05000456/LER-2002-002
| Braidwood, Unit 1 | |
| Event date: | 04-16-2002 |
|---|---|
| Report date: | 9-27-2002 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| Initial Reporting | |
| 4562002002R01 - NRC Website | |
B. Description of Event:
There were no systems or components inoperable at the beginning of this event that contributed to the severity of the event.
During refueling outage A1R09 the Operations Department executed 1BwOSR 3.4.11.3, `Pressurizer PORV Instrument Air Accumulator Check Valve Test'. This surveillance is required by Technical Specification Surveillance Requirement 3.4.11.3 which states:
"Perform a complete cycle of each solenoid air control valve and check valve on the air accumulators in PORV control systems". This surveillance is required to be performed once per 18 months. In addition to this requirement, in order to demonstrate operability of the pressurizer PORV [AB] Instrument Air (IA) [LD] accumulators, procedure 1BwOSR 3.4.11.3 also performs an as-found and as-left seat leakage test of the accumulator isolation check valves. The procedure demonstrates valve seating prior to cycling the valve. The check valves are then leak tested after the cycling to demonstrate an acceptable as-left condition.
The Unit 1 check valves, 1RY085A and 1RY086A (isolation valves for the train A accumulator tank) and 1RY085B and 1RY086B (isolation valves for the train B accumulator tank), failed to meet the acceptance criteria during the as-found seat leakage test. The check valves would have been incapable of maintaining pressurizer PORV IA accumulator pressure during a loss of IA supply to the containment building [NH] All four check valves were removed from the system and inspected and repaired. The inspection of the check valves found the disc seat o-ring partially dislocated from the o-ring groove. The partial dislocation of the o-ring was preventing the full closure of the check valve disc. The o-rings were replaced with new o-rings and the check valves were returned to service. Subsequent completion of 1BwOSR 3.4.11.3 verified proper operation of the check valves and as-left seat tightness.
This event, and the activities that took place during refueling outage A1R09 were documented in an Apparent Cause Evaluation (ACE). The ACE documented the cause of the surveillance failure as o-ring displacement. However, the ACE failed to determine the cause of the o-ring displacement. The ACE also reported a previous o-ring displacement that occurred in 1998. The o-ring displacement that occurred in 1998 was not documented in the Corrective Action Program at that time. A later review of this ACE determined that it needed to be re-opened for further investigation through the root cause process.
In an attempt to understand the cause of the failures, a spare check valve was inspected and tested. A physical examination of this check valve revealed that the o- ring had tendencies to impact the side, or drag across the edge, of the valve seat
C. Cause of Event
The root cause of the Unit 1 check valve failures was the incorrect link bushing gap that resulted in disc o-ring interference with the valve seat. This interference resulted in o-ring displacement from the disc o-ring groove. The o-ring displacement was caused by the combination of improper link bushing length and the large number of valve cycles that occurred during IA accumulator refill 0-ring displacement prevented proper closure of the check valve. The incorrect bushing gap was caused by past maintenance practices not using the correct maintenance procedure to verify an acceptable link bushing gap.
D. Safety Consequences:
The simultaneous failure of both Unit 1 pressurizer PORVs caused by a loss of IA has been evaluated. This failure degrades the ability to use the pressurizer PORVs for primary bleed and feed cooling following a dual unit loss of offsite power and failure of both Unit 1 Auxiliary Feedwater [BA] Pumps. Although IA can be restored to the pressurizer PORVs by loading a Station Air Compressor [LF] and Non-essential Service Water [KG] Pump onto an Emergency Diesel Generator [EK], this additional action considerably increases the probability of core damage for these scenarios. This failure also degrades the ability to depressurize the RCS following a Steam Generator Tube Rupture event as air must be restored to the containment building. For these reasons, the failure event is considered risk significant in accordance with the Significance Determination Process.
The failure of the pressurizer PORVs is considered a Safety System Functional Failure.
NRCFORM366A ql-noo U.S. NUCLEAR REGULATORY COMMISSION - FACILITY NAME (I) DOCKET (2) LER NUMBER 6) PAGE (3) Braidwood, Unit 1 STN 05000456 E. � Corrective Actions:
The corrective actions developed include:
a. Performing the necessary repairs to the Unit 1 valves in refueling outage A1R10 b. Revising 1BwOSR 3.4.11.3 to insure the as-left seat leakage test is performed after all maintenance is completed on the pressurizer PORV IA accumulators.
c. Revising the Maintenance procedure to include the proper guidance with respect to link bushing gap dimensions.
d. Replacing the check valve discs with an upgraded full-dovetail design to provide an additional barrier to o-ring displacement.
F. � Previous Occurrences:
Dating back to refueling outage A1R03 in 1992, the pressurizer PORV IA accumulator check valves on Braidwood Unit 1 have been repaired in 5 of the last 7 refueling outages.
The following is a summary of the history of the Braidwood Unit 1 check valves.
Fall 1992/A1R03 Spring 1994/A1R04 Fall 1995/A1R05 Spring 1997/A1R06 Fall 1998/A1R07 Spring 2000/A1R08 Fall 2001/A1R09 All1four check valves fail leakage surveillance Two of the four valves fail leakage surveillance All four check valves fail leakage surveillance All four check valves pass leakage surveillance All four check valves pass leakage surveillance All four check valves pass leakage surveillance All four check valves fail leakage surveillance G. � Component Failure Data:
Manufacturer � Nomenclature � Model � Mfg. Part Number Anderson and Co., Check Valve (Pressurizer � CV1B � N04-2480-520 Greenwood � Power Operated Relief Valve Accumulator Isolation)