05000261/LER-2002-001
| Docket Number | |
| Event date: | 10-09-2002 |
|---|---|
| Report date: | 12-09-2002 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 2612002001R00 - NRC Website | |
FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) H. B. Robinson Steam Electric Plant, Unit No. 2 05000261 I. � DESCRIPTION OF EVENT On October 9, 2002, the H. B. Robinson Steam Electric Plant (HBRSEP), Unit No. 2, reactor was operating at approximately 93% power. At 0829 hours0.00959 days <br />0.23 hours <br />0.00137 weeks <br />3.154345e-4 months <br />, testing of the main steam system [SB] safety valves [RV] was initiated in accordance with procedure EST-028, "Main Steam Safety Valve Testing (Refueling Shutdown Interval and As Needed After Maintenance).
By the completion of the testing on October 10, 2002, the entire set of 12 safety valves (four valves on each of the three main steam lines) had been tested. Four safety valves were found with setpoints above the test acceptance criteria.
The first safety valve test failure occurred at 0948 hours0.011 days <br />0.263 hours <br />0.00157 weeks <br />3.60714e-4 months <br /> on October 9, 2002, during testing of valve SV1-2B, which `is one of the four main steam system safety valves on the "B" steam generator [SG] main steam line.. .The applicable HBRSEP.
Unit No. 2. Technical Specifications limiting condition for operation (LCO) required .action for inoperability of SV1-2B had been..entered.at 0943 hoOrs to accommodate the valve test. The valve was adjusted and subsequently retested.. The valve was returned to operable status and the LCO action statement was exited at 1023 hours0.0118 days <br />0.284 hours <br />0.00169 weeks <br />3.892515e-4 months <br />.
This test failure required two additional valves, beyond the original four valves selected, to be tested in accordance with the inservice testing program.
The second safety valve test failure occurred at 1250 hours0.0145 days <br />0.347 hours <br />0.00207 weeks <br />4.75625e-4 months <br /> on October 9, 2002, during testing of valve SV1-2C, which is one of the four main steam system safety valves on the "C" steam generator main steam line. The applicable Technical Specifications LCO required action for inoperability of SV1-2C had been entered at 1247 hours0.0144 days <br />0.346 hours <br />0.00206 weeks <br />4.744835e-4 months <br /> to accommodate the valve test. The valve was adjusted and subsequently retested. The valve was returned to operable status and the LCO action statement was exited at 1327 hours0.0154 days <br />0.369 hours <br />0.00219 weeks <br />5.049235e-4 months <br />. This test failure required the entire set of main steam system safety valves to be tested.
The third safety valve test failure occurred at 1412 hours0.0163 days <br />0.392 hours <br />0.00233 weeks <br />5.37266e-4 months <br /> on October 9, 2002, during testing of valve SV1-1C, which is one of the four main steam system safety valves on the "C" steam generator main steam line. The applicable Technical Specifications LCO required action for inoperability of SV1-1C had been entered at 1407 hours0.0163 days <br />0.391 hours <br />0.00233 weeks <br />5.353635e-4 months <br /> to accommodate the valve test. The valve was adjusted and subsequently retested. The valve was returned to operable status and the LCO action statement was exited at 1445 hours0.0167 days <br />0.401 hours <br />0.00239 weeks <br />5.498225e-4 months <br />.
FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) H. B. Robinson Steam Electric Plant, Unit No. 2 05000261 The fourth safety valve test failure occurred at 1123 hours0.013 days <br />0.312 hours <br />0.00186 weeks <br />4.273015e-4 months <br /> on October 10, 2002.
during testing of valve SV1-1A, which is one of the four main steam system safety valves on the "A" steam generator main steam line. The applicable Technical Specifications LCO required action for inoperability of SV1-1A had been entered at 1122 hours0.013 days <br />0.312 hours <br />0.00186 weeks <br />4.26921e-4 months <br /> to accommodate the valve test. The valve was retested satisfactorily.
The valve was returned to operable status and the LCO action statement was exited at 1145 hours0.0133 days <br />0.318 hours <br />0.00189 weeks <br />4.356725e-4 months <br />.
A condition report (number 73940) was initiated and the corrective action program significant adverse condition investigation has been completed. This reportable event and the associated significant adverse condition investigation was reviewed by the Plant Nuclear Safety Committee on December 4. 2002.
