05000339/LER-2007-001
| North Anna Power Station , Unit 2 | |
| Event date: | 02-27-2007 |
|---|---|
| Report date: | 04-27-2007 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(A), Completion of TS Shutdown 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v), Loss of Safety Function 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability 10 CFR 50.73(a)(2)(v)(C), Loss of Safety Function - Release of Radioactive Material |
| 3392007001R00 - NRC Website | |
FACILITY NAME (1) DOCKET LER NUMBER (6) PAGE (3) 1.0 DESCRIPTION OF THE EVENT On February 27, 2007, at 1620 hours0.0188 days <br />0.45 hours <br />0.00268 weeks <br />6.1641e-4 months <br />, with North Anna Unit 2 operating at 100% power (Mode 1), Unit 2 Safeguards exhaust bypass dampers (EIIS System VF, Component DMP) 2-HV-AOD-228-1 and 2-HV-AOD-228-2 were found to have leak-by during the performance of Periodic Test (PT) 0-PT-77.14A, Emergency Core Cooling System (ECCS) Pump Room Exhaust Air Cleanup System (PREACS) Train A Filter In-Place Test (1-HV-FL-3A).
The ECCS PREACS is designed to filter air from the area of the active ECCS components during the recirculation phase of the loss of coolant accident (LOCA). The ECCS PREACS, in conjunction with other normal operating systems, also provides environmental controls of temperature in the ECCS pump room areas.
With leak-by of both Unit 2 Safeguards air operated dampers 2-HV-AOD-228-1 and 2-HV AOD-228-2, a potential path existed for both trains of ECCS PREACS to bypass the filter media (EIIS Component FLT). This condition rendered both trains of Safeguards exhaust inoperable. The Limiting Condition of Operation (LCO) for Technical Specification (TS) 3.7.12 does not provide an ACTION for both trains of ECCS PREACS being inoperable.
Therefore, TS LCO 3.0.3 applied which required the unit to be in Mode 5 (Cold Shutdown) in 37 hours4.282407e-4 days <br />0.0103 hours <br />6.117725e-5 weeks <br />1.40785e-5 months <br />.
Operations personnel began a unit ramp-down at 1947 hours0.0225 days <br />0.541 hours <br />0.00322 weeks <br />7.408335e-4 months <br /> and a 4-hour Non- Emergency Report was made to the NRC at 2215 hours0.0256 days <br />0.615 hours <br />0.00366 weeks <br />8.428075e-4 months <br />, in accordance with 10 CFR 50.72(b)(2)(i). At 2220 hours0.0257 days <br />0.617 hours <br />0.00367 weeks <br />8.4471e-4 months <br />, with Unit 2 at 32% power, the TS action was cleared after installation of a temporary modification that restored the system to OPERABLE.
During the review of this event, it was determined that a similar event occurred on February 6, 2006. Specifically, during the performance of 0-PT-77.14B, ECCS PREACS Train B Filter In-Place Test (1-HV-FL-3B) the as found leakage for Unit 2 Safeguards exhaust bypass dampers, 2-HV-AOD-228-1 and 2-HV-AOD-228-2, was out of specification high with the Unit 2 Safeguards exhaust aligned to the charcoal filters. This condition should have required entry into TS LCO 3.0.3. While Operations personnel discussed the issue and the requirement to enter TS LCO 3.0.3, the dampers were stroked and PT was subsequently performed satisfactorily. No actual entry into TS LCO 3.0.3 was made. TS LCO 3.0.3 entry should have been made at 1800 hours0.0208 days <br />0.5 hours <br />0.00298 weeks <br />6.849e-4 months <br />. Successful testing occurred within the first 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> (i.e., at 1826 hours0.0211 days <br />0.507 hours <br />0.00302 weeks <br />6.94793e-4 months <br />) of what should have been part of the TS LCO 3.0.3 entry time. Based on the PT being performed satisfactorily, approval of an operability determination with regards to ECCS leakage and dampers also being manually set, no further discussion of TS LCO 3.0.3 was performed. This event should have been reported pursuant to 10 CFR 50.73(a)(2)(i)(B), for any operation or condition which was prohibited by the plant's Technical Specifications.
DOCKETFACILITY NAME (1) LER NUMBER (6) 2.0 SIGNIFICANT SAFETY CONSEQUENCES AND IMPLICATIONS There were no nuclear safety consequences as a result of this event since the Emergency Core Cooling System (ECCS) Pump Room Exhaust Air Cleanup System (PREACS) ventilation filters were not called upon to perform their safety function. Furthermore, had a plant event occurred that would have necessitated use of the system, the bypass flow around the filters would not have caused the control room personnel or the public to receive radiation doses in excess of design limits since the total amount of ECCS leakage was well below the limit set for unfiltered leakage in design calculations. The only consequence of this event was a Unit ramp in an expeditious manner.
This event is reportable pursuant to 10 CFR 50.73(a)(2)(i)(A), for any operation or condition which was prohibited by the plant's Technical Specifications. The event is also reportable pursuant to 10 CFR 50.73(a)(2)(v)(C) for any event or condition that could have prevented the fulfillment of a safety function of structures or systems that are needed to control the release of radioactive material. In addition, the event is reportable pursuant to 10 CFR 50.73(a)(2)(vii) for any event where a single cause or condition caused at least one independent train or channel to become inoperable in multiple systems or two independent trains or channels to become inoperable in a single system designed to control the release of radioactive material. The event reportability for 10 CFR 50.73(a)(2)(v)(C) as a loss of safety function was not identified until after the event was initially reported pursuant to 10 CFR 50.72(b)(2)(i) on February 27, 2007. Thus, this event should have also been initially reported pursuant to 10 CFR 50.72(b)(3)(v) as a loss of safety function. Because this event is reportable per 10 CFR 50.73(a)(2)(v) it will also be reported as a Safety System Functional Failure for the NRC Performance Indicators in accordance with the guidance of NEI 99-02, revision 4.
