05000260/LER-2005-008
| Browns Ferry Unit 2 | |
| Event date: | 04-26-2005 |
|---|---|
| Report date: | 02-15-2007 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| Initial Reporting | |
| 2602005008R00 - NRC Website | |
I. PLANT CONDITION(S)
The subject past inoperability condition related to main steam relief valve (MSRV) lift setpoint drift was identified through bench testing following the pilot valves' removal from Unit 2. The condition is deemed to have existed during portions of Unit 2 operating cycles 11, 12, and 13. The accompanying plant conditions included all modes of operation from cold shutdown and refueling to full-power. Unit MSRVs is addressed separately in LERs 296/2004-003 and 296/2006-001. Unit 1 was shutdown and defueled over this interval and was not affected by the identified condition.
II. DESCRIPTION OF EVENT
A. Event:
Testing of MSRVs [SB] removed from Unit 2 following operating cycles 11, 12, and 13 revealed that, in each set of valves tested, multiple valves mechanically actuated at pressures greater than 3% above their nominal setpoint. The Unit 2 Technical Specifications (TS) Limiting Condition for Operation (LCO) 3.4.3 requires that 12 MSRVs be operable in reactor Mode 1 (power operation), Mode 2 (startup), and Mode 3 (hot shutdown). If less than 12 MSRVs are operable, the unit is to be placed into Mode 3 within 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> and Mode 4 (cold shutdown) within 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />. Since each BFN unit has 13 installed MSRVs, any concurrent MSRV inoperability would require the above actions to occur. While the setpoint-drift conditions were not identified until after the valves' removal from the plant, MSRV pilot valve disc-seat corrosion bonding in boiling water reactor applications is a known phenomenon, and the condition is deemed to have developed and existed while the valves were in service.
The exact intervals during which these MSRVs operated with lift setpoints beyond the TS allowable values is unknown, but, as the TS required action completion time is only 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />, it is probable that the allowed time frame was exceeded. Therefore, in accordance with 10 CFR 50.73(a)(2)(i)(B), WA is reporting this event as any operation or condition prohibited by the plant's Technical Specifications.
Similar MSRV performance prior to calendar year 2001 was reported by TVA to NRC via LER.
However, WA did not report the failure to meet TS following the Unit 2 cycles 11, 12, and 13 in 2001, 2003 and 2005. The failure to report the occurrences have been addressed by the TVA Corrective Action Program. This LER, 260/2005-008, addresses the past conditions (i.e., 2001, 2003, and 2005) which had not been previously reported.
B. Inoperable Structures, Components, or Systems that Contributed to the Event:
None.
C. Dates and Approximate Times of Major Occurrences:
March 2001 Unit 2 was shutdown for the Cycle 11 refueling outage.
During the outage, refurbished MSRV pilot cartridges with properly adjusted setpoints were installed on Unit 2.
April 2001 � Unit 2 Cycle 12 operation commenced.
June 2001 The relevant MSRV setpoint determination surveillance procedure for the removed pilot cartridges was formally completed, documenting that 4 as-found lift setpoints exceeded the allowable TS value during Cycle 11 operation.
February 2003 Unit 2 was shutdown for the Cycle 12 refueling outage.
During the outage, refurbished MSRV pilot cartridges with properly adjusted setpoints were installed on Unit 2.
March 2003 � Unit 2 Cycle 13 operation commenced.
June 2003 The relevant MSRV setpoint determination surveillance procedure for the removed pilot cartridges was formally completed, documenting that 5 as-found lift setpoints exceeded the allowable TS value during Cycle 12 operation.
March 2005 Unit 2 was shutdown for the Cycle 13 refueling outage.
During the outage, refurbished MSRV pilot cartridges with properly adjusted setpoints were installed on Unit 2.
April 2005 � Unit 2 Cycle 14 operation commenced.
April 2005 The relevant MSRV setpoint determination surveillance procedure for the removed pilot cartridges was formally completed, documenting that 6 as-found lift setpoints exceeded the allowable TS value during Cycle 13 operation.
D. Other Systems or Secondary Functions Affected
None.
E. Method of Discovery
The out-of-tolerance lift setpoints were discovered during bench testing at the Wyle Laboratories Huntsville, Alabama facility following the Unit 2 Cycle 11, Cycle 12, and Cycle 13 refueling outages.
F. Operator Actions
None.
G. Safety System Responses
None.
