05000338/LER-2014-003
| North Anna Power Station | |
| Event date: | |
|---|---|
| Report date: | |
| 3382014003R00 - NRC Website | |
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1.0 DESCRIPTION OF THE EVENT On December 22, 2014, Unit 1 reactor power was reduced from 100% to 30% to allow entry into the Reactor Coolant System (RCS) (EIIS System — AB) loop rooms to investigate an increased unidentified leak rate of 0.053 gallons per minute (gpm). At 2230 on December 22, 2014, with Unit 1 operating at 30% power, during a containment walkdown, steam was discovered coming from underneath the lagging on the "B" RCS intermediate loop. Further investigation identified a pressure boundary leak was on the "B" loop drain piping between the loop connection and 1-RC-68, the "B" Loop Cold Leg Drain Isolation Valve (EIIS System — AB, Component — V). At that time, the limiting action of Technical Specification (TS) 3.4.13, RCS Operational Leakage, Condition B was entered which required placing the unit in Mode 3 within 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> and Mode 5 within 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />. At 2345 on December 22, 2014, the power ramp to remove the unit from service commenced. At 0226 on December 23, 2014, the unit was placed in Mode 3 and the unit was placed in Mode 5 at 1629 on December 23, 2014.
Due to the pressure boundary leakage, this event was reported at 0057 on December 23, 2014, in accordance with 10 CFR 50.72(b)(2), for "Initiation of plant shutdown required by Technical Specifications" and 10 CFR 50.72(b)(3)(ii)(A) for "Any event or condition that results in the condition of the nuclear power plant including its principle safety barriers, being seriously degraded.
After Unit 1 was in Mode 5, further investigation, via Ultrasonic Testing (UT), of the other loop drain lines found that there was an indication on the corresponding "C" cold leg loop drain elbow, but no leakage was noted. Subsequently, this indication was evaluated as acceptable, per ASME code, until the next refueling outage in the spring of 2015.
2.0 SIGNIFICANT SAFETY CONSEQUENCES AND IMPLICATIONS No significant safety consequences resulted from this event because Unit 1 was promptly removed from service and the "B" cold leg loop drain elbow was replaced. The health and safety of the public were not affected by this event.
3.0 CAUSE The direct cause of the through-wall leak in the "B" loop drain line elbow was a thermal fatigue failure of the elbow. Limitations in the industry generic model used to predict swirl penetration thermal fatigue in stagnant RCS branch lines allowed the Non- North Anna Power Station Unit 1 05000338 Destructive Examination (NDE) test frequency of the "B" Loop drain line elbow to be set non-conservatively, causing thermal fatigue cracking to go unmonitored. The Root Cause is a legacy issue from 2009 due to the generic model, as developed by EPRI, not accurately predicting fatigue damage due to swirl penetration.
A contributing cause was the fact that Unit 1 chemistry samples of the RCS have been collected from the "B" cold leg loop drain line since 2004. Chemistry sampling added additional thermal stresses which accelerated the fatigue failure in the "B" loop drain line elbow. The screening calculation performed in 2010 to identify lines susceptible to thermal fatigue was performed by Site and Corporate Engineering. The screeners did not have a strong questioning attitude to identify chemistry sampling as a thermal fatigue contributor. A questioning attitude would have prompted a discussion with Chemistry personnel for additional information on sample duration and frequency. In 2011, the review process for calculations and other engineering products was improved after Engineering issued CM-AA-ECR-101, Engineering Challenge Reviews (ECR). This Guidance and Reference Document (GARD) improves the accuracy and completeness of engineering products by having a diverse review team. Engineering calculations are listed for consideration for and ECR at the discretion of Engineering Management.
4.0 IMMEDIATE CORRECTIVE ACTION(S) The drain line elbow was replaced while the unit was in Mode 5.
5.0 ADDITIONAL CORRECTIVE ACTIONS The Unit 1 "C" cold leg loop drain elbow will be replaced during the Unit 1 spring 2015 refueling outage. Engineering will then perform a Material Analysis of the elbows and implement any necessary changes to the Thermal Fatigue Management Program.
6.0 ACTIONS TO PREVENT RECURRENCE Baseline UT will be performed on all Unit 1 and Unit 2 hot and cold leg drain line elbows. The North Anna Augmented Inspection Program, ER-NA-AUG-101, will be revised to perform a UT examination on the cold leg drain elbows based upon guidance found in EPRI MRP-146S for locations where significant thermal fatigue is predicted.
Additionally, Engineering will work with EPRI and industry peers to develop a new model and/or new guidance to better predict the impact of thermal fatigue and other sources of thermal loading.
7.0 SIMILAR EVENTS No similar events have occurred at North Anna.
8.0 ADDITIONAL INFORMATION Unit 2 continued operating in Mode 1, 100 percent power during this event.