05000260/LER-2009-003
| Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber No. N/A 05000 | |
| Event date: | 06-08-2009 |
|---|---|
| Report date: | 07-28-2011 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 2602009003R01 - NRC Website | |
I. PLANT CONDITION(S)
At the time of discovery, Browns Ferry Nuclear Plant (BFN) Unit 2 was in Mode 4 (Cold Shutdown) and unaffected by the event since all 13 of the Safety/Relief Valves [SB] (S/RVs) had been refurbished during the Cycle 15 refueling outage. Unit 1,and Unit 3 were at 100 percent power (3458 Megawatts thermal) and unaffected by the event.
II. DESCRIPTION OF EVENT
A. Event
On June 8, 2009, the Tennessee Valley Authority (TVA) determined, as a result of offsite pressures greater than the allowed 3 percent above their Technical Specifications (TS) setpoint. The S/RVs were thus inoperable for an indeterminate period during Cycle 15 period of operation. Unit 2 TS Limiting Condition for Operation 3.4.3 requires the safety function of twelve (12) S/RVs to be operable in reactor modes 1, 2, and 3. With one or more required S/RVs inoperable, the unit is required to be placed in Mode 3 (Hot Shutdown) operated longer than allowed by the TS.
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 There were no inoperable structures, components, or systems that contributed to the event.
C. Dates and Approximate Times of Malor Occurrences February - April 2007� S/RV pilot cartridges with TS required setpoints were installed on Unit 2 during the Cycle 14 refueling outage.
April 25, 2009� Unit 2 Cycle 15 operation is concluded and the refueling outage is commenced.
June 8, 2009 � TVA determined that the as-found lift setpoint in 7 S/RVs exceeded the allowable TS limit during Cycle 15.
D. Other Systems or Secondary Functions Affected
There were no other systems or secondary functions affected.
E. Method of Discovery
The out-of-tolerance lift setpoints were identified during the performance of Surveillance Procedure 0-SR-3.4.3.1.B, Bench Test Relief Valves As-Found, at the test facilities of Wyle Laboratories, Huntsville, Alabama.
F. Operator Actions
There were no operator actions.
G. Safety System Responses
There were no safety system responses.
III. CAUSE OF THE EVENT
A. Immediate Cause The immediate cause for this event is undetectable, out-of-tolerance, high-lift setpoints on 7 of 13 S/RVs, which existed longer than allowed by the TS.
B. Root Cause The root cause of the S/RVs exceeding the setpoint lift tolerance is corrosion bonding between the disc and seat while the valve is in service, which is a generic industry issue.
C. Contributing Factors There were no contributing factors.
IV. ANALYSIS OF THE EVENT
The condition being reported is the operation of Unit 2 in a manner prohibited by the plants TS.
The as-found S/RV lift setpoints following Unit 2 Cycle 15 operation are summarized in the following table.
Unit 2 Cycle 15 As-Found Lift Setpoints(1) Valve Position Serial Number MSRV TS Setpoint 1st Test/Dev. 2nd Test/Dev. 3rd Test/Dev.
2-PCV-001-0004 1026 1155 1201/4.0% 1178/2.0% 1172/1.5% 2-PCV-001-0005 1061 1145 1181/3.1% 1175/2.6% 1179/3.0% 2-PCV-001-0018 1021 1145 1237/8.0% 1178/2.9% 1176/2.7% 2-PCV-001-0019 1060 1135 1137/0.2% 1136/0.1% 1135/0.0% 2-PCV-001-0022(2) 1065 1145 N/A N/A N/A 2-PCV-001-0023 1031 1135 1155/1.8% 1154/1.7% 1152/1.5% 2-PCV-001-0030 1130 1145 1174/2.5% 1157/1.0% 1149/0.3% 2-PCV-001-0031 1072 1135 1271/12.0% 1158/2.0% 1152/1.5% 2-PCV-001-0034 1063 1135 1163/2.5% 1167/2.8% 1151/1.4% 2-PCV-001-0041 1071 1155 1262/9.3% 1185/2.6% 1174/1.6% 2-PCV-001-0042 1073 1155 1145/-0.9% 1172/1.5% 1152/-0.3% 2-PCV-001-0179 1014 1155 1164/0.78% 1170/1.3% 1158/0.3% 2-PCV-001-0180 1029 1155 1254/8.6% 1179/2.1% 1174/1.6% (1) The shaded values indicate test results outside the TS required 3 percent tolerance.