III. CAUSE OF THE EVENT
A. Immediate Cause
The immediate cause of this reportable condition was an undetectable out-of-tolerance-high lift setpoint condition on required MSRVs which most likely existed for longer than allowed by the unit's TS during Unit 3 Cycles 11, 12, and 13 operations respectively.
B. Root Cause
The root cause of this condition was the long-identified issue of MSRV pilot valve disc-seat corrosion bonding. The corrosion bond is a metal oxide film that develops during normal reactor operations. Corrosion occurs because the MSRV pilot valve is located at a stagnant (non- flowing) high point in the main steam piping. The stagnant conditions allow steam to condense in the vicinity of the pilot valve, resulting in an environment consisting of moisture and non- condensable gases. The principal non-condensable gases present are oxygen and hydrogen.
The oxygen combines with the exposed internal metal surfaces to form corrosion products.
C. Contributing Factors
None.
IV. ANALYSIS OF THE EVENT
The condition being reported is operation of Unit 2 in a manner prohibited by the TS. The TS require the MSRV mechanical actuation as-found setpoint to be within +3% of the nominal actuation value.
The as-found valve lift setpoints following the Operating Cycles 11, 12, 13 operating cycles are summarized in the tables below. The shaded table values indicate test results outside the 3% tolerance.
Unit 2 Cycle 11 As-Found Lift Setpoints Service Dates 1st test/ 2nd 3rd 4th test/ Valve Position Nominal Serial # dev. test/ test/ dev.
Setpoint From To dev. dev.
3-PCV-01-004 1155 1026 4/25/1999 3/18/2001 1185/ 2.6% 1172/ 1.5% n/a 1171/ 1.4% 3-PCV-01-005 1145 1079 4/25/1999 3/18/2001 1143/ -0.2% 1137/ -0.7% n/a 1133/ -1.0% 3-PCV-01-018 1145 1017 4/25/1999 3/18/2001 1146/ 0.1% 1153/ 0.7% n/a 1152/ 0.6% 3-PCV-01-019 1135 1084 4/25/1999 3/18/2001 1169/ 3.0% 1152/ 1.5% 1146/ 1126/ -0.8% 1.0% 3-PCV-01-022 1145 1232 4/25/1999 3/18/2001 1258/ 9.9% 1161/ 1.4% 1150/ 1154/ 0.8% 0.4% 3-PCV-01-023 1135 1031 4/25/1999 3/18/2001 1179/ 3.9% 1164/ 2.6% 1140/ 1148/ 1.1% 0.4% 3-PCV-01-030 1145 1076 4/25/1999 3/18/2001 1112/ -2.9% 1112/ -2.9% n/a 1113/ -2.8% 3-PCV-01-031 1135 1072 4/25/1999 3/18/2001 1264/ 11.4% 1136/ 0.1% 1129/ -0.5% 1132/ -0.3% 3-PCV-01-034 1135 1022 4/25/1999 3/18/2001 1149/ 1.2% 1147/ 1.1% 1147/ 1.1% n/a Unit 2 Cycle 11 As-Found Lift Setpoints Service Dates 3-PCV-01-041 1155 1015 4/25/1999 3/18/2001 1170/ 1.3% 1170/ 1.3% n/a 1167/ 1.0% 3-PCV-01-042 1155 1032 4/25/1999 3/18/2001 1171/ 1.4% 1169/ 1.2% 1149/ -0.5% n/a 3-PCV-01-179 1155 1016 4/25/1999 3/18/2001 1167/ 1.0% 1160/ 0.4% 1158/ 0.3% 1158/ 0.3% 3-PCV-01-180 1155 1014 4/25/1999 3/18/2001 1193/ 3.3% 1157/ 0.2% 1152/ 1159/0 .3% -0.3% Unit 2 Cycle 12 As-Found Lift Setpoints Service Dates Valve Position Nominal Setpoint Serial # From To 1st test/ dev.
2nd test/ dev.
3rd test/ dev.
4th test/ dev.
