05000333/LER-2015-002
| Event date: | 06-01-2015 |
|---|---|
| Report date: | 07-30-2015 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| 3332015002R00 - NRC Website | |
Background
The James A. FitzPatrick Nuclear Power Plant (JAF) used eight (8) two-stage and three (3) three-stage Target Rock Safety / Relief Valves (S/RV) [SB] for emergency pressure relief through operating Cycle 21. These valves are located inside the primary containment, and relieve pressure from the main steam lines to the torus.
During each refueling outage, approximately one-third of the S/RV main body assemblies and all eleven of the pilot assemblies are removed and replaced with vendor tested and certified components. The main bodies and pilots that are replaced are sent to a vendor facility for testing, refurbishment, and certification. The test results for pilot assemblies removed in 2014, during Refueling Outage 21, identified seven (7) S/RV pilot assemblies that exhibited upward setpoint drift in excess of the Technical Specification (TS) setpoint tolerance.
JAF has extensive internal Operating Experience in this regard. Causal evaluations identified corrosion bonding as the cause for the upward setpoint drift. Corrosion bonding is a crevice corrosion phenomenon that occurs to highly polished metals in a wetted solution in close proximity to each other. This close proximity (usually a gap of between 0.1 and 100 ȝm) creates a crevice-like condition between the two wetted surfaces setting up the conditions for crevice corrosion to occur. An oxygen rich environment is created by the accumulation of oxygen in the area of the pilot disc due to the breakdown of water into hydrogen and oxygen.
The correct material in the right geometry with exposure to oxygen and high temperature are the conditions which cause corrosion bonding in JAF S/RVs. There is extensive industry experience with corrosion bonding in the Target Rock two-stage S/RVs pilot assemblies.
In order to address the concerns with corrosion bonding, JAF has begun a phased conversion to three-stage Target Rock S/RVs. Industry experience has shown that the three-stage S/RVs are less susceptible to corrosion bonding. The design of the three-stage S/RVs produces a greater mechanical force between the valve disc and seat, resulting in a greater likelihood of overcoming any potential effects of corrosion bonding that might occur.
Event Description
As-Found testing was performed on all eleven Main S/RV pilot assemblies removed in 2014, during Refueling Outage (RO) 21. The testing was conducted by NWS Technologies. The TS setpoint for each S/RV is 1145 psig +/- 3%. During the initial lift test, seven of the eleven pilot assemblies failed to open within the allowed setpoint tolerance of +/-3% (1110.7 to 1179.3 psig). As-Found test results are tabulated below. All three-stage S/RVs passed As-Found testing.
In-service Location Pilot Serial Number First Test (psig) Acceptance Range (1110.7 - 1179.3 psig) 02RV-71B 1045 1250 Unsat 02RV-71D 1052 1191 Unsat 02RV-71G 1110 1231 Unsat 02RV-71H 1195 1235 Unsat 02RV-71J 1218 1277 Unsat 02RV-71K 1217 1243 Unsat 02RV-71L 1235 1203 Unsat Subsequent tests performed on six of the seven pilot assemblies passed the acceptance criteria of +/-3%. Pilot number 1045 (02RV-71B) lifted at 3.5% over the acceptance criteria. The cause for the high lift setpoint on the seven pilot assemblies was determined to be corrosion bonding between the pilot valve disc and seat. Due to a history of failure for these S/RVs, it is assumed that they would have been inoperable at some point during the previous operating cycle.
TS Limiting Condition for Operation 3.4.3 requires nine operable S/RVs when in Modes 1, 2 or 3. Specifically, the LCO states: “the safety function of nine S/RVs shall be operable.” Since seven of the eleven pilot valves were not within the allowable setpoint tolerance, a condition prohibited by the plant's Technical Specification existed during operating Cycle 21.
Due to the Technical Specification inoperability of the seven S/RVs, a condition that could have prevented the fulfillment of a safety function existed during operating Cycle 21.
As described above, all eleven S/RV's in the plant had new (rebuilt and certified) pilots installed during the 2014 refueling outage. Certified pilot valves were installed and tested as required to verify operability. The currently installed S/RVs are operable.
