05000286/LER-2012-003
| Indian Point 3 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 2862012003R01 - NRC Website | |
Note: The Energy Industry Identification System Codes are identified within the brackets {}
DESCRIPTION OF EVENT
On May 1, 2012, Engineering was notified by Wyle Laboratories that one of three Pressurizer Code Safety Valves (SV) (RC-PCV-468) {RV} was outside its As-Found lift set point test acceptance criteria (2411 - 2559 psig) The As-Found set pressure testing acceptance criterion for operability is 2485 psig +/-3%. The SVs were removed during the last refueling outage (RO) in the spring of 2011 and sent offsite for testing. Testing was performed within one year of removal as required by the Inservice Testing Program. SV RC-PCV-468 lifted at 2617 psig which is outside its set pressure range. The initial As-Found lift was 5.3% above the 2485 psig set pressure therefore, the 110% design pressure limitation provided by the ASME OM Code-2001 requirement for Class 1 Pressure Relief Valves was not exceeded.
The remaining two SVs both tested satisfactorily. During the RO all three SVs were removed and replaced with certified pre-tested spare SVs. A11 three SVs were found with zero seat leakage. The SVs installed during the RO were As-Left tested to 2485 psig +/-1% with zero seat leakage in accordance with procedure 3-PT-RSA. The condition was recorded in the Indian Point Energy Center (IPEC) Corrective Action Program (CAP) as Condition Report CR-IP3-2012-01403 The pressurizer safety valves, in conjunction with the reactor protection system, provide overpressure protection for the reactor coolant system (RCS) {AB} The pressurizer safety valves (SVs) are totally enclosed pop type, spring loaded, self actuating 6 inch by 6 inch valves manufactured by Crosby Valve Company {C711}, Model HB-BP-86-E. The SVs are designed to prevent the system pressure from exceeding the system Safety Limit (SL) of 2735 psig, which is 110% of the design pressure. The SVs have an active safety function in the open position to prevent overpressure of the RCS. The SVs also have an active safety function in the closed position to prevent the loss of RCS inventory. Three pressurizer SVs (RC-PCV-464, RC-PCV-466, RC-PCV-468) discharge to the Pressurizer Relief Tank (PRT) and are designed with a set pressure of 2485 psig. Acceptable As-Left testing prior to installation requires the SVs to be set at 2485 psig +/-1% with zero seat leakage to compensate for set point drift over its operating cycle. The As-Found set pressure testing acceptance criterion for operability is 2485 psig +/- 3%. The pressureizer SVs are categorized as Class 1 safety relief valves in the Inservice Testing Program. A11 three pressurizer SVs are removed and replaced with pre-tested spares each refueling outage in order to satisfy the ASME OM Code-2001 requirement for Class 1 pressure relief valves. Upon replacement, all three of the removed SVs are tested within one year of removal from the system.
Technical Specification (TS) 3.4.10 (Pressurizer Safety Valves) requires three pressurizer safety valves to be operable with lift settings set at greater than 2460 psig and less than 2510 psig. TS Surveillance Requirement (SR) 3.4.10.1 requires each PSV to be verified operable in accordance with the Inservice Testing Program.
An extent of condition review determined that the condition is limited to the pressurizer SVs and Main Steam Safety Valves (MSSVs) Other than these valves, there are no other relief valves in the RCS or balance of plant that are subjected to the same temperatures, pressures and environment.
Cause of Event
The direct cause of SV RC-PCV-468 lifting greater than 3% of its nominal set point was spring fatigue. The spring assembly tested below the acceptable +/- 5% range.
When the spring rate is found to be lower than design, additional spring compression is required to achieve the same set-pressure. When additional compression is applied, it is possible for the spring-rate to become non-linear over time and the initial actuation would result in an elevated initial set-pressure, then once the spring deflects during the initial lift, the following actuations would be lower as found with RC-PCV-468. The apparent cause was a lack of periodic spring assembly inspection/testing/replacement Preventive Maintenance (PM) There was no periodic spring assembly inspection/test/replacement PM for the pressurizer SVs. The fleet PM template only provides direction to perform disassembly/inspection of spring actuated relief valves on an as required basis. There was no vendor specified maintenance provided.
The original apparent cause was believed to be internal friction within the valve.
However, this cause was ruled out based on the lack of visual evidence. There were no indications of side-loading, internal friction or contact along the length of the spindle, and disc and seat were found to be acceptable with no indications of bonding. Other potential causes that were ruled out by the Failure Modes Analysis were: set point drift, previous incorrect valve setting, over torque of the nozzle ring set screw, foreign material intrusion, damaged/broken internals and dissimilar metal corrosion.
