05000346/LER-2014-004
| Davis-Besse Nuclear Power Station | |
| Event date: | 11-25-2014 |
|---|---|
| Report date: | 01-23-2015 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| Initial Reporting | |
| ENS 50639 | 10 CFR 50.72(b)(3)(ii)(B), Unanalyzed Condition |
| 3462014004R00 - NRC Website | |
DESCRIPTION OF EVENT:
On October 20, 2014, the fuel vendor (AREVA) for the Davis-Besse Nuclear Power Station (DBNPS) notified the FirstEnergy Nuclear Operating Company (FENOC) of a potential issue regarding Loss of Coolant Accident (LOCA) modeling. This issue, which potentially affected the operations of all Babcock and Wilcox (B&W) designed plants, involved the fuel pellet thermal conductivity model and associated conservatisms in the Large Break LOCA model. For the DBNPS, which was operating in Mode 1 at approximately 100 percent full power for the duration of this issue, adequate margin existed in the reactor core operating limits to accommodate temporary compensatory measures, in the form of restricted operating limits, while further evaluations were being performed.
On October 21, 2014, the fuel vendor transmitted temporary conservative limits on Power Peaking Factor Fq and Axial Power Imbalance (as measured by the Out-of-Core Neutron Detectors) for the DBNPS. The guidance provided by the fuel vendor recommended the Fq limits for higher burnup fuel assemblies be reduced, confirmed the limits on Axial Power Imbalance (as determined by the Incore Detector System) did not need to be changed due to conservatism built into the limits, and recommended that at less than 92 percent full power the limits on Axial Power Imbalance as determined from the out-of-core neutron detectors be reduced. This guidance was implemented at the DBNPS on October 23, 2014.
On November 25, 2014, the fuel vendor notified FENOC of the final evaluation results for the deficiency discovered in the thermal conductivity model computer codes. Accounting for the deficiency resulted in the predicted analytical Peak Cladding Temperature (PCT) increasing to 2513 degrees F during Large Break LOCA conditions. This 2513 degrees F exceeded the DBNPS licensing basis PCT of 2200 degrees F for compliance with 10 CFR 50.46(b)(1). Because of the recommended compensatory measures implemented on October 23, 2014, the deficiency had no impact on current plant operation or public health and safety.
CAUSE OF EVENT:
The cause of this event was related to the evolution of the fuel vendor's modeling. The fuel vendor's previous models for fuel rod thermal mechanical performance inadequately addressed the thermal conductivity degradation effect. The deficiency was identified while developing new computer models, which utilized updated data and incorporated a greater understanding of the fuel rod thermal mechanical performance.
ANALYSIS OF EVENT:
A methodology was developed by FENOC in collaboration with the fuel vendor to evaluate past core operating data. Implementation of this methodology determined that there were no actual fuel assembly power peaks observed during the last three years that would have resulted in peak clad temperatures in excess of analysis of record peak clad temperatures. Therefore, this issue was of very low safety significance.
Reportability Discussion:
The NRC was verbally notified of this event per 10 CFR 50.72(b)(3)(ii)(B) at 1632 hours0.0189 days <br />0.453 hours <br />0.0027 weeks <br />6.20976e-4 months <br /> on November 25, 2014, via Event Number 50639. This issue is being reported in accordance with 10 CFR 50.73(a)(2)(ii)(B) as an unanalyzed condition that significantly degraded plant safety. Notification of the Defect in the LOCA Analysis in accordance with 10 CFR 21 was completed by the fuel vendor by letter dated December 16, 2014 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML14351A308).
CORRECTIVE ACTIONS:
On December 19, 2014, notification of the error and change that affected the Large Break LOCA analysis for the DBNPS was submitted to the NRC in accordance with 10 CFR 50.46(a)(3)(ii) via FENOC letter L-14-403 (ADAMS Accession No. ML14353A228). This letter includes the schedule for the LOCA reanalysis and submittal to the NRC as required by 10 CFR 50.46.
PREVIOUS SIMILAR EVENTS:
There have been no Licensee Event Reports submitted for the DBNPS in the past three years documenting deficiencies in the Loss of Coolant Accident Analysis.