05000298/LER-2007-002
| Docket Number | |
| Event date: | 03-05-2007 |
|---|---|
| Report date: | 04-30-1200 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 2982007002R00 - NRC Website | |
PLANT STATUS
Cooper Nuclear Station (CNS) was in Mode 1 at 100% power at the time of discovery.
BACKGROUND
The pressure relief system includes three American Society of Mechanical Engineers (ASME) code safety [EIIS: SB] and eight safety relief valves (SRVs) [EIIS: SB], all of which are located on the main steam lines [EIIS: SB] within the drywell [EllS: NH], between the reactor vessel [EIIS: RPV] and the first main steam isolation valve [EIIS: ISV]. The safety valves provide protection against overpressurization of the nuclear system and discharge directly into the interior space of the drywell. The relief valves discharge to the suppression pool [EIIS: BT] and provide three main functions: overpressurization relief operation to prevent safety valve opening, overpressure safety operation to prevent nuclear system overpressurization, and depressurization operation (opened automatically or manually) as part of the emergency core cooling system [EIIS: BM,BO]. The nominal set pressure and tolerances for these valves is established in CNS Technical Specification (TS) Surveillance Requirement (SR) 3.4.3.1.
The SRVs installed at CNS are Target Rock Model 7567F two stage pilot actuated valves with pilot assemblies comprised of Stellite 21 pilot discs and Stellite 6 body seats.
During Refueling Outage (RFO) 23, three complete SRVs and five SRV pilot assemblies were removed from the plant and replaced with refurbished certified spares. The SRVs removed from service were shipped to Wyle Laboratories for required testing.
EVENT DESCRIPTION
On February 28 through March 2, 2007, the CNS Main Steam SRVs (three entire valves and 5 pilot assemblies) removed during RFO 23 were as-found tested at Wyle Laboratories, Inc. As-found pressure set point testing of one SRV pilot assembly (serial number 383) exceeded the TS limit. This pilot was formerly installed in functional location MS-RV-71ERV [EIIS: SB,RV].
The pressure set point of this pilot assembly is 1090 psig. The TS SR 3.4.3.1 as-found limits of acceptance are +/- 32.7 psig (1057.3 psig to 1122.7 psig), a 3% band. The first actual safety lift pressure of this pilot assembly was recorded as 1127 psig, approximately 3.4% above the set point. A second lift came in at 1102 psig, which is approximately within 1.1% of the set point.
The other seven SRV pilot assemblies tested satisfactory for the as-found set pressure tests. Based on the test evidence, the failure mechanism is the same as previously reported in LER 2005-002; that is, pilot disc-to-seat corrosion bonding.
The affected SRV pilot assembly is not currently installed in the plant; it was removed in RFO 23 from MS-RV-71ERV. All eight SRV pilot assemblies currently installed in the plant during RFO 23 were refurbished and certified to lift within 1% of set point acceptance criteria prior to installation. Therefore, all currently installed SRVs are capable of performing their intended function.
This phenomenon is an industry-wide issue which has been previously evaluated within the CNS corrective action program. The corrective actions involving installation of a new Stellite 21 pilot disc, improvements in refurbishment practices and CNS Engineering oversight were not yet fully incorporated into pilot assembly 383 during its refurbishment prior to installation in the plant in RFO 22 for Cycle 23.
Pilot assembly 383 was refurbished using its existing pilot disc prior to installation.
BASIS FOR REPORT
This event is being reported as a condition prohibited by plant Technical Specifications per 10CFR50.73(a)(2)(i)(B).
SAFETY SIGNIFICANCE
The actual lift at a pressure of 1127 psig is bounded by the Reload Licensing Report of Cooper Nuclear Station Reload 22 Cycle 23. This analysis assumed 1210 psig for the opening pressure of all eight SRVs and demonstrated that for the limiting transient the peak vessel pressure would be 1311 psig, which is below the ASME code pressure limit of 1375 psig. All eight SRVs lifting well below 1210 psig provides assurance that the ASME code over pressure requirement of 1375 psig will not be exceeded.
The RFO 23 as-found test failure of pilot assembly 383 does not present a challenge to the function of the SRVs to provide their safety-related objective of overpressure protection for the reactor vessel. The SRV pilot assemblies currently installed during RFO 23 have all been refurbished under the corrective actions associated with LERs 2003-002 and 2005-002. These pilots were certified to lift within +/- 1% of set point prior to installation.
This event is considered to have no safety significance from a probabilistic safety assessment risk evaluation standpoint. This event does not create a core damage scenario. There is no change to the CNS core damage frequency or the large early release frequency. This condition also has no impact on the reactor pressure vessel pressure relief function capability. Even under postulated failure conditions, there is no associated risk increase to the plant.
CAUSE
The cause for the SRV not opening at its set point is corrosion bonding between the Stellite 21 pilot disc and the Stellite 6 pilot body seat, which prevented the valve from actuating within its TS tolerance.
This condition is an industry-wide issue which also applies to CNS. This condition had been most recently reported by CNS per LER 2005-002-00. The corrective actions associated with LER 2005-002 had not been fully implemented for the specific SRV pilot assembly at the time of its test failure.
CORRECTIVE ACTION
The corrective action identified in CNS LER 2003-002, to replace the SRV pilot valve assemblies with certified pilot valve assemblies containing Stellite 21 pilot discs and Stellite 6 pilot body seals, has been fully implemented during RFO 21. The corrective actions identified in CNS LER 2005-002, to implement appropriate elements of the Target Rock Users Group industry best practices for refurbishment on each refurbishment, including installation of a new Stellite 21 pilot disc during each refurbishment, and to have a CNS Engineering representative present during refurbishment of each SRV, had not been fully applied to pilot assembly 383 prior to its installation during RFO 22 for Cycle 23. However, it is part of the population of pilot valve assemblies, sent to the vendor for refurbishment following RFO 23, which will undergo these corrective actions. The corrective actions represent an ongoing effort for future SRV pilot valve refurbishments.
Based on the test evidence, the evaluation of the failure mechanism, and the corrective actions currently in place, no additional corrective actions are required.
PREVIOUS EVENTS
Laboratories, determined that three of eight SRV pilot valve assemblies failed to lift within their TS lift set point. Examination determined that sufficient corrosion bonding existed between the SRV pilot valve assembly Stellite 21 disc and the pilot valve Stellite 6 in-body seat to cause the SRV pilot valves to lift outside TS set point tolerances.
determined that four of eight SRV pilot valve assemblies failed to lift within their TS lift set point.
Examination determined that sufficient corrosion bonding existed between the SRV pilot valve assembly Stellite 21 disc and the pilot valve Stellite 6 in-body seat to cause the SRV pilot valves to lift outside TS set point tolerances.