05000259/FIN-2015001-07
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Finding | |
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Title | Inaccurate Assumptions used for Past Operability analysis of a Probable Maximum Flood |
Description | The NRC identified an Unresolved Item (URI) for the licensees inaccurate assumptions used to generate their flooding analysis in the event of a probable maximum flood (PMF) event at the Browns Ferry Nuclear Plant. On December 30, 2009, Tennessee Valley Authority (TVA) completed the installation of HESCO flood barriers on the embankments of four dams as an interim and immediate correction to prevent overtopping flows if the probable maximum flood (PMF) occurs. The HESCO is a commercial brand of sand baskets used as flood barriers against flood overtopping at a dam. Hereafter, the pre-HESCO flood level is defined as a result from the conditions of upstream overtopping flow and dam failures during PMF event, but not including HESCO flood barriers installed in 2009 and dam modifications performed between 1982 and 1997. The U.S. Nuclear Regulatory Commission (NRC) Region II Office questioned the validity of the re-calculated PMF elevation using the Hydrologic Engineering Centers River Analysis System (HEC-RAS) model, as the aforementioned pre-HESCO flood level. As indicated in the Licensee Event Report (LER) 50-259/2013-001-01, the pre-HESCO flood level at Browns Ferry Nuclear Station (BFN) is 571.5 feet. By memorandum dated August 18, 2014 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML14227A671), the NRC Region II Office requested technical assistance from the Office of Nuclear Reactor Regulation (NRR) to conduct a technical assessment on the pre-HESCO flood level at BFN. The NRR staff found that the HEC-RAS model calibration for BFN was not completed and the model is lacking simulations for a few dam failures and tributary flows in some upstream extensions for only the analysis prior to the installation of the HESCO barriers. The NRC does not have a concern with the present day ability of BFN to cope with a PMF. The complete calibration of the HEC-RAS model for BFN will be reviewed in another work scope after TVA corrects for the inadequate setup of the HEC-RAS model for BFN. URI 05000259/260/296/2015-001-07 (Inaccurate Assumptions used for Past Operability Analysis of a Probable Maximum Flood) was opened to allow the NRC to review the results of the updated setup of the HEC-RAS model to determine if the inaccurate assumptions used to generate the past operability BFN flooding analysis was a more than minor performance deficiency. |
Site: | Browns Ferry |
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Report | IR 05000259/2015001 Section 4OA3 |
Date counted | Mar 31, 2015 (2015Q1) |
Type: | URI: |
cornerstone | Miscellaneous |
Identified by: | NRC identified |
Inspection Procedure: | IP 71153 |
Inspectors (proximate) | A Ruh D Dumbacher D Retterer L Pressley M King T Stephen |
INPO aspect | |
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Finding - Browns Ferry - IR 05000259/2015001 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Finding List (Browns Ferry) @ 2015Q1
Self-Identified List (Browns Ferry)
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