05000423/LER-2008-002
| Millstone Power Station - | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 4232008002R00 - NRC Website | |
1. Event Description:
With the plant in MODE 1 at 100% power on October 9 and 10, 2008 set pressure "simmer" testing of MPS3 MSSVs [SB, RV] was conducted per plant procedures. During the conduct of testing, four MSSVs failed to lift within the (+/- 3%) acceptance criteria.
Plant TS 3.7.1.1 requires all MSSVs be OPERABLE with lift settings as specified on Table 3.7-3. Table 3.7-3 "Steam Line Safety Valves Per Loop" provides the valve numbers and lift settings. TS 3/4.7.1.1 BASES includes (in part):
"The OPERABILITY of the MSSVs is defined as the ability to open upon demand within the setpoint tolerances, relieve steam generator overpressure, and reset when pressure has been reduced." Table 3.7-3 "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 lift settings are within +/- 3% of the required lift setting. A footnote to Table 3.7-3 requires that the lift setting be restored to within +/- 1% of the required lift setting following testing to allow for drift during the next operating cycle.
Since the as-found lift pressures for 3MSS*RV22C, 3MSS*RV22D, 3MSS*RV24A, and 3MSS*RV25C exceeded the +/- 3% TS allowable values these valves were declared INOPERABLE.
As multiple MSSVs (four) exceeded the allowable TS limits and the cause evaluation indicates this condition 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," Example (3,) regarding multiple test failures. The actual time the valves were inoperable is not known as this condition occurred over the period of time the valves were installed or since they were last tested.
2. Cause:
The four failures (3MSS*RV22C, 3MSS*RV22D, 3MSS*RV24A, and 3MSS*RV25C) 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.
The MSSVs installed on MPS3 are Dresser 3707R Safety Relief Valves equipped with stainless steel materials for discs and nozzle seats. Based on an evaluation, Dominion Nuclear Connecticut Engineering concluded these valves were subject to the known phenomenon of micro bonding.
3. Assessment of Safety Consequences:
This condition is judged to be of very low safety significance. Five steam line safety valves are installed on each of the unit's four main steam lines with nominal lift settings increasing in 10 psi increments from 1185 to 1225 psig. The safety valves protect the SG and portions of the main steam [SB] and feedwater systems [SJ] from overpressure conditions. The valves also serve as a heat sink for the reactor coolant system if the main condenser [COND] is unavailable and the atmospheric steam dump valves cannot relieve pressure following a reactor trip or secondary system accident. The limiting FSAR events with respect to main steam and reactor coolant overpressurization are those presented in FSAR Section 15.2, which involve a decrease in heat removal by the secondary system. The accidents presented in FSAR Section 15.2 assume all MSSVs open at a lift setting 3% higher than their nominal set pressure. An additional conservatism is added to this value for accumulation plus a conservative estimate of the pressure drop to the relief valve. For the limiting Section 15.2 event, all the safety relief valves are assumed open at the time of peak secondary pressure. Since the relief valves all opened during the simmer testing, the same relief capacity would exist with all the relief valves open for the event. Therefore, it is concluded the overall MSSV response of the as-found MSSV condition is equivalent to the MSSV response assumed in the FSAR Section 15.2 Safety Analyses.
Additionally, the micro bonding phenomenon has been limited to static, in-situ testing. The EPRI report indicates that under actual transient high pressure conditions, disk movement/flexure breaks the micro bonding allowing the MSSVs to lift uninfluenced by this phenomenon. Additionally, the EPRI Report indicates that, within the limits of detectability, no history of the seat sticking phenomena had been identified during actual plant transients resulting in MSSV lifts.
4. Corrective Action:
All twenty MSSVs were left within +/- 1% of TS acceptance criteria to account for potential future drift.
Additional corrective actions to address micro bonding will be evaluated in accordance with the station's Corrective Action Program.
5. Previous Occurrences:
LER MPS3 2007-001-00.
On September 30, 2005, during the performance of set pressure "simmer" testing of MSSVs, valve 3MSS*RV25C failed to lift within the +/- 3% acceptance criteria. The allowable as-found set pressure range is 1179-1251 psig. The valve lifted at 1252.9 psig. The valve was lifted two additional times with results within the required as-left criteria of +/- 1% with no adjustments required. Two additional MSSVs were set pressure tested, to meet the American Society of Mechanical Engineers/Operation and Maintenance code requirement for test expansion. These valves (3MSS*RV23C and 3MSS*RV24C) were satisfactorily tested. The as-left "simmer" tests for both valves were within the required as-left criteria of +/- 1%.
Energy Industry Identification System (EIIS) codes are identified in the text as [XX].