05000247/LER-2006-007
| Indian Point 2 | |
| Event date: | 12-05-2006 |
|---|---|
| Report date: | 02-05-2007 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 2472006007R00 - NRC Website | |
Note: The Energy Industry Identification System Codes are identified within the brackets ().
DESCRIPTION OF EVENT
On December 5, 2006, at approximately 0923 hours0.0107 days <br />0.256 hours <br />0.00153 weeks <br />3.512015e-4 months <br />, while at 100% steady state reactor power, Engineering determined that it was likely that the Containment {NH} isolation valves (ISV) used for pressure relief (PCV-1190, PCV-1191, and PCV-1192) had stroked greater than the maximum allowed value of 60 degrees imposed by Technical Specification (TS) Surveillance Requirement (SR) 3.6.3.7.
TS SR 3.6.3.7 requires that the isolation valves be blocked to restrict opening to less than or equal to 60 degrees from the fully closed position. As a result, Condition A and B of TS 3.6.3 requiring valve closure and deactivation, was not entered. Also, this condition had likely existed while the isolation valves were opened for pressure relief without the administrative controls required by TS 3.6.3, Note 1. Condition Report CR-IP3-2006-006322 documents this event. These conditions are operation outside TS, reportable under 10 CFR 50.73(a)(2)(i)(B).
The pressure relief system consists of three containment isolation valves that are in series. These valves (PCV-1190, -1191, and -1192) are opened in order to relieve the Containment pressure to keep internal pressure within normal operating limits. This assures the Containment pressure remains within applicable safety analysis limits. The valves are quick-closing butterfly valves, one inside and two outside Containment, actuated by a Containment isolation signal, a high radiation signal, or by manual isolation. The PCVs are 10 inch Butterfly valves, Model 9211, manufactured by Fisher (F127) with a Fisher 486U-15-60 actuator with link and lever. The valves are kept from opening more than 60 degrees by a travel stop assembly that consists of two travel stop guides (threaded rods) with stop jam nuts mounted on the valve actuator. The travel stop assembly is mounted on the valve piston rod and moves with valve travel. The travel stop consists of a clevis (screwed on the piston rod and held by a jam nut), an upper retaining clip (absorbs force when travel stop reaches jam nuts), the travel stop plate or bar (travels along travel stop guides and impacts the jam nut at 60 degrees), and bottom retaining clip (carries travel stop bar weight during movement). The following summarizes the various condition reports documenting the problems identified with the pressure relief valves.
1.CR-IP2-2006-04723, dated August 8, 2006, reports the failure of PCV-1191 to close in the required time. The valve was declared inoperable and evaluation determined the traveler stop plate on PCV-1191 as well as PCV-1192 were raised and cocked due to damage on the bottom retaining clip. The cause was attributed to outage work that removed but did not disassemble or inspect the travel stop allowing the damage to occur over time. This type of damage had not been previously seen with years of usage.
2.CR-IP2-2006-06307, dated October 28, 2006, reports PCV-1192 had dual indication after a pressure relief. The valve stroked satisfactorily and was not declared inoperable. During corrective action on October 31, the valve failed during stroke time testing and was declared inoperable. This CR was evaluated as part of CR-IP2-2006-06322.
3.CR-IP2-2006-06322, dated October 29, 2006, reports the failure of PCV-1191 to close in the required time. The valve was declared inoperable. The valve stroked satisfactorily after repair of a bent guide rod. The cause of this and conditions reported in CR-IP2-2006-06307, CR-IP2-2006-06426 and CR-IP2- 2006-06481 were evaluated. The travel stop design uses a single retaining clip installed onto a clevis to prevent the actuator from stroking more than 60 degrees open. The travel stop can be disturbed, with any kind of major maintenance on the valve or actuator assembly.
Misalignment of the travel stop bar or the jam nut results in an uneven load on the retaining clip. Repeated uneven loads could cause the retaining clip to be pushed out of the groove in the clevis where it sits. This results in damage to the lips of the groove (identified by inspection) which reduces the forces holding the retaining clip in place. This condition is not readily detectable by valve stroke tests unless there is binding between the travel stop bar and the guide rod. These critical parts were not required to be inspected in past maintenance activities.