Cause
The cause for the upward setpoint drift was determined to be corrosion bonding between the S/RV pilot disc and seat [Cause Code B]. Conditions conducive to corrosion bonding exist in the S/RV pilot disc and seat at JAF; the correct material in the right geometry with exposure to oxygen and high temperature. With a bond forming between the pilot disc and seat greater force is required to raise the pilot disc off its seat. Since the normal balance of pilot assembly spring force and steam pressure force necessary to lift the pilot disc corresponds to the nominal setpoint of the S/RV, the pilot disc to seat bond results in a higher pilot lift setpoint.
Extent of Condition All two-stage S/RVs are susceptible to upward setpoint drift due to corrosion bonding between the pilot valve disc and seat. This is a recurring industry issue that has been the subject of both Nuclear Regulatory Commission (NRC) and Boiling Water Reactor Owner's Group (BWROG) generic assessments.
Failed Component Identification Manufacturer: Target Rock Corporation Manufacturer Model Number: 7567F-10 NPRDS Manufacturer Code: T020 FitzPatrick Component ID: 02RV-71B, -D, -G, -H, -J, -K, -L
Corrective Actions
Previous corrective actions taken by JAF to address the corrosion bonding issue include:
1. Installed Stellite 21 discs in all eleven S/RVs pilot assemblies during refurbishment at the vendor facility; 2. Installed the electric lift system recommended by the BWROG; 3. Installed enhanced insulation on the S/RVs; 4. Phased replacement of 2-Stage S/RVs with 3-Stage (3 replacements installed during RO-19), which is scheduled for completion prior to exiting Refueling Outage 23.
Safety Significance
There were no actual consequences to the general safety of the public, nuclear safety, industrial safety, or radiological safety associated with this event.
The potential consequences of this event are associated with the over-pressurization of the Reactor Coolant Pressure Boundary (RCPB). The S/RVs provide overpressure protection for the Reactor Coolant Pressure Boundary (RCPB) as required by the ASME Boiler and Pressure Vessel Code. Events similar to the one reported herein may be significant if design limits are challenged. The potential consequences of this event are considered low based on a) the operation and availability of the Electric Lift System, and b) the expected pressure response relative to the As-Found setpoints of all eleven S/RV pilots.
The Electric Lift System is required by Technical Requirement for Operation (TRO) 3.3.I, and ensures that all S/RVs actuate at or below the TS acceptance criteria. This system energizes solenoid valves which supply pneumatic pressure to assist in actuating the S/RVs. The Electric Lift System was verified to be functional for the entire operating cycle.
The RCPB Overpressure Analysis is performed each fuel cycle based on the worst case anticipated transient with nine S/RVs opening at an analyzed upper limit pressure of 1195 psig, and two S/RVs out of service. As- Found testing confirmed that all S/RVs installed during Cycle 21 would have actuated. Therefore, the transient analysis results are very conservative relative to the expected plant response should a limiting transient have occurred. Even if the two S/RVs with the highest As-Found test results (Serial Nos. 1045 and 1218) are assumed to not actuate, the transient pressure response would be milder than analyzed since the average setpoint of the remaining nine S/RV pilots is 15 psi less than the nominal analyzed setpoint.
Similar Events 1. JAF LER-11-003 “Safety Relief Valve Setpoints Outside Allowable Tolerances,” August 8, 2011 2. JAF LER-09-005 “Safety Relief Valve Setpoint Drift,” June 22, 2009.
3. JAF LER-07-001 "Safety Relief Valve Setpoint Drift," August 6, 2007.
4. JAF LER-05-002 “Safety Relief Valve Setpoint Drift,” June 6, 2005.
5. JAF LER-03-002 “Safety Relief Valve Setpoint Drift,” October 16, 2003.
6. JAF LER-01-005 “Safety Relief Valve Setpoint Drift,” August 17, 2001.
7. JAF LER-99-003 “Safety Relief Valve Setpoint Drift,” March 16, 1999.
References
1. JAF Condition Report CR-JAF-2015-02493, Root Cause Evaluation, Seven S/RV Pilots Failed As- Found Testing 2. JAF Condition Report CR-JAF-2013-04098, Two S/RV Pilots Failed As-Found Testing.
3. JAF Condition Report CR-JAF-2007-02108, Root Cause Analysis Report, Seven of ten S/RV pilots failed as-found testing (testing high out of tolerance).
4. JAF Technical Specification 3.4.3 5. JAF Technical Specification Bases 3.4.3