013F Inspections of the 2-B21-F013F pilot valve assembly identified multiple indications of misalignment between the pilot rod and set spring, with the pilot rod being scored on one side. The scoring combined with the misalignment created excessive friction between the pilot rod and pilot guide. In addition, the pilot seat was slightly ditched which caused a pinch between the pilot disc and seat. These two factors combined caused the setpoint drift. The cause of the SRV 2-B21-F013F setpoint drift was misalignment causing mechanical binding between the pilot rod and guide.
Event Cause (continued) SRV 2-B21-F013K SRV pilot discs at BSEP are coated with platinum using the ion beam assisted deposition (IBAD) process.
The purpose of the coating is to eliminate the oxide bonding between the stellite disc and the stellite seat.
The oxide bond was responsible for significant setpoint drift issues across the industry.
The pilot disc for valve 2-B21-F013K was platinum coated during a rebuild in 1996. The pilot valve assembly was rebuilt again in 1999; without platinum recoating. The pilot valve assembly for SRV 2-B21-F013K was installed in Unit 1 and operated for one cycle until it was certified during as-found testing on March 14, 2002. It was recertified on July 20, 2004; without being rebuilt. The pilot valve assembly was installed in Unit 2 in the spring 2005 refueling outage.
The duration of time between the platinum coating application and the complete rebuilding of the pilot valve assembly allowed the accumulation of deposits/tarnish on the pilot disc which lead to the formation of an oxide bond between the pilot disc and seat. The cause of the:valve 2-B21-F013K setpoint drift was lack of maintenance on the pilot valve assembly.
Safety Assessment The safety significance of this condition is considered minimal. The as-found condition of the Unit 2 SRVs was compared to the current overpressure analysis prepared in support of extended power uprate and it was concluded that this analysis remained bounding. As such, the applicable acceptance criteria for design basis events would have been met and the SRVs remained capable of performing their intended safety function.
Corrective Actions
The 11 SRV pilot valve assemblies were replaced with certified spares during the B218R1 refueling outage.
To address lapping concerns, guidance from the EPRI Target Rock SRV Model 67F Maintenance Guide was incorporated into the SRV pilot rebuild procedure (i.e., OCM-VSR509). This was completed as a corrective action for LER 1-2006-004, Supplement 1. Use of the lapping practices outlined in the guide will minimize ditching of the pilot disc to preclude failures as well as minimize pilot leakage.
To address the mechanical binding of the 2-B21-F013F pilot valve assembly, the pilot valve assembly has been rebuilt.
To address concerns with rebuild intervals for pilot valve assemblies, OCM-VSR509 will be revised to allow a pilot valve assembly to be recertified as-found only one time before a mandatory rebuild is performed. This revision is expected to be completed by January 4, 2008.
Previous Similar Events
A review of LERs and corrective action program condition reports for the past three years identified the following similar event.
- LER 1-2006-004, dated July 26, 2006, as supplemented on November 17, 2006, documents operation prohibited by TSs that occurred as a result of as-found testing which indicated that four of the 11 valves actuated at pressures outside of the 3 percent tolerance allowed by Technical Specification 3.4.3. Since this was the first Unit 2 refueling outage after LER 1-2006-004, the corrective actions associated with LER 1-2006-004 could not have reasonably been expected to prevent the condition reported in this LER.
Commitments No regulatory commitments are contained in this report.