05000296/LER-2008-002
| Browns Ferry Unit 3 | |
| Event date: | 06-02-2008 |
|---|---|
| Report date: | 07-31-2008 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 2962008002R00 - 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 June 2, 2008, TVA determined that 7 of the 13 Main Steam Relief Valves (MSRVs) [SB], removed from Unit 3 following Cycle 13 operation mechanically actuated at pressures greater than 3 percent above their Technical Specifications (TS) setpoint, thus inoperable. One valve exhibited leakage past the seat, and the lift pressure could not be verified. Unit 3 TS limiting condition for operation (LCO) 3.4.3 requires that twelve (12) MSRVs be 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 3 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 Major Occurrences:
March 2006 � Refurbished MSRV pilot cartridges with TS required setpoints are installed on Unit 3 during a scheduled refueling outage.
March 2008 � Unit 3 Cycle 13 operation is concluded and the refueling outage is commenced.
June 2, 2008� TVA determined that the as-found lift setpoint in 7 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 facility following the Unit 3 Cycle 13 refueling outage.
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 3 in a manner prohibited by the TSs. The as-found valve lift setpoint s following Unit 3 Cycle 13 operation in summarized in the following table.
Unit 3 Cycle 13 As-Found Lift
MSRV
Setpoint Setpointsw 1st test/ dev.
2nd test/ dev.
Valve Position Serial Number 3rd test/ dev.
3-PCV-01-004 1016 1155 1145 12027 4.1% 1194? 3.4% 11747 1.6% 3-PCV-01-005 1018 Valve could not be tested dim to leakage past seat.
3-PCV-01-018 1076 1145 11-71/ 2.4% 114131 0.3% I 1147/ . 0.2% , 3.7% 3-PCV-01-019 1034 1135 If 1191/ 4.9% 1177/ 1183/ 4.2% 3-PCV-01-022 1028 1145 1165/ 1.7% 1164/ 1.7% 1159/ 1.2% 2.5% 3-PCV-01-023 1059 1135 11867 4.5% 1163/ 1167/ 2.8% 3-PCV-01-030 1075 1145 12017 4.9% 11631 11537 1.6% 0.7% 3-PCV-01-031 1020 1135 11
- 17 5_1$% 1151)/ 1.3% I 11497 1.2% 3-PCV-01-034 1033 1135 1141/ 0.5% 1141/ 113ff 0.5% 0.1% 3-PCV-01-041 1032 1155 1208/ 4.6% 1190/ 1182/ 3.0% 2.3% 3-PCV-01-042 1027 1155 1157/ 0.2% 1162/ 1156/ . 0.6% . 0.1% 1131/ 1131/ -2.1% -2.1% 3-PCV-01-179 1085 1155 1162/ 0.6% 3-PCV-01-180 1068 1155 1246/ 1164/ 1160/- 7.9% 0.8% 0.4% (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 the 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 requires tolerance.
V. ASSESSMENT OF SAFETY CONSEQUENCES
The safety consequences of this event were not significant. An evaluation, using the Unit 3 Cycle Cycle 13 MSRVs was performed. The evaluation did not include the valve that exhibited seat leakage. The evaluation found a lower plenum pressure of 1311 psig and a steam dome pressure of 1280 psig. These pressures are less than the limiting pressures of 1375 and 1325 psig for the reactor vessel heads. 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, WA 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 3 Cycle 14 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 will install platinum coated pilot valves in the discs on the thirteen Unit 1 MSRVs in the upcoming fall outage in 2008. WA 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.
(2) TVA does not consider the corrective action a regulatory requirement. The completion of the action will be tracked in TVA's Corrective Action Program.
VII. ADDITIONAL INFORMATION
A. Failed Components
None.
B. Previous LERs on Similar Events Numerous previous reports on similar events have been made by BFN. Starting with Unit 1 in the fall of 2008, BFN will implement corrective actions that have been successful at other Boiling Water Reactors. This should help correct the issue at BFN.
C. Additional Information
Corrective action document for this report is PER 146189.
D. Safety System Functional Failure Consideration:
This event is not a safety system functional failure according to NEI 99-02.
E. Scram With Complications Consideration:
This event was not a complicated scram according to NEI 99-02.
VIII. COMMITMENTS
None.