(2) Valve failed to lift at 1300 psig test pressure.
The BFN S/RVs are Target Rock Model 7567F two-stage S/RVs. The valve is a leak tolerant valve; however, it exhibits significant in-service setpoint drift because of corrosion bonding between the valve seat and pilot disc. The 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. When installed in an operating environment typical of a boiling water reactor, the steam exposed surfaces can oxidize, forming a surface corrosion film. This corrosion film 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 pressure required to actuate the valve. Generally, once the pilot valve is actuated, the corrosion bond is broken; the subsequent lift setpoint is 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 2 using the Unit 2 Cycle 15 Reload American Society of Mechanical Engineers (ASME) Overpressure and Plant Transient Analysis at 3458 MWT demonstrates compliance with the ASME upset limit of 1375 psig for peak vessel pressure and Cycle 15 S/RVs realized a peak reactor vessel pressure of 1347 psig in the vessel lower plenum and a maximum steam dome pressure of 1316 psig. The anticipated transient without scram overpressure analyses performed using the limiting Unit 2 event resulted in a peak vessel pressure of 1474 psig in the vessel lower plenum and a maximum steam dome pressure of 1455 psig which demonstrates compliance with the ASME Service Level C Limit of 1500 psig.
Therefore, the pressure relief safety objective of the S/RVs was satisfied during the operating cycle.
BFN has previously installed an electronic logic which automatically opens the S/RVs as appropriate during pressurization transients. The electronic logic, although not safety related, utilizes high-quality instrumentation that has proven to be very reliable. During a pressure transient event in the relief mode, safety grade pressure sensors actuate the S/RVs. When the relief mode is actuated, the setpoint spring preload is removed from the pilot disc, and full differential pressure is then present across the pilot disk. This electronic logic largely negates the impact on safety presented by this condition.
Therefore, WA concluded that there was no significant reduction in the health and safety of the public for this event.
VI. CORRECTIVE ACTIONS - The corrective actions are being managed by TVA's corrective action program.
A. Immediate Corrective Actions
All S/RV pilot cartridges were replaced during the Unit 2 Cycle 15 refueling outage. Prior to installation, each of the replacement cartridges demonstrated a lift setpoint within the TS requirements during bench testing.
B. Corrective Actions to Prevent Recurrence
Industry experience has demonstrated that the use of platinum coating on the S/RV pilot valve discs helps mitigate corrosion bonding and associated setpoint drift problems. The S/RV pilot cartridges replaced during the Unit 2 Cycle 15 refueling outage include platinum coated pilot discs, with the exception of 2-PCV-001-0023. A cartridge with a platinum coated disc was unavailable for this S/RV, so a pilot assembly without platinum coating was installed for 2-PCV-001-0023.
VII. ADDITIONAL INFORMATION
A. Failed Components
The failed components were the 7 S/RVs.
B. PREVIOUS LERs or SIMILAR EVENTS TVA has submitted previous reports on similar events at BFN: LERs 50-259/2008-003-00, 50-260/2007-002-00, and 50-296/2008-002-00.
C. Additional Information
Problem Evaluation Report (PER) 175990 documents the results from Unit 2 Cycle 15 for the S/RVs. However, PER 175990 corrective actions taken state that all S/RVs were platinum coated for Unit 2 Cycle 16. This statement was incorrect. S/RV 2-PCV-001-0023 initially had a platinum coated cartridge until troubleshooting exhausted all spares. BFN then installed a pilot assembly without platinum coating. PER 372047 documents the non-platinum coated pilot assembly that was installed during Unit 2 Cycle 15 refueling outage that actuated above its TS setpoint plus the 3 percent allowance.
D. Safety System Functional Failure Consideration:
This event is not a safety system function 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
There were no commitments.