3.0 CAUSE The cause of the excessive damper leakage was a combination of improper adjustment of the actuator damper linkage and degraded damper seals. External adjustment of the damper linkage without performing an internal verification of damper position does not ensure proper damper operation. In addition, the damper linkages were not set to pre load the actuator springs in accordance with manufacturers recommendations.
Degradation of the damper blade seals over time prevented a tight seal with the blades in the closed position.
Several contributing causes were also identified. These contributing causes included inadequate preventive maintenance on the seals, difficulty in performing seal maintenance due to access limitations, limited actuator design margin, and inadequate damper testing.
FACILITY NAME (1) DOCKET LEIR NUMBER 6) PAGE (3) 4.0 IMMEDIATE CORRECTIVE ACTION(S) Operations personnel began a unit ramp-down at 1947 hours0.0225 days <br />0.541 hours <br />0.00322 weeks <br />7.408335e-4 months <br /> and a 4-hour Non- Emergency Report was made to the NRC at 2215 hours0.0256 days <br />0.615 hours <br />0.00366 weeks <br />8.428075e-4 months <br />, in accordance with 10 CFR 50.72(b)(2)(i). At 2220 hours0.0257 days <br />0.617 hours <br />0.00367 weeks <br />8.4471e-4 months <br />, with Unit 2 at 32% power, the TS action was cleared after installation of a temporary modification that restored the system to OPERABLE.
5.0 ADDITIONAL CORRECTIVE ACTIONS Work Orders (WOs) 00737900-01 and 02 were completed to replace damper seals, including the jamb seals and to perform a thorough inspection of dampers 2-HV-AOD 228-1 and 2. This work also directed adjustment of the actuator linkage and established proper spring preload to help ensure damper closure.
Design Change (DC)07-117 was issued and implemented to provide a method for quickly isolating the Unit 2 Safeguards bypass path should future damper leakage occur. The DC installed a structural framework that supports metal blanks that may be used to isolate flow through the bypass while permitting access to the bypass dampers for inspection/maintenance. A similar change is being developed for North Anna Unit 1.
The damper actuators for 2-HV-AOD-228-1 and 2 were refurbished. The refurbishment included replacement of the diaphragm and actuator spring.
A partial performance of 0-PT-77.14A was conducted for the Unit 2 Safeguards bypass dampers. Damper leakage was satisfactory, approximately 0.8% vs. acceptance criteria of less than 1%.
The preventive maintenance (PM) frequency and scope for damper seal replacements was revised.
Changes to component classifications for the dampers and actuators were implemented.
Procedure changes were completed to ensure adequate contingency measures are staged prior to testing.
6.0 ACTIONS TO PREVENT RECURRENCE Maintenance procedures will be revised to 1) ensure that actuator linkages are not disconnected/reconnected without visual verification of the damper position internal to the ductwork and 2) ensure actuator preloading.
FACILITY NAME (1) DOCKET LER NUMBER 6) PAGE (3) An evaluation will be completed to address potential solutions for damper design deficiencies. Changes will be implemented as appropriate.
Maintenance training programs will be reviewed and revised as required to strengthen maintenance work practices on dampers, damper linkages, and damper actuators.
0-PT-77.14A and B will be revised to include individual testing of PREACS bypass dampers.
These actions will be tracked in the Corrective Action Program to completion.
7.0 SIMILAR EVENTS Licensee Event Report No. 50-338/1999-006-00 dated September 28, 1999, documents finding a damper associated with the Auxiliary Building General Area Exhaust Ventilation System degraded due to missing damper seals.
Plant Issue N-2006-0504, dated February 6, 2006, documents excessive leakage as measured by 0-PT-77.14B by 2-HV-AOD-228-1 and 2.
8.0 ADDITIONAL INFORMATION At the time of this event North Anna Unit 1 was operating at 100 percent power and was not affected by this event.
The extent of condition was reviewed and it was determined that it is limited to ventilation system dampers that are required to provide essentially leak-tight sealing, which includes the following:
- 1-HV-A0D-228-1, -2 (Unit 1 Safeguards Exhaust Bypass Dampers) - 1-HV-AOD-103-1, -2 (Auxiliary Building Central Exhaust Bypass Dampers) - 1-HV-A0D-102-3, -4 (Auxiliary Building General Exhaust Filter Dampers) - 1-HV-A0D-104-3, -4 (Containment Purge Exhaust Filter Dampers) - 1-HV-AOD-107-3, -4 (Fuel Building Exhaust Filter Dampers) - 1-HV-A0D-113-3, -4 (Waste Solids/Decontamination Building Exhaust Filter Dampers) - 1-HV-AOD-160-1, -2 (Control Room Normal Supply Isolation Dampers) - 1-HV-AOD-161-1, -2 (Control Room Normal Exhaust Isolation Dampers) Previous ventilation system testing for North Anna Units 1 and 2 has demonstrated acceptable damper performance.