05000334/LER-2006-001
| Beaver Valley Power Station | |
| Event date: | 02-12-2006 |
|---|---|
| Report date: | 04-10-2006 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 3342006001R00 - NRC Website | |
PLANT AND SYSTEM IDENTIFICATION
Westinghouse-Pressurized Water Reactor {PWR} Main Steam System {SB}
CONDITIONS PRIOR TO OCCURRENCE
Unit 1: Mode 1 at 60 percent power There were no systems, structures, or components that were inoperable at the start of the event that contributed to the event other than as described below.
DESCRIPTION OF EVENT
On February 12, 2006, Beaver Valley Power Station (BVPS) Unit No. 1 was in Mode 1 and holding at approximately 60 percent (%) reactor power during the downpower prior to entering its 1R17 refueling shutdown which was scheduled for the following day. Technical Specification surveillance testing of the Main Steam Safety Valves (MSSVs) was then performed using the Trevitesting method in accordance with site testing procedure 1BVT 1.21.2. This test determines each valve's actual lift setting using normal system pressure with assistance from a hydraulic testing device.
The initial lift tests on the five "C" Main Steam System header Main Steam Safety Valves [SV-1 MS-101 C, 102C, 103C, 104C and 105C] did not meet BVPS Unit No. 1 Technical Specification 3.7.1.1 required acceptance criteria (+1%/-3%) for as-found lift pressures.
These five valves were new valves that had been installed as replacements during the previous BVPS Unit No. 1 refueling outage (1R16 in November 2004). Due to the number of valves not meeting ASME acceptance criteria (+3%/-3%) for as-found lift pressures, the testing was expanded to include all 15 MSSVs on the three steam generators. The ten MSSVs associated with the "A" and "B" steam headers, which are original plant valves, all successfully passed the testing, without any adjustments.
The MSSV tests, and any required valve adjustments, were performed on only one valve at a time. Valves not meeting acceptance criteria were adjusted (if required) and satisfactorily tested before proceeding to the next valve. Operations personnel entered the applicable Technical Specification 3.7.1.1 action statement until each valve was tested with satislactory results. Since reactor power was at 60%, no reactivity changes were required in order to comply with the Technical Specification 3.7.1.1. action for one MSSV inoperable. Because the MSSVs were tested sequentially, only one valve was known to be inoperable at any one time.
DESCRIPTION OF EVENT (Continued) The following provides the setpoint values found and the actions taken for the five "C" steam header MSSVs:
[SV-1MS-101C] - Setpoint is 1075 psig +1/-3% (1042.8 to 1085.7 psig). The initial as-found lift pressure was 1115.4 psig (3.8% above set pressure) and the second as-found lift pressure was 1047.7 psig (2.6% below set pressure). The valve was adjusted and re-iested until 2 consecutive lifts within +1/-1% of set pressure were achieved. The final as-left lit pressures were 1067.7 psig and 1073.5 psig.
[SV-1MS-102C] - Setpoint is 1085 psig +1/-3% (1052.5 to 1095.8 psig). The initial as-found lift pressure was 1134.5 psig (4.6% above set pressure). The second and third as-found lift pressures were 1074.9 psig (0.9% below set pressure) and 1074.5 psig (0.98% below set pressure) respectively. Thus this valve did not require adjustment to achieve 2 consecutive lifts within +1/-1% of set pressure.
[SV-1MS-103C] - Setpoint is 1095 psig +1/-3% (1062.2 to 1105.9 psig). The initial as-found lift pressure was 1158.1 psig (5.8% above set pressure) and the second as-found lift pressure was 1078.6 psig (1.5% below set pressure). The valve was adjusted and re-tested until 2 consecutive lifts within +1/-1% of set pressure were achieved. The final as-left lift pressures were 1091.1 psig and 1091.7 psig.
[SV-1MS-104C] - Setpoint is 1110 psig +1/-3% (1076.7 to 1121.1 psig). The initial as-found lift pressure was 1153.4 psig (3.9% above set pressure) and the second as-found lift pressure was 1091.9 psig (1.6% below set pressure). The valve was adjusted and re-tested until 2 consecutive lifts within +1/-1% of set pressure were achieved. The final as-left lift pressures were 1115.3 psig and 1114.5 psig.
[SV-1MS-105C] - Setpoint is 1125 psig +1/-3% (1091.3 to 1136.2 psig). The initial as-found lift pressure was 1169.7 psig (3.97% above set pressure). The second and third as-found lift pressures were 1117.8 psig (0.6% below set pressure) and 1120.1 psig (0.4% below set pressure) respectively. Thus this valve did not require adjustment to achieve 2 consecutive lifts within +1/-1% of set pressure.
