05000259/LER-2008-003
| Browns Ferry | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 2592008003R00 - NRC Website | |
I. PLANT CONDITION(S)
At the time the inoperability condition was identified, Units 1, 2, and 3 were at 100 percent power (3458 Megawatts thermal) and unaffected by the event.
II. DESCRIPTION OF EVENT
A. Event:
On December 12, 2008, TVA determined that 10 of the 13 Main Steam Relief Valves (MSRVs) [SB], removed from Unit 1 following Cycle 7 operation mechanically actuated at pressures greater than 3 percent above their Technical Specifications (TS) setpoint, thus were inoperable for an unknown time frame during Cycle 7 operation. Unit 1 TS limiting condition for operation (LCO) 3.4.3 requires that twelve (12) MSRVs are operable in reactor modes 1, 2, and 3. If less than twelve MSRVS are operable, the unit is to be placed in Mode 3 hot shutdown within 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />. As such, it is probable that Unit 1 operated outside the TSs longer than allowed by the TSs. Therefore, TVA is submitting this report in accordance with 10 CFR 50.73(a)(2)(i)(B), 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 Maior Occurrences:
MSRV pilot cartridges with TS required setpoints were installed on Unit 1 during the Unit 1 recovery effort.
October 25, 2008� Unit 1 Cycle 7 operation is concluded and the refueling outage is commenced.
December 12, 2008� TVA determined that the as-found lift setpoint in 10 valves exceeded the allowable TS value during the operating cycle.
D. Other Systems or Secondary Functions Affected None.
E. Method of Discovery The out-of-tolerance lift setpoints were identified during the performance of 0-SR-3.4.3.1.b, Bench Test Relief Valves As-Found, at Wyle Laboratories, Huntsville, Alabama.
F. Operator Actions, None.
G. Safety System Responses None.
III. CAUSE OF THE EVENT
A. Immediate Cause
The immediate cause for this reportable condition is an undetectable out-of-tolerance high-lift setpoint drift condition on the MSRVs which existed for longer than allowed by the TSs.
B. Root Cause
The root cause of this condition is a generic industry issue, MSRV pilot valve disc-seat corrosion bonding. A metal oxide film that develops during normal reactor operation which results in a bonding between seat and the disc, which adds resistance to the pressure needed to open the relief valve.
C. Contributing Factors
None.
IV. ANALYSIS OF THE EVENT
The condition being reported is the operation of Unit 1 in a manner prohibited by the TSs. The as-found valve lift setpoints following Unit 1 Cycle 7 operation in summarized in the following table.
Unit 1 Cycle 7 As-Found Lift Setpointsm Valve Position Serial MSRV TS 1st test/dev. 2nd test/dev. 3rd test/dev.
Number Setpoint 1-PCV-001-0004 1024 1155 1186/2.7% 1185/2.6% 1179/2.1% 1-PCV-001-0005 1268 1145 1177/2.8% 1162/1.5% 1158/1.1% 1-PCV-001-0018 1266 1145 1171/2.3% 1160/1.3% 1-PCV-001-0019 1234 1135 1151/1.4% 1141/0.5%was% 1-PCV-001-0022 1070 1145 1158/1.1% 1144/-0.1% 1-PCV-001-0023 1022 1135 1166/2.7% 1163/2.5% 1-PCV-001-0030 1258 1145 "" 1163/1.6% 1155/0.9% 1-PCV-001-0031 1023 1135 1159/2.1% 1141/0.5% 1145/0.9% 1-PCV-001-0034 1084 1135 1134/-0.1% 1131/-0.4% 1-PCV-001-0041 1261 1155 SUM 1175/1.7% 1-PCV-001-0042 1064 1155 1182/2.3% 1184/2.5% 1-PCV-001-0179 1254 1155 f5FAM::„ 1164/0.8% 1153/-0.2% 1-PCV-001-0180 1265 1155 1165/0.9% 1158/0.3% (1) The shaded values indicate test results outside the TSs required 3 percent tolerance.
The setpoint drift is the result of corrosion bonding at the pilot valve disc-seat interface. The material construction of the pilot valve seat and disk is Stellite 6B. This material has exceptional hardness and wear characteristics. However, when placed in an operating environment typical of a Boiling Water Reactor, the steam exposed surfaces can oxidize forming a surface corrosion film.
This corrosion forms a bond between the valve seat and disc. The bond adds to the resistance of the setpoint adjustment spring pressure necessary to open the valve and increases the as-left setpoint of the valve. Generally, once the pilot valve is actuated the corrosion bond is broken, subsequent lift setpoints are within the TS required tolerance.
V. ASSESSMENT OF SAFETY CONSEQUENCES
The safety consequences of this event were not significant. Reactor Vessel overpressure evaluation performed for Unit 1 using the Unit 1 Cycle 7 Reload ASME overpressure and plant transient analysis at 3458 MWT demonstrates compliance with the ASME upset limit of 1375 psig for peak vessel pressure and dome pressure Safety Limit of 1325 psig. The evaluation of the as-found data from the Unit 1 Cycle 7 MSRVs realized a peak reactor vessel pressure of 1332 psig and a maximum steam dome pressure of 1304 psig. An anticipated transient without scram overpressure analyses performed using the limiting Unit 1 event resulted in a peak vessel pressure of 1463 psig demonstrated compliance with the ASME Service level C Limit of 1500 psig. As such, the pressure relief safety objective of the MSRVs was satisfied during the operating cycle.
BFN has previously installed an electronic logic which automatically opens the MSRVs as appropriate during pressurization transients. The electronic logic, although not safety related utilizes high-quality instrumentation that has proven to be very reliable, largely negates the impact on safety presented by this condition. Based on the above, TVA concludes that the health and safety of the public was not affected by this event.
VI. CORRECTIVE ACTIONS
A. Immediate Corrective Actions
All MSRV pilot cartridges were replaced during the Unit 1 Cycle 7 refueling outage. 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 (2) The BFN MSRVs are Target Rock Model 7567F two-stage safety/relief valves. The valve is a leak tolerant valve; however, it exhibits significant in-service setpoint drift due to corrosion bonding to the pilot disc to seat. The current pilot valve seats are constructed from erosion and wear resistant Stellite 6B. The Stellite alloy develops a hard, metal-oxide corrosion layer on the pilot disc. The corrosion bonding requires additional force to break the crystalline structure. The additional force adds to the valves mechanical setpoint and is observed as setpoint apparent drift.
To combat the setpoint drift problem, BFN installed platinum coated pilot valves in the discs on the thirteen Unit 1 MSRVs during the fall outage in 2008. TVA plans to install platinum coated discs on Unit 2 in the spring of 2009 and Unit 3 in the spring of 2010 on all MSRVs. Brunswick and Fermi plants have upgraded to the platinum plated pilot disc which has helped mitigate the corrosion bonding between the pilot discs and improved the setpoint lift values.
VII. ADDITIONAL INFORMATION
A. Failed Components
None.
2 NA does not consider the Corrective Actions to Prevent Recurrence regulatory requirements. The completion of the corrective actions is tracked in TVA's Corrective Action Program.
B. PREVIOUS LERS ON SIMILAR EVENTS
Numerous previous reports on similar events have been made by BFN. BFN plans to implement corrective actions that have been successful at other boiling water reactors.
C. Additional Information
Corrective action document for this report is PER 159200.
D. Safety System Functional Failure Consideration:
This event is not a safety system functional failure according to NEI 99-02.
E. Scram With Complications Consideratio_n:
This event was not a complicated scram according to NEI 99-02.
VIII. COMMITMENTS
None.