05000298/LER-2011-005
| Cooper Nuclear Station | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 2982011005R00 - NRC Website | |
PLANT STATUS
Cooper Nuclear Station (CNS) was in Mode 1, Power Operation, at 100 percent power when the event was discovered; i.e., June 22, 2011. Additionally, CNS was in a Notice of Unusual Event due to Missouri River level above 899 feet.
BACKGROUND
The pressure relief system includes three American Society of Mechanical Engineers code safety valves (SV) [EllS: SB] and eight safety relief valves (SRV) [EllS: RV], all of which are located on the main steam lines [EllS: SB] within the drywell [EllS: NH], between the reactor vessel [EllS: RPV] and the first main steam isolation valve [EMS: ISV]. The safety valves provide protection against over pressurization of the nuclear system and discharge directly into the interior space of the drywell. The SRVs discharge to the suppression pool and provide three main functions: overpressure relief operation to limit the pressure rise and prevent safety valve opening, overpressure safety operation to prevent nuclear system over pressurization, and depressurization operation (opened automatically or manually) as part of the emergency core cooling system [EllS: BJ, BM, BO].
Technical Specification (TS) Limiting Condition for Operation (LCO) 3.4.3 requires the safety function of eight SRVs and three SVs to be operable. The nominal set pressure and tolerances for these valves are established in CNS 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 6B pilot body seats.
During Refueling Outage 26 (RE26), three complete SRVs (pilot assemblies and main bodies) and five SRV pilot assemblies were removed from the plant and replaced with refurbished certified spares. The SRVs and pilot assemblies removed from service were shipped to Wyle Laboratories, Inc. for required surveillance testing.
EVENT DESCRIPTION
On June 20 through June 22, 2011, three complete SRVs and five SRV pilot assemblies, removed during RE26 in the Spring of 2011, were as-found tested at Wyle Laboratories, Inc.
On June 22, 2011, as-found pressure setpoint testing for one SRV pilot assembly (serial number 1242) exceeded the TS limit. The SRV pilot assembly was installed during RE25 at functional location MS-RV-71HRV and operated through the entire cycle.
The pressure setpoint for SRV pilot assembly serial number 1242 is 1090 psig +/-3%. The TS SR 3.4.3.1 as-found limit of acceptance is 1090 +/- 32.7 psig (1057.3 psig to 1122.7 psig).
The first actual lift pressure of this SRV pilot assembly was recorded as 1199 psig, 10% above the pressure setpoint. For informational purposes, the technicians performed a second and third lift. The results were 1095 psig and 1102 psig, both within 1.1% of the pressure setpoint.
The lift patterns are characteristic of corrosion bonding.
The remaining seven SRVs and SRV pilot assemblies tested satisfactory for TS lift setpoint requirements. The affected SRV pilot assembly is not currently installed in the plant.
All eight SRV pilot assemblies installed during RE26 were refurbished and certified to lift within +1- 1% of as-left setpoint acceptance criteria prior to installation. Therefore, all currently installed SRVs are capable of performing their intended function of providing protection against over pressurization.
The failure mechanism is pilot disc-to-seat corrosion bonding. As reported in Licensee Event Reports (LER) 2008-002-00 and 2010-001-00, the performance of the two-stage Target Rock pilot-actuated valves in the current design application at CNS is unsuccessful in consistently meeting the as-found lift test requirements contained within the current TS due to corrosion bonding. SRV corrosion bonding is an industry-wide issue which has been evaluated within CNS' corrective action program. Past corrective actions taken by CNS have reduced the number of failures due to corrosion bonding. They include replacement of the SRV pilot valve assemblies with certified SRV pilot valve assemblies containing Stellite 21 pilot discs instead of platinum-impregnated Stellite 6B pilot discs; and installation of new Stellite 21 pilot discs during each refurbishment. This is the third set of SRVs tested with new Stellite 21 pilot discs.
BASIS FOR REPORT
CNS is reporting this event as an operation or condition prohibited by plant TS per 10 CFR 50.73(a)(2)(i)(B).
SAFETY SIGNIFICANCE
This event has negligible safety significance. One SRV opening at 1199 psig does not impact the ability to provide adequate overpressure protection for the reactor pressure vessel (RPV).
Seven of the eight SRVs tested satisfactorily for as found set pressure. Analysis has shown that three SRVs could fail to lift completely and still meet RPV overpressure protection requirements. Therefore, this event does not create a core damage scenario. The change in core damage frequency or large early release frequency is insignificant. This event did not compromise overpressure protection for the RPV.
CAUSE
The mechanistic cause is pilot disc-to-seat corrosion bonding. The root cause of the event is corrective actions to implement a Licensing Amendment, related to TS 3.4.3 and the number of SRVs required to be operable, were not completed prior to SRV testing in June 2011.
CORRECTIVE ACTIONS
CNS submitted a license amendment request (LAR) to the Nuclear Regulatory Commission (NRC) on January 5, 2011, to revise the number of SRVs required to be operable. The station is engaged with the NRC and is working through the LAR review and approval process.
Implementation of this LAR, if approved by the NRC, will reduce entries into TS LCO 3.4.3 and requisite LERs in the future.
PREVIOUS EVENTS
failed to lift within TS lift setpoint requirements. Wyle Laboratories performed this testing. The pressure setpoint for the first pilot assembly is 1100 +/- 33.0 psig; the SRV pilot assembly lifted at 1166 psig. The pressure setpoint for the second pilot assembly is 1090 +/- 32.7 psig; it lifted at 1139 psig. Two subsequent informational lifts were performed for both SRV pilot assemblies and were within the TS pressure setpoint tolerances. The mechanistic cause was the same as reported in previous LERs, pilot disc-to-seat corrosion bonding.
performed at Wyle Laboratories identified that one of eight SRV pilot assemblies failed as- found pressure setpoint testing. The SRV pilot assembly lifted at 1165 psig, outside its TS setpoint tolerance of 1100 +/- 33.0 psig. The mechanistic cause was pilot disc-to-seat corrosion bonding between the Stellite 21 pilot disc and Stellite 6B pilot body seat to cause the SRV pilot assembly to lift outside its TS setpoint tolerance.
tests performed at Wyle Laboratories identified that one of eight SRV pilot valve assemblies failed to lift within its TS lift setpoint of 1090 +/- 32.7 psig. The failure was a result of sufficient corrosion bonding between the SRV pilot valve assembly Stellite 21 disc and the pilot valve Stellite 6B body seat to cause the SRV pilot valve to lift outside its TS setpoint tolerance.
provided by Wyle Laboratories, determined that three of eight SRV pilot valve assemblies failed to lift within their TS lift setpoint. Examination determined that sufficient corrosion bonding existed between the SRV pilot valve assembly Stellite 21 disc and the pilot valve Stellite 6B in- body seat to cause the SRV pilot valves to lift outside TS setpoint tolerances.
Laboratories, determined that four of eight SRV pilot valve assemblies failed to lift within their TS lift setpoint. Examination determined that sufficient corrosion bonding existed between the SRV pilot valve assembly Stellite 21 disc and the pilot valve Stellite 6B in-body seat to cause the SRV pilot valves to lift outside TS setpoint tolerances.