This condition was determined to be reportable based on guidance contained in NUREG-1022, "Event Reporting Guidelines 10 CFR -50.72 and 50.73." Revision 2.
Example 3 in Section 3.2.2 of NUREG-1022. Revision 2, states that multiple test failures are reportable under 10 CFR 50.73(a)(2)(i)(B), "Any operation or condition prohibited by the plant's Technical Specifications," if the existence of similar discrepancies in multiple valves is an indication that the discrepancies may well have arisen over a period of time.
HBRSEP, Unit No. 2. Technical Specifications LCO Section 3.7.1 provides the operability requirements for the Main Steam Safety Valves (MSSVs). LCO 3.7.1 requires the MSSVs to be operable in MODES 1, 2, and 3. Inoperability of the MSSVs, as determined during the testing, required entry into Condition A of LCO 3.7.1. The required action for LCO 3.7.1. Condition A, required that thermal power be reduced to during the testing, because the valves were restored to operable status prior to the required completion time. Therefore, the reportability assessment of this condition is conservatively based on the guidance of NUREG-1022, Revision 2, which indicates that multiple test failures of this type represent a condition that, if known, would have required the reactor power level to be reduced and, hence, was a condition prohibited by the Technical Specifications.
An evaluation of the safety consequences of the condition was completed. The safety consequences were determined to be minimal, and the overpressure protection of the steam generators, main steam lines, and reactor coolant system [AB] would not be H. B. Robinson Steam Electric Plant, Unit No. 2 05000261 exceeded due to the higher lift pressures. Therefore, the failure of these four main steam safety valves did not trigger any other reporting criteria.
The corrective actions, as described in the corrective actions section of this report, were completed during Refueling Outage (RO) 21, which started on October 12, 2002, and ended on November 13, 2002. Work was performed on the entire set of 12 MSSVs during RO-21. The work scope required post-maintenance testing on seven of the 12 valves (SV1-1A. 1B, 1C, 2B, 2C, 3A, and 4C).
II. CAUSE OF EVENT
The root cause of the out of tolerance lift pressures for these four main steam safety valves was attributed to mechanical component failure/degradation due to ,slight binding of the spindle on the guide bearing. Wear was observed on the spindle'that may have contributed to the interference.between the guide bearing and spindle: It was judged that binding of-the spindle on the thrust bearing and spring-drift (i.e., minor change in the spring characteristics based on environmental conditions) were likely contributing causes. (A valve diagram is provided as Figure 1).
III. ANALYSIS OF EVENT
Technical Specifications Table 3.7.1-2 lists the main steam safety valve lift setting requirements as follows:
VALVE NUMBER LIFT SETTING
(psig +/- 3%) A
STEAM GENERATOR
SV1-1A SV1-1B SV1-1C 1085 SV1-2A SV1-2B SV1-2C 1110 SV1-3A SV1-3B SV1-3C 1125 SV1-4A SV1-4B SV1-4C 1140 I FACILITY NAME (1) LER NUMBER (6) DOCKET NUMBER (2) PAGE (3) H. B. Robinson Steam Electric Plant, Unit No. 2 05000261 The four valves that failed to meet the +/- 3% acceptance criteria were SV1-1A, SV1-2B, SV1-1C, and SV1-2C. The results of the "as-found" test for each valve is provided in the following table:
VALVE.
'NUMBER :AS-FOUND LIFT PRESSURE(PSIG)' PERCENT � ' -EXCEEDANCE .
' :.STEAM -
GENERATOR
SV1-1A 1119 3.1% A SV1-2B 1156 4.1% B SV1-1C 1126 3.8% C SV1-2C 1187 6.9% C As stated in the bases for LCO 3.7.1, the primary purpose of the MSSVs is to provide overpressure, protection for the secondary system. The MSSVs also provide protection against overpressurizing the reactor coolant pressure boundary (RCPB) by providing a heat sink for the removal of energy from the Reactor Coolant System (RCS) if the preferred heat sink, provided by the Condenser and Circulating Water System, is not available.
Four MSSVs are located on each main steam header, outside containment, upstream of the main steam isolation valves, as described in the Updated Final Safety Analysis Report (UFSAR). Section 10.3.2. The MSSVs must have sufficient capacity to limit the secondary system pressure to order to meet the requirements of the ASME Code.Section III. The MSSV design includes staggered setpoints, according to Technical Specifications Table 3.7.1-2 in the accompanying LCO, so that only the needed valves will actuate. Staggered setpoints reduce the potential for valve chattering that could result from steam pressure insufficient to fully open all valves following a turbine or reactor trip.
