05000298/LER-2005-002
| Docket Number | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 2982005002R00 - NRC Website | |
PLANT STATUS
Cooper Nuclear Station (CNS) was in Mode 1 at 100% power at the time of discovery.
BACKGROUND
The Safety Relief Valves (SRVs) [EIIS: SB] installed at CNS are Target Rock Model 7567F two stage pilot actuated valves which are common for this type of application at boiling water reactors (BWR). Failure of the Target Rock SRV pilot assemblies to lift within their upper set point tolerance of + 3.0% due to corrosion bonding is a long-standing industry problem.
In 1997, CNS installed 0.3% Platinum-Stellite pilot discs in all eight SRVs installed in the plant. While improved performance was noted, data showed that the BWR Owners Group-recommended 0.3% Platinum-Stellite pilot discs did not completely solve the corrosion bonding problem. Stellite 21 was reported to have better corrosion resistance characteristics than 0.3% Platinum-Stellite. A review of SRV pilot assembly lift data from another BWR plant which has had Stellite 21 pilot discs installed since 1984 indicated minimal set point drift problems. As a result, in March 1998, CNS installed eight SRVs pilot assemblies comprised of Stellite 21 pilot discs and Stellite 6 in-body seats.
During Refueling Outage (RFO) 22, three complete SRVs and five SRV pilot assemblies were removed from the plant and replaced with refurbished/certified spares. The SRVs removed from service were shipped to Wyle Laboratories for required testing.
EVENT DESCRIPTION
Three Target Rock SRVs failed to automatically actuate at the required Technical Specification (TS) set point during as found testing. The remaining valves tested within their set point tolerances.
In accordance with testing program requirements, four lifts were performed on each pilot assembly. For the three SRVs that failed the initial lift test, the remaining subsequent lifts were within 1.0% of the set point tolerances. A Nitrogen (N2) lift test was performed on each pilot valve assembly as a diagnostic test for indication of corrosion bonding prior to the set point lift test (except for one SRV where leakage was found out of the pilot stage exhaust port, MS-RV-71ERV). A pilot disc found to be stuck during the N2 lift test is a positive indication of a corrosion bond.
The results of the testing on the three valves that failed their initial lift tests are as follows:
CIC Set Point Pilot Assembly AS FOUND Lift Pilot Disc Stuck Serial Number 1st, 2nd, 3rd, & 4th During N2 Lift Test (S/N) MS-RV-7ICRV 1090 385 1173 (+7.6%), 1096 (+0.5%), YES 1088 (-0.2%), 1090 (0.0%) MS-RV-71ERV 1090 1241 1132 (4-3.9%), 1097 (+0.6%), 1092 N/A (+0.2%), 1093 (0.3%) MS-RV-71HRV 1090 1242 1127 (+3.4%), 1101 (+1.0%), 1097 YES (+0.6%), 1097 (+0.6%) The test data showed that two discs tested positive for corrosion bonding prior to lift testing. The valve pilots that failed to meet their nameplate set points after the initial lift tests exhibited test pressures within 1.0% of nameplate set point in subsequent tests. Corrosion bonding occurs when a Target Rock SRV pilot disc-seat interface oxidizes, sticking the two parts together. The initial lift test is followed by three additional lift tests. If the first lift test results in a SRV not meeting its lift set point and the three successive lift tests are very close to the set point, it is believed that a corrosion bond had formed between the pilot disc and seat. The first lift broke the corrosion bond.
The second, third and fourth lifts are not impacted by the corrosion bond. The three SRVs that failed the initial lift test had successive lift tests within 1.0% of the set points.
Historically, the SRVs have performed very well. CNS has previously experienced only five AS FOUND lift test failures of a SRV prior to the three from RFO 22 since the installation of the Stellite 21 pilot discs. Even with the three failures from RFO 22, all the SRVs removed/tested from RFO 22 lifted below 1210 psig, which is the value assumed in the Supplemental Reload Licensing Report for the opening pressure of all SRVs.
BASIS FOR REPORT
This event is being reported as a condition prohibited by plant Technical Specifications per 10 CFR 50.73(a)(2)(i)(B).
SAFETY SIGNIFICANCE
This event is considered to have no safety significance from a probabilistic safety assessment risk evaluation standpoint. This event does not create a core damage scenario. There is no change to the CNS core damage frequency or the large early release frequency. This condition also has no impact on the Reactor Pressure Vessel pressure relief function capability. Even under postulated failure conditions, there is no associated risk increase to the plant.
CAUSE
The root cause for the safety relief valves not opening at their set point is corrosion bonding between the Stellite 21 pilot discs and the Stellite 6 pilot body seats prevents the valves from actuating at or below their TS set point limit.
A similar condition was evaluated in CNS Significant Condition Report (SCR) 99-0346 (LER 1999-004). SCR 99 0346 referenced a General Electric analysis that demonstrates that as long as the SRVs lift at or below 1210 psig, there will be adequate margin available to avoid any potential plant safety concerns. The evaluation performed for the Supplemental Reload Licensing Report uses the value of 1210 psig for the opening pressure of the SRVs. The three SRVs that failed the initial lift test lifted well below the 1210 psig value. Even if the SRVs open at 1210 psig and the Safety Valves (SVs) open at 1277 psig, there is still 68 psi margin between the calculated CNS peak pressure (1307 psig) and the ASME code over pressure protection limit of 1375 psig. This margin (SRVs lifting well below 1210 psig and the SVs historically lifting well below 1277 psig) provide assurance that the ASME code over pressure requirement of 1375 psig would not be exceeded.
CORRECTIVE ACTION
The eight SRV pilot valve assemblies that were removed during CNS RFO 22 for testing were replaced with certified spare pilot valve assemblies containing Stellite 21 pilot discs during CNS RFO 22. In addition, the following corrective actions shall be undertaken prior to and following CNS RFO 23:
1)Implementation of appropriate elements of the Target Rock Users Group industry best practices guide for Target Rock SRV refurbishment during each refurbishment.
2)Presence of a CNS Engineering representative during refurbishment of each SRV pilot assembly to provide oversight and support.
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