CAUSE OF EVENT
The root cause of the high initial lift setpoint has been determined to be oxide bonding between the discs and nozzles of the new MSSVs with 422 Stainless Steel (SS) discs installed during the previous refueling outage 1R16. This material is susceptible to the oxide bondin
- )henomenon during long continuous runs of plant operation.
REPORTABILITY
The existence of similar discrepancies believed to be due to oxide bonding between the discs and nozzles of the new MSSVs installed during 1R16 is an indication that the condition likely developed during the Unit 1 Cycle 17 operating cycle and that the condition existed prior to the surveillance testing. Thus, this event is reportable pursuant to 10 CFR 50.73(a)(2)(i)(B) due to the assumption that the plant inadvertently operated with multiple MSSVs inoperable due to set points outside of their required lift setting tolerance band, without taking the actions specified in Technical Specification 3.7.1.1.
SAFETY IMPLICATIONS
The primary purpose of the MSSVs is to provide overpressure protection for the secondary system. These valves also provide protection against over-pressurizing the reactor coolant system pressure boundary by providing a heat sink for the removal of energy from the reactor coolant system if other preferred heat sink methods (i.e., the condenser or atmospheric steam dump valves) are not available. The MSSVs also serve as containment isolation valves.
The risk significance of the BVPS Unit No. 1 "C" Steam Generator MSSVs all lifting above their setpoint tolerance of +1% is considered to be of very low risk significance, given that the remaining MSSVs on the "A" and "B" Steam Generators were within tolerance. This is based on an Engineering evaluation of the consequences of the identified higher safely valve setpoints. This evaluation performed a review of the Westinghouse analyses for the BVPS Ulit No. 1 design basis accidents which credit Main Steam Safety Safety Valve operation. These accidents include the Large Loss of External Load with Turbine Trip (Load Rejection) and Small Break Loss of Coolant Accident (SBLOCA), which are the limiting transients. Assessments against the analyses of record indicate that the existing input conservatisms in both calculations are sufficient to offset the effect of the MSSVs lifting above their Technical Specification 3.7.1.1 nominal setpoint to the extent experienced on February 12, 2006. These conservatisms include 1) an assumed 3% tolerance for MSSV opening (for all valves in the Load Rejection and all except the first MSSV in the LOCA), 2) a 5 psi valve accumulation on all MSSVs, and 3), for the SBLOCA, a conservative Moderator Temperature and Doppler coefficients, along with a conservative High Head Safety Injection flow. While the MSSV with the lowest setpoint has a direct impact on the SBLOCA transient behavior, the valve capacity does not, because the heat load on the valve is small in comparison to its design capacity.
Therefore, only one out of the three first MSSVs and its associated Steam Generator is required to effectively dictate early transient Reactor Coolant System conditions during the SBLOCA.
SAFETY IMPLICATIONS (Continued) As a result of the assessments against the analyses of record, it is concluded that even with the "C" Steam Generator MSSVs all lifting above their setpoint tolerance to the extent described earlier, the peak secondary pressure in the Load Rejection would remain below the 110% limit, while peak clad temperature in the SBLOCA would remain below 2200 °F.
In the risk assessment, since all MSSVs were available on the "A" and "B" Steam Generators at their Technical Specification setpoints, along with the Atmospheric Steam Dump Valves, and Residual Heat Release Valve, there is a high likelihood that the secondary and primary pressure boundary would be maintained during all transient conditions. Additionally, since the tested MSSVs properly reseated following their Trevitesting, the probability of them not reseating would not be increased over the current values assumed in the PRA model. Therefore, the impact on risk is considered to be very low.
CORRECTIVE ACTIONS
1. Immediate actions included adjusting SV-1MS-101C, 103C and 104C to within -FM % of their required set lift pressure.
2. The `C" steam header MSSVs, SV-1MS101C, 102C, 103C, 104C, and 105C, will be removed and replaced with safety valves that have improved pre-oxidized Inconel X-750 discs and passivated nozzles during this 1R17 refueling shutdown.
3. An action plan will be developed to perform mid-cycle testing of one MSSV on the "C" Main Steam System.
4. The Engineering Change process will be revised to enhance the integration of Operating Experience Program requirements.
5. An Operating Experience report on this subject will be issued to the industry.
Completion of the above and other corrective actions are being tracked through the BVPS corrective action program.
PREVIOUS SIMILAR EVENTS
A review found no prior BVPS Unit 1 and no prior BVPS Unit 2 Licensee Event Reports within the last five years involving a steam generator safety valve problem.
COMMITMENTS
There are no new commitments made by FirstEnergy Nuclear Operating Company (FENOC) for BVPS Unit No. 1 in this document.