Corrective Actions
The following corrective actions will be performed under Entergy's Corrective Action Program to address the cause and prevent recurrence:
- Valve RC-PCV-468 was shipped to the valve vendor, disassembled and inspected.
A detailed internal examination and failure analysis of RC-PCV-468 was performed by Wyle Labs. The spring assembly was tested at Wyle Labs in accordance with the applicable Crosby spring data sheet. The cause of the failure was identified and findings provided in a Crosby field report.
- SV purchase orders will be revised to include testing spring assemblies.
- A revised inspection and repair plan will be developed to include, in addition to the existing testing/maintenance practices, SV disassembly and complete spring assembly inspection that includes spring measurements, inspection of overall spring condition, and performance of a spring-rate test every 10 years or as directed by Engineering.
- The Relief Valve Component Monitoring database was revised to provide a place to track spring assembly performance and spring-rate testing results.
Event Analysis
The event is reportable under 10CFR50.73(a)(2)(i)(B) The licensee shall report any operation or condition which was prohibited by the plant TS. TS 3.4.10 (Pressurizer Safety Valves) Limiting Condition for Operation (LCO) requires three pressurizer safety valves to be operable with lift settings set greater than 2460 psig and less than 2510 psig. During testing, SV RC-PCV-468 lifted at 2617 psig which is outside its set pressure range acceptance criterion. TS 3.4.10 Condition A (One pressurizer safety valve inoperable) required action A.1 is to restore the inoperable valve to operable status in 15 minutes. This TS action was not performed and the actions of Condition B not implemented. As the cause was determined not to be set point drift, the condition is a TS prohibited condition.
Past Similar Events
A review was performed of Licensee Event Reports (LERs) for the past three refueling cycles for any events reporting TS prohibited conditions due to pressurizer SVs outside their As-Found lift set point test acceptance criteria. No LERs were identified. A review of CRs during this period (2007 - 2011) recording pressurizer SV failures identified CR-IP3-2007-00679 which recorded a failure of PCV-464 whose as-found pressure set point was outside the acceptable range.
Testing of MSSVs during the review period identified MSSV failures to meet as-found lift test acceptance criteria. LER-2011-004 reported the failure of valves MS-47-4 and MS-48-4 to meet test acceptance criteria due to spindle wear and spring skew.
In 2009, LER-2009-002 reported two valves (MS-45-1 and MS-48-3) failed their As- Found lift set point test. The cause was indeterminate but most likely due to set point drift. The Original Equipment Manufacturer (OEM) could not directly relate the indications discovered on the valves' spindles to the As-Found test results.
LERs were reported for Unit 3 MSSV failures in 2007.
No
Safety Significance
This event had no effect on the health and safety of the public. There were no actual safety consequences for the event because there were no events that required the pressurizer SVs. An evaluation was performed on the potential impact of the condition on the accident analysis in the UFSAR and realistic plant response. The realistic plant response to transients would not be impacted, since the pressurizer pressure control system would be available for relieving RCS pressure which includes the pressure spray valves and power operated relief valves (PORVs) The non-Loss of Coolant Accident (non-LOCA) analyses in the UFSAR that could be impacted by the condition were: 1) Loss of load/Turbine Trip (LOL/TT), 2) Loss of normal feedwater/loss of non-emergency AC power (LONF/LOAC), 3) Loss of flow/locked rotor (LOF/LR), 4) Rod withdrawal at power (RWAP) An assessment of the impact of the condition on these non-LOCA transients concluded they are either negligible or insignificant. Pressurizer SV RC-PCV-468 lifted at approximately 2617 psig, which is 5.31% above the nominal set point of 2485 psig.
Before RC-PCV-468 reaches its opening set point of 5.31% (2617 psig), the other two pressurizer SVs (RC-PCV-464, RC-PCV-466) will lift at +0.32% (2493 psig) and +1.69% (2527 psig) and relieve pressure from the RCS. The non-LOCA UFSAR transients all conservatively assume a +4% opening set point, instead of the +/- 3% testing acceptance criterion. A 1.31% increase in RC-PCV-468 opening set point over what was assumed in the UFSAR analysis would be compensated by approximately a 3.6% and 2.3% margin in the other two pressurizer SVs. Thus, the UFSAR Chapter 14 analysis remains valid with the SV condition.