3-PCV-01-004 1155 1233 4/28/2001 2/24/2003 1171/ 1.4% 1136/ -1.6% 1145/ -0.9% n/a 3-PCV-01-005 1145 1070 4/28/2001 2/24/2003 1184/ 3.4% 1171/ 2.3% 1175/ 2.6% n/a 3-PCV-01-018 1145 1021 4/28/2001 2/24/2003 1150/ 0.4% 1143/ -0.2% 1140/ -0.4% n/a 3-PCV-01-019 1135 1060 4/28/2001 2/24/2003 1202/ 5.9% 1141/ 0.5% 1132/ 1133/ -0.2% -0.3% 3-PCV-01-022 1145 1240 4/28/2001 2/24/2003 1145/ 0.0% 1142/ -0.3% 1136/ -0.8% n/a 3-PCV-01-023 1135 1069 4/28/2001 2/24/2003 1122/ -1.1% 1101/ -3.0% 1095/ -3.5% n/a 3-PCV-01-030 1145 1030 4/28/2001 2/24/2003 1202/ 5.0% 1168/ 2.0% 1157/ 1.0% n/a 3-PCV-01-031 1135 1234 4/28/2001 2/24/2003 1306/ 15.1% 1121/ -1.2% 1123/ -1.1% n/a 3-PCV-01-034 1135 1033 4/28/2001 2/24/2003 1239/ 9.2% 1111/ -2.1% 1106/ -2.6% n/a 3-PCV-01-041 1155 1071 4/28/2001 2/24/2003 1180/ 2.2% 1176/ 1.8% 1165/ 0.9% n/a 3-PCV-01-042 1155 1078 4/28/2001 2/24/2003 1181/ 2.3% 1160/ 0.4% 1150/ -0.4% n/a 3-PCV-01-179 1155 1019 4/28/2001 2/24/2003 1143/ -1.0% 1165/ 0.9% 1155/ 0.0% n/a 3-PCV-01-180 1155 1064 4/28/2001 2/24/2003 1171/ 1.4% 1145/ -0.9% 1145/ -0.9% 1149/ -0.5% (continued on next page) Unit 2 Cycle 13 As-Found Lift Setpoints Service Dates Valve Position Nominal Setpoint Serial # From To 1st test/ dev.
2nd test/ dev.
3rd test/ dev.
4th test/ dev.
3-PCV-01-004 1155 1073 3/15/2003 3/21/2005 1180/ 2.2% 1179/ 2.1% 1164/ 1165/ 0.9% 0.8% 3-PCV-01-005 1145 1018 3/15/2003 3/21/2005 1205/ 5.2% testing terminated due to high pilot leakage 3-PCV-01-018 1145 1065 3/15/2003 3/21/2005 1242/ 8.5% 1138/ - 0.6% 1138/ 1138/ -0.6% -0.6% 3-PCV-01-019 1135 1034 3/15/2003 3/21/2005 1159/ 2.1% 1162/ 2.4% 1143/ 0.7% 1143/ 0.7% 3-PCV-01-022 1145 1028 3/15/2003 3/21/2005 1157/ 1.0% 1146/ 0.1% 1143/ 1145/ 0.0% -0.2% 3-PCV-01-023 1135 1023 3/15/2003 3/21/2005 1199/ 5.6% testing terminated due to high pilot leakage 3-PCV-01-030 1145 1075 3/15/2003 3/21/2005 1285/ 12.2% 1154/ 0.8% 1148/ 0.3% 1143/ -0.2% 3-PCV-01-031 1135 1059 3/15/2003 3/21/2005 1148/ 1.1% 1052/ -7.3% 1056/ -7.0% n/a 3-PCV-01-034 1135 1063 3/15/2003 3/21/2005 1247/ 9.9% 1142/ 0.6% 1135/ 1139/ 0.4% 0.0% 3-PCV-01-041 1155 1027 3/15/2003 3/21/2005 1161/ 0.5% 1180/ 2.2% 1155/ 0.0% n/a 3-PCV-01-042 1155 1085 3/15/2003 3/21/2005 1204/ 4.2% 1180/ 2.2% 1180/ 2.2% n/a 3-PCV-01-179 1155 1024 3/15/2003 3/21/2005 1178/ 2.0% 1165/ 0.9% 1161/ 1165/ 0.9% 0.5% 3-PCV-01-180 1155 1068 3/15/2003 3/21/2005 1181/ 2.3% 1165/ 0.9% 1158/ 1158/ 0.3% 0.3% The setpoint drift is the result of corrosion at the pilot valve disc-seat interface. The material of construction for the pilot valve seat and disc is Stellite 6B. This material has exceptional hardness and erosion characteristics. However, when placed into an operating environment typical of a boiling water reactor, the steam exposed surface areas can oxidize and form a surface corrosion film. The surface corrosion film has a crystalline structure. At the point of contact between the pilot valve seat and disc, the crystalline matrix on the two surfaces can merge and form what is referred to as a corrosion bond between the disc and seat. This bond adds to the resistance of the setpoint adjustment spring which system pressure must overcome to mechanically actuate the pilot valve; therefore, the system pressure necessary to open the valve increases above the as-left setpoint in some proportion to the magnitude of the corrosion present. Generally, once this bond has been broken by opening the pilot valve, the lift setpoints observed on subsequent lifts are within the nominal 3% tolerance.
