Information Notice 2012-02, Potentially Nonconservative Screening Value for Dam Failure Frequency in Probabilistic Risk Assessments
| ML090510269 | |
| Person / Time | |
|---|---|
| Issue date: | 03/05/2012 |
| From: | Mcginty T, Camper L, Laura Dudes, Kinneman J, Weaver D Division of Policy and Rulemaking, NRC/FSME/DWMEP, NRC/NMSS/FCSS, NRC/NMSS/SFST, Office of New Reactors |
| To: | |
| Ferrante, Fernando NRR/DRA, 415-8385 | |
| References | |
| FOIA/PA-2012-0325 IN-12-002 | |
| Download: ML090510269 (6) | |
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
OFFICE OF NUCLEAR MATERIAL SAFETY AND SAFEGUARDS
OFFICE OF FEDERAL AND STATE MATERIALS AND
ENVIRONMENTAL MANAGEMENT PROGRAMS
OFFICE OF NEW REACTORS
WASHINGTON, DC 20555-0001 March 5, 2012 NRC INFORMATION NOTICE 2012-02: POTENTIALLY NONCONSERVATIVE
SCREENING VALUE FOR DAM FAILURE
FREQUENCY IN PROBABILISTIC RISK
ASSESSMENTS
ADDRESSEES
All holders of an operating license or construction permit for a nuclear power reactor under
Title 10 of the Code of Federal Regulations (10 CFR) Part 50, Domestic Licensing of
Production and Utilization Facilities.
All holders of or applicants for an early site permit, standard design certification, standard
design approval, manufacturing license, or combined license under 10 CFR Part 52, Licenses, Certifications, and Approvals for Nuclear Power Plants.
All holders of and applicants for a fuel cycle facility license under 10 CFR Part 70, Domestic
Licensing of Special Nuclear Material.
All holders of and applicants for an independent spent fuel storage installation license under
10 CFR Part 72, Licensing Requirements for the Independent Storage of Spent Nuclear Fuel, High-Level Radioactive Waste, and Reactor-Related Greater Than Class C Waste.
All holders of and applicants for a gaseous diffusion plant certificate of compliance or an
approved compliance plan under 10 CFR Part 76, Certification of Gaseous Diffusion Plants.
All holders of and applicants for a specific source material license under 10 CFR Part 40,
Domestic Licensing of Source Material.
All Agreement State Radiation Control Program Directors and State Liaison Officers.
PURPOSE
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to alert
addressees of a potentially nonconservative screening value for dam failure frequency that
originated in 1980s reference documents which may have been referenced by licensees in their
probabilistic risk assessment (PRA) for external events. Using a nonconservative screening
value for dam failure frequency to evaluate the need for an additional detailed analysis may
result in underestimating the risks to the plant associated with external flooding or loss of heat
sink from the failure of upstream and downstream dams or levees. The NRC expects that
recipients will review the information for applicability to their facilities and consider actions, as
appropriate, to avoid similar problems. Suggestions contained in this IN are not NRC
requirements; therefore, no specific action or written response is required.
BACKGROUND
The NRC staff has identified a potentially nonconservative screening value for dam failure
frequency contained in Nuclear Safety Analysis Center (NSAC) report NSAC-60, A Probabilistic
Risk Assessment of Oconee Unit 3, issued June 1984. NSAC-60 calculated a value for the
dam failure frequency of the Jocassee Dam, a value referenced by other licensees in their
flooding analyses. The NSAC-60 PRA study determined the failure frequency for the Jocassee
Dam by compiling data for dams with similar attributes. NSAC-60 then performed a Bayesian
analysis using historical dam failures to estimate the annual failure frequency at the time the
analysis was performed (1981) and an associated uncertainty range for dams built within
different time periods: 1900-1981, 1940-1981, and 1960-1981. For these time periods, the
NSAC-60 PRA analysis determined that the median annual failure frequencies for the Jocassee
Dam based on each of these data ranges were 2.3x10-5/year, 1.6x10-5/year, and 1.4x10-5/year, respectively, from causes other than earthquakes and overtopping (which NSAC-60 treated
separately).
The NSAC-60 approach estimated the failure frequency for the Jocassee Dam by screening the
available historical data for United States (U.S.) dams at the time of the analysis (1981) and
using only the data deemed applicable to the Jocassee Dam, based on characteristics such as
(1) dam composition, (2) construction completion date, and (3) failure modes. NSAC-60
acknowledged the challenge in collecting sufficient historical information based on the scarcity
of the data applicable to the specific dam characteristics considered, as well as the complexity
of the actual phenomena controlling dam failures and their potential impacts on a nuclear power
plant site.
