05000260/LER-2007-002
| Browns Ferry Unit 2 | |
| Event date: | 05-16-2007 |
|---|---|
| Report date: | 07-16-2007 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 2602007002R00 - NRC Website | |
I. PLANT CONDITION(S)
At the time of the subject past operability condition, Units 2 and 3 operated at 100 percent thermal power (approximately 3458 megawatts thermal). Unit 3 was unaffected by the operability condition.
Unit 1 was shutdown and was not affected by the identified condition.
II. DESCRIPTION OF EVENT
A. Event:
Following the completion of 0-SR-3.4.3.1.b, Bench Test Relief Valves As-Found, on the main steam relief valves (MSRV) [SB] removed from Unit 2 following Cycle 14 operation, TVA has determined that four of the thirteen tested valves mechanically actuated at pressures greater than 3 percent above their Technical Specifications (TS) setpoint, thus; deemed inoperable.
Unit 2 TS Limiting Condition for Operation (LCO) 3.4.3 requires that 12 MSRVs be operable in reactor modes 1, 2, and 3. If less than 12 MSRVs are operable, the unit is to be placed into Mode 3 (hot shutdown) 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 BFN Unit 2 has 13 installed MSRVs, the inoperability of more than one MSRV would require the above required actions be implemented. The setpoint drift condition was not identified while the reactor was in reactor modes 1, 2, or 3, however; the MSRV valve disk-seat bonding issue in boiling water reactors is a known phenomenon, and is deemed to have developed while the valves were in service during Unit 2 Cycle 14 operations. 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.
B. Inoperable Structures, Components, or Systems that Contributed to the Event:
None.
C. Dates and Approximate Times of Major Occurrences:
May 2006� Refurbished MSRV pilot cartridges with TS required setpoints were installed on Unit 2 during mid-cycle outage.
February 2007 Unit 2 Cycle 14 operation is concluded and refueling outage for Cycle 14 is commenced. During the refueling outage, refurbished MSRV pilot cartridges with TS required setpoints were installed on Unit 2.
May 2007 TVA determined the as-found lift setpoint on four valves exceeded the allowable TS value during Cycle 14 operation.
D. Other Systems or Secondary Functions Affected
None.
E. Method of Discovery The out-of-tolerance lift setpoints determined during performance of 0-SR-3.4.3.1.b at the Wyle Laboratories, Huntsville, Alabama facility following the Unit 2 Cycle 14 refueling outage.
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 TSs.
B. Root Cause The root cause of this condition is the generic 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 condition allows 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 Cycle 14 operation is summarized in the following table.
The shaded table values indicate test results outside the 3% tolerance.
MSRV Pilot Valve As-Found Setpoint Data for Unit 2 Cycle 14 Service Dates Actual Setpoint/ Percent Drift Valve Position Nominal Setpoint Pilot Valve Serial # From To 1st test 2nd test 3rd test 2-PCV-01-004 1155 1255 05/24/2006 02/21/2007 1162/ 0.6% 1153/ -0.2% 1153/ -0.2% 2-PCV-01-005 1145 1253 05/24/2006 02/21/2007 1191/ 4.0% 1160/ 1.3% 1160/ 1.3% 2-PCV-01-018 1145 1240 05/24/2006 02/21/2007 1160/ 1.3% 1148/ 0.3% 1141/ -0.3% 2-PCV-01-019 1135 1256 05/24/2006 02/21/2007 1113/ -1.9% 1118/ -1.5% 1123/ -1.1% 2-PCV-01-022 1145 1017 05/24/2006 02/21/2007 1143/ -0.2% 1142/ -0.3% 1147/ 0.2% 2-PCV-01-023 1135 1259 05/24/2006 02/21/2007 1136/ 0.1% 1130/ -0.4% 1131/ -0.4% 2-PCV-01-030 1145 1232 05/24/2006 02/21/2007 1 1204/ 5.2% 1149/ 0.3% 1161/ 1.4% 2-PCV-01-031 1135 1260 05/24/2006 02/21/2007 1116/ -1.7% 1110/ -2.2% 1097/ -3.3% 2-PCV-01-034 1135 1264 05/24/2006 02/21/2007 i 1259/ 10.9% 1132/ -0.3% 1132/ -0.3% 2-PCV-01-041 1155 1019 05/24/2006 02/21/2007 1216/ 5.3% 1161/ 0.5% 1159/ 0.3% 2-PCV-01-042 1155 1262 05/24/2006 02/21/2007 1158/ 0.3% 1150/ -0.4% 1149/ -0.5% 2-PCV-01-179 1155 1263 05/24/2006 02/21/2007 1152/ -0.3% 1147/ -0.7% 1138/ -1.5% 2-PCV-01-180 1155 1078 05/24/2006 02/21/2007 1153/ -0.2% 1159/ 0.3% 1170/ 1.3% 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 forming 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 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 the pilot valve is actuated the bond is broken and 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.
Areva NP Browns Ferry Unit 2 Cycle 14 Plant Transient Analysis was used to verify that the reactor vessel overpressure protection function of the MSRVs was acceptable. The analysis assumed 6 setpoint is assumed out of service). TVA assumed that 3 valves opened before 1203.1 psig, 4 valves opened before 1213.7 psig, and 5 valves opened before 1224.3 psig.
Considering the as-found test data for Unit 2 Cycle 14 MSRVs, 10 valves opened before 1203.1 psig, and an additional valve each opened at 1204 psig and 1216 psig. The remaining valve is assumed to not open. The Unit 2 Cycle 14 Cycle reload analysis encompasses the as found condition of the Unit 2 MSRVs. As such, the pressure relief safety objective for the MSRVs would have been satisfied, with no part of the Unit 2 reactor pressure boundary exceeding relevant pressure limits.
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 as part of the site response to the pilot valve disc-seat bonding phenomenon. Since this automatic opening circuitry has been in place at BFN, there has been no observed pressurization transient events (e.g., turbine trip events) in which an MSRV has failed to open when required.
The as-found test data indicates that the Safety Objective of the Nuclear Pressure Relief System was met. The automatic opening circuitry provides additional defense-in-depth to assure the pressure relief function. The subject past-inoperability condition 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 Unit 2 Cycle 14 refueling outage in February of 2007. Each of the replacement cartridges demonstrated a lift setpoint within the TS requirements during bench-testing prior to installation.
B. Corrective Actions to Prevent Recurrence) Work orders tracked refurbishment or replacement of the pilot valves having lift setpoint out of tolerance.
(1)TVA does not consider these corrective actions regulatory commitments. The completion of these actions will be tracked in TVA's Corrective Action Program.
VII.�ADDITIONAL INFORMATION A. Failed or Degraded Components Target Rock MSRV model No. 7567F 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.
Though this phenomenon has only a relatively small impact on the MSRV function and because of the compensatory hardware mitigation which has been installed at BFN, BFN is continuing to work with other industry stakeholders toward the total elimination of this issue.
C. Additional Information
Browns Ferry corrective action document PER 12944.
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; therefore, did not involve a loss of normal heat removal consideration.
VIII. COMMITMENTS
None.