05000278/LER-2005-004
| Peach Bottom Atomic Power Station Unit 3 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability |
| 2782005004R00 - NRC Website | |
Unit Conditions Prior to Discovery of the Event Unit 3 was in Mode 5 for its 15'h Refueling Outage when the event was discovered on 10/2/05. The condition was discovered during routine laboratory as-found testing for Safety Relief Valves (SRVs) removed during the 15 h` Unit 3 Refueling Outage. There were no structures, systems or components out of service that contributed to this event.
Description of the Event
Based on information received on 10/2/05 from a laboratory performing SRV (EIIS: RV) as-found testing, Site Engineering personnel determined that SRV set point and performance deficiencies existed with five SRVs that were installed during the 15th operating cycle for Unit 3. These five SRVs had been removed during the 15th Refueling Outage for Unit 3 and were sent to an off-site laboratory for as-found testing and routine refurbishment. Four of the SRVs were determined to have their as-found set points in excess of the Technical Specification allowable ± 1% tolerance. All four SRVs were well within their ASME Code allowable ±3% tolerance. The four SRVs as-found set points were as follows:
SRV Serial Required Set As-Found Set % Outside of Technical Number (SN) Point (psig) Point (psig) Specification Allowable Tolerance 179 1143 - 1167 1142 - 0.13% 174 1143 - 1167 1136 - 0.64% 23 1123 - 1147 1114 - 0.85% 21 1133 - 1157 1129 - 0.40% In addition, one additional SRV (S/N 193) was found with performance concerns. Specifically, the SRV would not properly re-close when tested. This SRV was found to have its setpoint within its + 1% tolerance lift pressure tolerance. Normally, SRVs should re-close shortly after the first stage pilot disc fully re-seats.
The five SRVs were replaced with different SRVs for the 16th Unit 3 operating cycle.
This report is being submitted pursuant to:
1. 10CFR 50.73(0(2)(1)(B) - Conditions Prohibited by Technical Specifications - Technical Specification Limiting Condition for Operation (LCO) 3.4.3 requires that 11 of the 13 installed SRVs / Safety Valves (SVs) be operable. Contrary to this requirement, four SRVs were found with set points outside of the Technical Specification requirements.
2. 10CFR 50.73(a)(2)(vii) — Multiple Inoperable Trains in a Single System — Technical Specification LCO 3.4.3 requires that 11 of the 13 installed SRVs / SVs be operable. PBAPS has 11 SRVs and 2 SVs installed on the main steam system. Because four SRVs were found outside of the ± 1% SRV set point tolerance, this resulted in a condition where two of the 'required' SRVs were considered to be outside of their +1% set point tolerance during plant operations based on a common cause.
Analysis of the Event
There were no actual safety consequences associated with this event.
The ASME Boiler and Pressure Vessel Code requires that the Reactor Pressure Vessel (EIIS: RCT) be protected from overpressure during upset conditions by self-actuated relief valves. As part of the nuclear pressure relief system, the size and number of SRVs and SVs are selected such that the peak pressure in the nuclear system will not exceed the ASME Code limits for the Reactor Coolant Pressure Boundary. The 11 installed SRVs exhaust steam through discharge lines to a point below the minimum water level in the Suppression Pool. The 2 installed SVs discharge steam directly to the Drywell. The SRVs and SVs are located on the four main steam lines (EIIS: SB) within Primary Containment. The SRVs are 'three-stage' valves consisting of a main valve disc and piston (third stage) operated by a second stage disc and piston displaced by either a first stage pressure-sensing pilot (for overpressure protection) or a pneumatically-operated mechanical push rod (for remote-manual operation).
During Unit 3 Cycle 15 operations, there were no plant transients that required automatic SRV/SV operation.
The as-found set points for the four SRVs that tested outside of their Technical Specification allowable range were slightly low. There were a total of five SRVs and one SV removed for testing and replacement during the 15th Refueling Outage. All four SRVs outside of their Technical Specification allowable range were well within the ASME Code allowable of ±3%. The low as-found set points are conservative with respect to the over-pressure protection safety function of the SRVs. The cycle-specific transient analysis for Unit 3 Cycle 15 operations bounded this discovered condition. Two of the four SRVs (i.e. SRVs S/N 179 and S/N 174) were also Automatic Depressurization System (ADS) valves. The set point drift had no impact on the ADS function of the valves.
Concerning the SRV S/N 193 (non-ADS SRV) failure to re-close during laboratory testing, it was determined that there were no actual safety consequences associated with this condition. As stated previously, there were no plant transients that required automatic SRV operation during Cycle 15 operations. The SRV S/N 193 was installed during the 13th Refueling Outage in 2001. On 9/15/03 (during Cycle 14 operations), this SRV automatically opened on high pressure and properly re-closed in response to a unit scram associated with a loss of grid event (LER 2-03-04) that occurred just before the 14th Refueling Outage. The SRV was not challenged subsequent to the 9/15/03 event. If the SRV had been challenged by a plant transient during Cycle 15 operations and the valve did not re-close, the event would be bounded by the design basis event entitled, 'Inadvertent Opening of a Relief or Safety Valve.' If this event had occurred during Cycle 15 operations, a pressure — temperature transient would have occurred on the RPV.
This event is not considered risk significant.
NRC FORM 366AU.S. NUCLEAR REGULATORY COMMISSION (1-2001) FACILITY NAME (1) DOCKET 12) LER NUMBER (6) PAGE (3)
Cause of the Event
The cause of the four SRVs being outside of their allowable as-found set points is due to set point drift. A historical review of SRV as-found test set points indicates that approximately 20% of valves tested over time do not meet the ± 1% Technical Specification set point.
Concerning the failure of SRV S/N 193 to re-close, laboratory failure analysis identified that the main valve (EIIS: V) disc (third stage) had not properly re-seated when closing. Preliminary analysis indicates that a misalignment of the main valve disc spring is the likely cause. This valve was last refurbished in February 2001. Finalization of the causal analysis is in progress.
Corrective Actions
The five SRVs were replaced with different SRVs for the 16th Unit 3 operating cycle.
Additional assessment and appropriate corrective actions concerning SRV refurbishment vendor work practices will be further assessed as part of the Corrective Action Program.
Previous Similar Occurrences There was one previous LER identified involving the failure of an SRV to re-close following automatic operation. LER 2-03-04 identified an event on Unit 3 that involved a failure of SRV S/N 18 to re-close. A laboratory failure analysis determined that tightly adhered foreign material on the pilot valve disc may have prevented the first stage pilot valve disc from properly re-closing. Corrective actions identified through the Corrective Action Program included the improved decontamination, cleaning, and examination processes.
These corrective actions are still in progress and primarily involved the first stage piston work practices.
Therefore, these corrective actions would not be expected to have prevented the SRV S/N 193 failure to re close concern.