05000336/LER-2008-001
| Millstone Power Station - | |
| Event date: | 04-03-2008 |
|---|---|
| Report date: | 05-30-2008 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| LER closed by | |
| IR 05000245/2008004 () | |
| 3362008001R00 - NRC Website | |
1. Event Description With the plant in MODE 1 at 100% power on April 3 and 4, 2008, set pressure "simmer" testing of Millstone Power Station Unit 2 (MPS2) main steam safety valves (MSSVs) [SB, RV] was conducted per plant procedures. Eight valves did not meet acceptance criteria. The failures were consistent in that all eight valves lifted high.
Plant Technical Specification (TS) 3.7.1.1 requires that all MSSVs be OPERABLE with lift settings as specified on Table 4.7-1. OPERABILITY of the MSSVs is the ability to open within the setpoint tolerances, relieve steam generator overpressure, and reset when pressure has been reduced. Table 4.7-1 allows a +/- 3% setpoint tolerance (allowable value) on the lift setting for OPERABILITY to account for drift over an operating cycle. During this testing, the MSSVs are OPERABLE provided that the actual as-found lift settings are within +/- 3% of the required lift setting. A footnote to Table 4.7-1 requires that the lift setting shall be within +/- 1% of the required lift setting following testing (as-left).
Since the as-found lift pressures for the eight valves exceeded the +/- 3% TS allowable values, these valves were not OPERABLE. In accordance with TS Action Statement 3.7.1.1.a.1, each valve was adjusted to a lift pressure +/-1% of set pressure within four hours and declared operable before the next valve was tested. The valves that exceeded their TS set pressure are 2-MS-252, 2-MS-242, 2-MS-248, 2-MS-249, 2-MS-239, 2-MS-241, 2-MS-245 and 2-MS-246.
Since multiple MSSVs exceeded the allowable TS limits and the cause indicates this occurred during operation, this condition is reportable under 10 CFR 50.73(a)(2)(i)(B), "any operation or condition prohibited by the plant's technical specifications". This is consistent with the guidance provided in NUREG 1022 Rev. 2 Section 3.2.2 Operation or Condition Prohibited by Technical Specifications regarding multiple test failures. The actual time the valves were inoperable is not known as this condition has arisen over the period of time the valves have been installed.
2. Cause Two failures (2-MS-252 and 2-MS-242) were attributed to micro bonding, as evidenced by greater than or equal to 2% difference between the first and second lift, with the first lift being higher and the second satisfactory. Electric Power Research Institute (EPRI) Report TR-113560, (Investigation of MSSV High First Lift Phenomenon in Dresser 3700 Series Steam Relief Valves, dated September 2000) suggests that a bonding mechanism exists between two stainless steel (SS) materials. MPS2 MSSVs have 422 SS discs and 316 SS nozzle seats, which creates the potential for micro bonding occurrences in a MSSV static controlled in-situ test environment.
Six failures (2-MS-248, 2-MS-249, 2-MS-239, 2-MS-241, 2-MS-245 and 2-MS-246) are attributed to differences between test methods. A Crosby Set Pressure Verification Device (SPVD) was being used for the first time at MPS2 for MSSV testing. Prior testing and "as-left" settings utilized a Dresser Hydroset system. Although both methods are Appendix B approved, there are key differences between the test methods. The primary difference between the Hydroset and the SPVD test methods involves determining when a valve's "simmer" (lift) point is reached. Using the Hydroset at MPS2, operators rely on a number of visual and auditory cues in a hot, noisy environment to determine valve lift. The use of a manual hand pump with the Hydroset system at. MPS2 also introduces inconsistency in the rate of pressurization (ramp rate) up to the valve lift point. The SPVD measures the spindle movement and the force that was applied to achieve this movement and is not subject to the environmental conditions at MPS2. Testing with the SPVD test method found the lift point of these valves to be at a higher pressure than the lift point established with the Hydroset system.
3. Assessment of Safety Consequences This event is of low safety significance. The Final Safety Analysis Report (FSAR) Chapter 14 limit on main steam system pressurization is 110% of the design pressure or 1100 psia. No actual event has occurred at MPS2 that challenged this pressure limit. From the as-found testing results with deviations up to 5.38%, the MSSV 2-MS-248 failure is the most significant. Valve 2-MS-248 would have opened at 1103 psia, which is above the FSAR Chapter 14 design limit. The FSAR Chapter 14 single main steam isolation valve (MSIV) closure event reports a maximum main steam pressure of 1093 psia, which is 7 psi less than 110% of design pressure limit. This event relies on the opening of all eight MSSVs on the steam generator with the isolated MSIV to limit the main steam pressurization. Since valve 2-MS-248 would not have opened until the main steam system pressure exceeded 110% of design, the FSAR Chapter 14 acceptance criterion would have been exceeded for this event. Using the as-found MSSV settings, should the single MSIV closure event have occurred, it is expected that the maximum main steam system pressurization would have exceeded the 1100 psia design limit by less than 2% or 20 psi. Based on an engineering evaluation, DNC concludes that the integrity of the steam generator secondary side and main steam system would have been maintained.
4. Corrective Action The current as-left settings of the valves were performed using the Crosby SPVD method. Consistent use of the Crosby SPVD going forward, will eliminate the subjectivity associated with identifying the correct MSSV lift points.
After testing, all MSSVs were left within +1- 1% of TS acceptance criteria to account for future drift. Additional corrective actions to address micro bonding and the in-situ testing process will be evaluated in accordance with the station's Corrective Action Program.
5. Previous Occurrences Micro bonding has occurred previously during simmer testing at MPS2. Valve 2-MS-241 failed high (3.4%) during 2R16 simmer testing on April 07, 2005.
No previous similar events were identified related to test methodology, as this was the first time the Crosby SPVD has been used at MPS2.
Energy Industry Identification System (EllS) codes are identified in the text as [XX].