05000389/LER-2006-002
| Event date: | 02-15-2006 |
|---|---|
| Report date: | 1-7-2006 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 3892006002R00 - NRC Website | |
Description of the Event
While performing a Local Leak Rate Test (LLRT) on Unit 2's penetration P-56 (Mini Purge), using Operation procedure "Purge Valve Leak Rate Test," leakage exceeded the TS defined limits. Initial troubleshooting determined that FCV-25-36 (EIIS:BF:JM), the inside 8-inch containment isolation valve, was not fully closed. Given the suspected failure mode of the isolation valve, and the obstacles to repair the valve within I'S limits, a NOED (NOED-06-2-01) was pursued and granted. The valve was removed from the system, a blind flange installed, and the LLRT successfully performed.
Cause of the Event
Following removal of the valve, an Apparent Cause Evaluation (ACE) was conducted.
The apparent cause of the failure was attributed to inadequate corrective maintenance. Investigation of the valve indicated FCV-25-36 failed to close as a result of binding between the valve stem and body bushings caused by galling and "balling" of material. This restriction lead to an increase in friction that could not be overcome by the actuator spring to provide a leak-tight seal for the LLRT, but closed enough to indicate closure to the Operators.
Analysis of the Event
A review of the valve's history showed the valve had been inspected and overhauled during the previous outage (SL2-15). At that time, a new disk and stem were scheduled to be installed, however, like-for-like replacements were not available.
Although the stem had signs of wear damage, the decision was made to the repair and reuse the existing stem.
Further investigation indicates the galling between the valve stem and body bushings was mos: likely initiated by damage that occurred during the SL2-15 overhaul of the valve or ineffective repair of the valve during the overhaul. Reuse of the refurbished stem and/or damaged body bushings may have contributed to the failure.
This event is reportable under 10 CFR 50.73(a)(2)(i)(B) as "any operation or condition which was prohibited by the plant's Technical Specifications..." because the valve could not be returned to service within TS allowable limits.
Analysis of Safety Significance The containment purge valves are part of the containment purge and the continuous purge/hydrogen purge systems. The 8-inch purge (or mini-purge) system is not required for safe shutdown of the reactor or to mitigate the consequences of a design basis accident. The containment purge system is designed to reduce the level of radioac:ive contamination in the containment atmosphere below the limits of 10 CFR 20 so as to permit personnel access to the containment during shutdown and refueling.
It is a:Lso used to relieve the building of pressure in the containment caused by leak-by of instrument air-operated components. The continuous purge/hydrogen purge system can be used as a not-nuclear-safety backup to the redundant safety-related hydrogen recombiners, which maintain containment hydrogen concentration below 4 percent after a postulated accident. The only portions of the mini-purge system that are safety-related are the containment penetrations and the isolation valves.
St. Lucie Unit 2 05000389
Corrective Actions
The proposed corrective actions and supporting actions listed below are entered into the site corrective action program. Any changes to the proposed actions will be managed under the commitment management change program.
- Replace the valve stem and body bushings and reinstall the valve (Complete)
- Exigent Technical Specification Change Amendment Number 142 for Unit 2, approved March 17, 2006. (Implemented)
- Will develop a repair procedure for the FCV-25-36 valve prior to the next scheduled valve overhaul.
Similar Events A review of industry Operating Experience (OE) identified no common failure issues.
The review identified only one similar OE, regarding a Contromatics Motor Operated Valve (MOV) that failed to fully close as a result of seat degradation and wear
- Df bronze bearings. However, because of differences in materials and failure mode (seat vs. stem galling), this OE is not considered relevant to this event.
Failed Components
Contromatics Valve, Tag Number, FCV-25-36, Model, Control-Seal Type II valve with SRMSV Actuator and Mastergear MCF-60 manual operator.