The design basis for the MSSVs comes from ASME Code,Section III, and their purpose is to limit the secondary system pressure to anticipated operational occurrence (A00) or accident considered in the design basis accident (DBA) and transient analysis.
The events that challenge the relieving capacity of the MSSVs, and thus RCS pressure, are those characterized as decreased heat removal events, which are presented in the UFSAR. Section 15.2. Of these, the loss of external electrical LER NUMBER (6) DOCKET NUMBER (2) PAGE (3) FACILITY NAME (1) H. B. Robinson Steam Electric Plant, Unit No. 2 05000261 load is the limiting A00. This event also terminates normal feedwater flow to the steam generators.
An evaluation of the effect of the as-found lift pressures for the MSSVs was performed. This evaluation concluded that the as-found lift pressures for the identified MSSVs would not cause any safety analysis limits to be exceeded.
IV. CORRECTIVE ACTIONS
The following table summarizes the work performed on the MSSVs during RO-21:
VALVE.NUMBER‘' - WORK COMPLETED DURING RO-21 SV1-1A The valve was disassembled, inspected, the spindle was replaced, and the valve seats were lapped.
SV1-3A The valve was disassembled, inspected, the spindle was replaced, and the valve seats were lapped.
SV1-1B The valve was disassembled, inspected, the spindle was replaced, the guide bearing was machined, and the valve seats were lapped.
SV1-2B The valve was disassembled, inspected, the spindle was replaced, the guide bearing was machined, and the valve seats were lapped.
SV1-1C The valve was disassembled, inspected, the spindle was replaced, and the valve seats were lapped.
SV1-2C The valve was disassembled, inspected, the guide bearing was machined, the spindle was polished, and the valve seats were lapped.
SV1-4C The valve was disassembled, inspected, the spindle was replaced, and the valve seats were lapped.
Entire Set of 12 Valves Removed spindle nut and cotter pin. � This change is considered an enhancement and is not expected to significantly affect valve performance.
The work performed on the valves, in conjunction with the testing performed prior to and during RO-21, provides assurance that the valves are operable. No additional corrective actions are required. The test failure of four MSSVs has caused the maintenance rule (10 CFR 50.65) reliability criterion for the MSSVs to be exceeded, which in turn requires monitoring the MSSVs in accordance with 10 CFR 50.65(a)(1). The system will be returned to 10 CFR 50.65(a)(2) status in FACILITY NAME (1) LER NUMBER (6) DOCKET NUMBER (2) PAGE (3) H. B. Robinson Steam Electric Plant, Unit No. 2 05000261 accordance with the guidance contained in the Progress Energy Nuclear Generation Group procedure. ADM-NGGC-0101, "Maintenance Rule Program," which requires corrective actions, goal setting, and performance monitoring.
V. ADDITIONAL INFORMATION
A. Failed Component Information:
The MSSVs [System Code: SB and Component Code: RV] are style HC-65W, Type C.
manufactured by the Crosby Valve and Gage Company. The SV1-1 and 2 valves are type 6Q8, with a mean seat area of 15.473 square inches. The SV1-3 and 4 valves are type 6R10, with a mean seat area of 22.438 square inches.
B. Previous Similar Events:
On April 5, 2001, SV1-1C failed the as-found lift pressure test acceptance criteria of +/- 3% The valve lift pressure was measured as 1211 psig. The maximum acceptable as-found lift pressure for this valve is 1108 psig. The valve spindle was replaced and the guide bearing clearance was increased to reduce the possibility of valve binding. The valve performance, although found to be out of tolerance during testing on October 9, 2002, was substantially improved as demonstrated by the as-found lift pressure test data (i.e., in 2001 the as-found lift pressure was 1211 psig and in 2002 the as-found lift pressure was 1126 psig).
FACILITY NAME (1) PAGE (3) LER NUMBER (6) DOCKET NUMBER (2) H. B. Robinson Steam Electric Plant, Unit No. 2 05000261 Figure 1 Main Steam Safety Valve Diagram Spindle Nut and Cotter Pin Thrust Bearing Spindle Rod Spring Guide Bearing Guide -al-- Valve Outlet Valve Inlet