V. ASSESSMENT OF SAFETY CONSEQUENCES
As stated in the BFN Updated Final Safety Analysis Report (UFSAR) paragraph 4.4.1, the safety objective of the Nuclear System Pressure Relief System is to prevent overpressurization of the nuclear system; this protects the nuclear system process barrier from failure which could result in the uncontrolled release of fission products. The Power Uprate Transient Analysis Task Report, GE-NEB13-01866-05, for BFN determined that the reactor vessel overpressure protection function of the MSRVs is provided if 2 MSRVs open at or before 1184 psig and with an additional 5 (for a total of 7) opening at or before 1194 psig.
Considering the as-found test data for Unit 2 Cycle 11 cited above, it can be seen that 9 valves opened before 1184 psig, and an additional 2 valves opened between 1185 and 1194 psig. Likewise, considering the as-found test data for Unit 2 Cycle 12 cited above, it can be seen that 9 valves opened at pressures at or below 1184 psig. The as-found data for Unit 2 Cycle 13 cited above shows that 7 valves opened before 1184 psig. The cumulative test data shows a few instances beyond the initial opening test where the subject valve opened more than 3% lower than the nominal setpoint.
Since the valves' safety function is to protect against overpressurization, out-of-tolerance low values have no significance in the safety analysis. For all three of the affected operating cycles the as-found performance of the MSRVs is within the relevant analysis.
In addition to the mechanical actuation of the pilot valves via increasing process steam pressure, each MSRV pilot valve can also be actuated by control air pressure via a solenoid valve. The solenoid valve can be energized either manually via control panel hand-switches or by automatic circuitry driven by pressure-sensing logic. The automatic actuation method utilizes high-quality instrumentation, and the circuits have demonstrated high reliability in service. This circuitry was installed on BFN Units 2 and 3 as part of the site response to the pilot valve disc-seat bonding phenomenon, and a design to accomplish this function will be installed on BFN Unit 1 before it returns to power operation. Since this automatic opening circuitry has been in place at BFN, there have been no observed pressurization transient events (e.g., turbine trip events) in which an MSRV has failed to open when demanded.
The formal BFN design analysis requires the opening of 7 MSRVs as described above, and the as found test data shows that this analysis was met. The automatic opening circuitry provides additional defense-in-depth to assure the pressure relief function. The subject past-inoperability conditions did not have any significant impact on the health and safety of the public.
VI. CORRECTIVE ACTIONS
A. Immediate Corrective Actions
All MSRV pilot valve cartridges were replaced during the associated refueling outages. Each of the replacement cartridges demonstrated a properly adjusted lift setpoint during bench-testing prior to installation.
B. Corrective Actions to Prevent Recurrence) Work orders tracked refurbishment of the pilot valves having excessive leakage and/or lift setpoint out of tolerance.
VII. ADDITIONAL INFORMATION
A. Failed or Degraded Components Target Rock MSRV model No. 7567F (1)� TVA does not consider this corrective action a regulatory commitment. The completion of this action will be tracked in TVA's Corrective Action Program.
B. Previous LERs on Similar Events Numerous previous reports on similar events have been made from BFN and other nuclear plants. The physical phenomenon affecting MSRV lift setpoints which results in this reportable condition is well-understood, and it has been the subject of much industry study. Different mitigative approaches have been tested, but none have successfully eliminated the issue. The installation of the instrumentation logic/circuitry which will automatically open the MSRVs as appropriate during pressurization transients largely negates the condition's safety impact. This phenomenon has only a relatively small impact on the MSRV function. BFN has installed compensatory mitigation hardware. BFN is continuing to work with other industry stakeholders toward the total elimination of this issue.
C. Additional Information
Browns Ferry corrective actions for the rework of the valves in Unit 2 Cycles 11, 12 and 13 are documented TVA corrective action documents BFPER 961764, PER 50084 and PER 81376.
The failure to report MSRV setpoint drift is documented in PER 112190.
D. Safety System Functional Failure Consideration:
The condition being reported involves only setpoint drift of varying numbers of MSRVs on Unit 2. The safety/relief function provided by these valves was not compromised at any time. A safety system functional failure did not result from this condition.
E. Loss of Normal Heat Removal Consideration:
The condition being reported did not involve a reactor scram.
VIII.COMMITMENTS None.