ML13066A760

Executive Team Briefing on Draft Information Notice Potential Nonconservation Screening Value for Dam Failure Frequency in Pras.

ML13066A760
Person / Time
Site:	Oconee
Issue date:	05/19/2011
From:	Ferrante F NRC/NRR/DRA/APOB
To:
References
FOIA/PA-2012-0325
Download: ML13066A760 (2)

Text

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• Purpose To inform NRR Senior Management about a draft Information Notice (IN) being prepared by NRR/DRA to alert addressees of a potentially nonconservative screening value for dam failure frequency.

Expected Outcome 1 An understanding of the background, basis, and intent of the IN NRRIMay P0( Process Background - How issue originated and its generic implications May 19, 2011  ?.. U.SN fi Discussion - NRC evaluation and findings Current status of IN- concurrence process I Conclusions and recommendations 2 Background NSAC60 Analysis

• White Finding at Oconee (2006) related to a performance deficiency due to maintenance at a flood barrier protection

• Estimated the failure frequency for Jocassee Dam by screening the available US historical data at the time of the analysis (1981)

• Best estimate flood was found to be nonconservative - Data based on: height, composition, construction period,

• Catastrophic dam failure identified as flood risk contributor failure date, and failure modes

• Analysis supporting dam failure rate estimates in Nuclear

• Acknowledge challenge in collecting representative data Safety Analysis Center (NSAC) report NSAC-60, "A

• Considered failure modes separately Probabilistic Risk Assessment of Oconee Unit 3," issued June 1984 - Seismic, overtopping (via Probable Maximum Precipitation estimate), and "random" failure

• Used Bayesian updating analysis to estimate 'random" failure 5

- In the range of 1.4 to 2.3x10- /year NRC Staff ActionsC NRC Staff Findings 'U.S.NRC

• Revisited assumptions and analysis in NSAC-60

• Assumptions in the dam failure rate estimation approach are

• Assessed currently available databases for U.S. dams, in order to strongly dependent on the data completeness, accuracy determine generic dam failure frequencies based on:

• Current databases still include a significant amount of

- historical failures of dams (Stanford University) completeness and accuracy issues

- years of operation for dams (Army Corps of Engineers)

• Flooding hazard was not considered in an integrated manner

• Evaluated dam failure rate estimation In the available literature, (certain hazard categories excluded from further consideration) including state-of-art dam risk assessment methods

• Use of worst-case deterministic scenarios to exclude hydrologic

• Developed internal assessments of dam failure rates using failures may underestimate the risk contribution from current information in cooperation with RES combinations of less severe but more frequent events

- Paper presented at ANS PSA 2011 Intemational Topical Meeting on Combination of generic and site-specific information may not be Probabilistic Safety Assessment and Analysis applicable for screening sites with different characteristics

• NRRJDE and NRR/DRA submitted a request to the Generic Issue (i.e., hydrologic, geologic, operational)

Program (GIP) for further evaluation of generic implications Most dam failure rate estimates are an order of magnitude highpr 1

• 4  %,~

~~V~II4FQRMA14ON Applications of NSAC60 U*Aj-C Information Notice RC After NSAC-60 issuance, dam failure rates were revisited in

• An order of magnitude lower estimates may not be justified by NUREG/CR-5042 'Evaluation of External Hazards to Nuclear historical data alone Power Plants in the United States" (1987), which states that

• Applying the NSAC-60 estimate to other dams with different bounding values for dam failure rate ... characteristics may be inappropriate

... are quoted as being in the range of lO/yeer or even smaller, especially

• Using a nonconservative screening dam failure frequency to for modem welI-engineered dams [Ref. 5.7 Oconee PRA, 1984]". evaluate the need for additional detailed analysis may result in NSAC-60 estimate was used as a reference (directly or indirectly) underestimating the risks to the plant associated with external by other licensees as part of their Individual Plant Examination of flooding or loss of heat sink from the failure of upstream and External Events (IPEEE) submittal downstream dams or levees.

• NSAC-60 and NUREG/CR-5042 may have percolated further as Staff will evaluate if NUREG/CR-5042 needs to be revisited.

a screening value through the industry (most IPEEEs screened out dam failures)

Information Notice

• IN Status

- Currently in concurrence

• Conclusions and Recommendations

- Events in Japan have increased general interest on external flooding risk. However, the focus of the IN was not impacted by the accident.

- GIP screening assessment reviews support the risk significance of the dam failure issue.

- Staff recommends issuing the IN.

- Follow-up generic implications expected to be addressed through the GIP 2