4 CR-IP2-2006-06426, dated November 3, 2006, reports the retaining clip on valve PCV-1191 not seated properly. This was identified while taking corrective action on PCV-1192 for slow stroke time (CR-IP2-2006-06307) and the valve was declared inoperable. The clevis assemblies for valves PCV-1191 and PVC-1192 were replaced.� This CR was evaluated as part of CR-IP2-2006- 06322.
5 CR-IP2-2006-06481, dated November 7, 2006, reports the retaining clip on valve PCV-1190 dislodged. The valve was declared inoperable and the clevis assembly was replaced on November 8, 2006. This CR was evaluated as part of CR-IP2-2006-06322.
6 CR-IP2-2007-00014, dated January 2, 2007, reported that during an inspection of PCV-1191 and 1192, the travel stop retaining clip was found out of its groove on PCV-1192 while PCV-1191 was intact. The valve was declared inoperable. The initial evaluation concluded that the poor design could cause uneven loading of the valve plate leading to a displaced retaining clip if maintenance activities during repair did not align the plate perfectly. The evaluation and installation of a more reliable travel stop design was expedited on PCV-1192.
Cause of Event
The direct cause of the TS prohibited condition was failure of the containment relief valves to be limited to the TS maximum stroke of 60 degrees open and failure of the containment relief valves to close within the specified Inservice Testing Program stroke time requirement. The apparent cause is the dislodging of the travel stop retaining clip due to a poor design that can lead to uneven loading of the travel stop plate. The retaining clip could be displaced following maintenance or from the wear of the retaining rings and grooves during normal usage.
Corrective Actions
The following corrective actions have been or will be performed under Entergy's Corrective Action Program to address the cause and prevent recurrence
- Re-adjusted the travel stop jam nuts to assure even pressure on the travel stop bar.
- Augmented the maintenance procedure details to require inspection of the travel stop assembly upon completion of work and included a detailed drawing of the travel stop assembly.
- Implemented a Modification of the travel stop design on PCV-1192 to provide more reliable operation. Valves PCV-1190 and PCV-1191 are scheduled for modifications to address the condition. Implementation of the modification for these valves is scheduled to be completed by March 1, 2007.
Event Analysis
This event is reportable under 10 CFR 50.73(a)(2)(i)(B), any event or condition that resulted in the nuclear power plant operating in a condition prohibited by TS. The Containment pressure relief valves were found in an inoperable condition that is only detected when performing inspections or timing of strokes. This appears to have occurred several times since at least August 8, 2006, when stroke timing anomalies were identified during the quarterly stroke testing per 2-PT-Q13. The valves were declared inoperable and repaired. The diagnosis that the identified condition allowed the pressure relief valves to open 60 degrees or more was reached on December 5, 2006. Since the pressure relief valves may be opened to relieve containment pressure every few days for approximately 2 -4 hours at a time, and the event condition is not always identifiable, the valves have most likely been inoperable during a time that the pressure control valve(s) had power and that pressure has been relieved without proper administrative controls. This is a condition prohibited by Technical Specifications. The time that this has occurred is not identifiable to establish the period of non-compliance with the Technical Specifications.
A review was conducted of Licensee Event Reports (LER) in the past two years for non-compliance with TS. IP2 reported three events resulting in operations prohibited by Technical Specifications. LER-2005-001 reported inoperable Component Cooling Water check valves that resulted in an inoperable Emergency Core Cooling train. LER 2005-002 reported an inoperable Safety Injection pump due to gas binding. LER 2006-002 reported two inoperable Post Accident monitors. No common cause was identified.
Safety Significance
This event had no significant effect on the health and safety of the public.
There were no actual safety consequences because Containment integrity was not required by an actual event. Additionally, there was no significant effect on the public health and safety as no loss of safety function for the Containment isolation valves occurred. The ability of the pressure relief valves to close from a full open position was evaluated and operability calculation IP-CALC-06-00373 demonstrates that the valves were capable of closing on demand.