05000324/LER-2015-002
| Brunswick Steam Electric Plant (Bsep) Unit 2 | |
| Event date: | 03-10-2015 |
|---|---|
| Report date: | 06-26-2015 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 3242015002R01 - NRC Website | |
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2. DOCKET 6. LER NUMBER 1 3. PAGE Brunswick Steam Electric Plant (BSEP) Unit 2 2015 - 002 001 Energy Industry Identification System (EIIS) codes are identified in the text as [XX].
Introduction Initial Conditions At the time the condition was found, Unit 2 was in a refueling outage with the reactor vessel defueled.
Reportability Criteria valves (SRVs) to be operable. Per Surveillance Requirement (SR) 3.4.3.1, each valve is required to open within +/-3 percent of its opening setpoint. As-found testing of the valves indicated that three of the valves had lift setpoints outside this tolerance. Based on the causes of the inoperability described below, it was concluded that at least two SRVs were inoperable when the unit was in operation.
Consequently, the plant operated in a condition which is prohibited by the TS, that is, with fewer than the required number of SRVs having lift setpoints within the 3 percent tolerance. Therefore, the condition is being reported per 10 CFR 50.73(a)(2)(i)(B) for operation in a condition prohibited by the plant TSs.
Event Description
During the spring 2015 Unit 2 refueling outage, all 11 Model 7657F Target Rock Two-Stage pilot valve assemblies in the SRVs were replaced with certified spares. The removed SRV pilot valves were sent to National Technical Systems (NTS) to determine the as-found set pressure. On March 10, 2015, test results were reported to BSEP. The test results showed that two of the 11 valves actuated at pressures outside of the 3 percent tolerance allowed by TS 3.4.3. A third valve was reported out of tolerance on the following day, March 11, 2015. The test data for the valves found out of tolerance are shown below:
Valve Identification TS Setpoint (psig) As-Found Lift Pressure (psig) Percent Difference S/N 1082 1150 1191 +3.6 S/N 1086 1130 1166 +3.2 S/N 1097 1130 1085 —4.0 Event Cause The elevated lift setpoints of SRV S/Ns 1082 and 1086 resulted from slight corrosion bonding between the pilot disc and seat. The corrosion bonding increases the breakaway force needed to open the pilot and actuate the main valve stage.
Corrosion bonding of the pilot disc and seat resulted from discontinuities in the platinum coating on the pilot valve which allow oxygen and moisture to contact the Stellite 6B base metal. The discontinuities in the platinum occurred most frequently in localized sites where microcracking was present on the base metal of the disc. The microcracking, in turn, was a consequence of carbide inclusions inherent in the Stellite 6B. Carbide inclusions are typically less than 10 micrometers in length. Machining and polishing techniques used in surface preparation of the pilot disc occasionally caused the carbide inclusions to crack or pull out of the base metal rather than forming a uniformly planar surface. The resulting surface irregularities in the base metal were typically less than 10 micrometers in length and became stress risers when the platinum coating was applied. The probability of developing a discontinuity in the platinum coating was slightly higher in the vicinity of these stress risers.
The cause of the decreased initial lift setpoint in SRV S/N 1097 is not known. After the initial test, four subsequent tests resulted in valve lifts that were within the TS +/-3 percent tolerance. The valve was disassembled and all internals inspected. No conditions were identified which could account for the initial lift setpoint being out of tolerance.
Safety Assessment The purpose of the safety/relief valves is to provide overpressure protection for the reactor coolant system. The as-found condition of the Unit 2 SRVs was compared to the Brunswick Unit 2 Cycle 21 Reload Safety Analysis. The most limiting case of the overpressurization analysis is for main steam isolation valve closure. It assumes one SRV is inoperable; two SRVs drift to 6 percent above setpoint and one SRV drifts to. 10 percent above setpoint. The maximum predicted steam dome pressure in this situation is 1315 psig, which is below the BSEP Unit 2 safety limit of 1325 psig. In addition, the analysis bounds the as-found condition in which two valves had setpoints between 3 and 4 percent high, and one valve had a setpoint at 4 percent low. Therefore, the SRVs always remained capable of performing their safety function of preventing overpressurization of the reactor coolant system.
Based on the foregoing analysis, it is concluded that this event had no adverse impact on nuclear safety.
Corrective Actions
Any changes to the corrective actions and schedules noted below will be handled in accordance with the site's corrective action program.
All 11 Unit 2 SRV pilot valves were replaced with refurbished and certified spares during the spring 2015 refueling outage.
For pilot valve 1097, which experienced a decreased lift setpoint, BSEP will replace all internal parts that affect the set pressure of the valve. This action will be completed before the valve is re-installed on either unit.
BSEP will revise procedure OCM-VSR509 to improve pilot valve disc surface preparation and perform a verification of the surface characteristics to reduce the probability of microcracking which can lead to degradation of the platinum coating. This action will be completed by July 31, 2015.
BSEP will also assess the possible use of Stellite-21 material for the pilot disc base metal. The assessment will be completed by September 30, 2015.
Previous Similar Events
A review of LERs and corrective action program condition reports identified two previous similar occurrences within the past three years.
- LER 2-2013-003, dated July 22, 2013, Adams Accession Number ML13212A170
- LER 1-2014-005, dated July 21, 2014, Adams Accession Number ML14212A031 These events also resulted from corrosion bonding between pilot discs and seats, which is a recurring problem throughout the boiling water reactor industry. Previous corrective actions did not prevent this event from occurring because the effect of carbide inclusions in pilot disc base metal upon the quality of surface preparation was not previously known or accounted for in the Ion Beam Enhanced Deposition (IBED) process.
Commitments No regulatory commitments are contained in this report.