The NRC subsequently included the NSAC-60 dam failure frequency results in
NUREG/CR-5042, Evaluation of External Hazards to Nuclear Power Plants in the United
States, which was initially published in 1987. NUREG/CR-5042 reported bounding calculations
with results of 10-6/year or even smaller for modern well-engineered dams and a range of
values between 10-4/year and 10-5/year, referring to NSAC-60 in both cases
(NUREG/CR-5042, page 5-8, Agencywide Documents Access and Management System
(ADAMS) Accession No. ML111950285). Some licensees referred directly to NSAC-60 as part
of their Individual Plant Examination of External Events submittal in response to Supplement 4 to Generic Letter 88-20, Individual Plant Examination for Severe Accident Vulnerabilities, dated November 23, 1988. In other cases, licensees may have indirectly used NSAC-60 via
references such as NUREG/CR-5042.
Bounding values for dam failure frequency, such as those mentioned in NUREG/CR-5042 were
not the result of a dam-specific analysis but instead utilized a generic dam failure database to
determine a bounding generic estimate of dam failure frequency. The information used to
determine the generic estimate can be a starting point to estimate bounding values of dam
failure frequency for another dam by adjusting for dam-specific differences involving design, operation, and potential failure mechanisms. For example, including historical dam failure
events that failed in a manner that could not occur at the dam being analyzed would increase
the estimated dam failure frequency. Also, consideration of site-specific characteristics such as meteorology and hydrology may either increase or decrease the likelihood of a dam failure with
respect to a generic estimate.
DESCRIPTION OF CIRCUMSTANCES
After reviewing the dam failure frequency estimates in NSAC-60, the NRC staff noted that many
assumptions in the dam failure rate estimation approach used in NSAC-60 are strongly
dependent on the completeness and accuracy of the dam data used and on the criteria for
including or excluding certain failure events and operational years deemed to be applicable to
characteristics specific to the dam considered in NSAC-60 (i.e., type, height, construction year, and years of operation). The NRC staff also noted that NSAC-60 did not consider hazards in an
integrated manner and instead considered several hazard categories separately, leading to
certain hazard categories being completely excluded from the calculation of dam failure
frequency. In particular, the dam failure frequency derived in NSAC-60 was representative of
failure modes resulting from causes other than hydrologic (e.g., severe precipitation) and
seismic events, which NSAC-60 considered in separate analyses. The NSAC-60 method
considered severe precipitation events to be negligible contributors to downstream impacts and
screened them out from further consideration, although these types of events are responsible
for the majority of the dam failures recorded in historical data. Additionally, failure modes
associated with nonhydrologic, nonseismic phenomena (e.g., internal erosion/degradation) were
screened out because they were deemed not applicable based on site-specific design
considerations made in NSAC-60. Based on the NRC staffs assessment, screening failure
modes from the historical data and special treatment of failure modes may require additional
detailed analyses beyond the consideration of the specific dam design features in order to
establish sufficient technical bases. Additionally, the NRC staff believes that excluding credible
hazard categories such as hydrologic events without an integrated assessment could lead to an
underestimation of the true dam failure frequency.
To assess the impact of the NSAC-60 screening assumptions, the NRC staff reviewed currently
available databases for U.S. dams. In order to determine generic dam failure frequencies, the
NRC staff used the databases to find (1) the number of historical failures of dams of a particular
characteristic, such as dam type, and (2) the total number of years of operation for dams of the
same characteristic. The NRC staff reviewed databases of historical dam failure events, primarily the National Performance of Dams Program by Stanford University, http://npdp.stanford.edu, and a database of the existing U.S. dam population, the National
Inventory of Dams maintained by the U.S. Army Corps of Engineers (USACE),
http://nid.usace.army.mil 1. While these databases contain valuable information and are more
complete and accurate than information available when NSAC-60 was prepared, the NRC staff
still found challenges in using the databases to justify with sufficient technical basis a very low
value of dam failure frequency using only historical data. The databases were not created for
the specific purpose of performing dam failure frequency calculations and were not designed to
be fully consistent with each other. Notwithstanding, these databases are still the primary
source of information on existing dams and events. The discussion contained in this IN does
not intend to express judgment on the quality of the efforts to develop these databases; instead, it highlights the challenges in the input and categorization of data for such a wide population that
potential users also need to take into account when deriving estimates for low-probability
events. For example, the NRC staff found that, for historical dam failure events, there is, as
1 Although the USACE Web site states, Non-government users are no longer able to directly download
any data from this site, nongovernment users can request access to the data by contacting the USACE
staff member specified on this Web site. indicated by NSAC-60, an inherent challenge in the completeness of failure event accounts
(e.g., construction year of failed dam and failure mode) and the consistency of definitions used
on both failed and operating dams (e.g., dam types). In particular, eliminating selected failure
modes from consideration without sufficient technical basis while retaining the population
contribution for total number of years can produce an artificially low dam failure frequency.
To understand the impact of these data challenges, the NRC staff analyzed the ranges of
generic dam failure frequency estimates that can be derived from these databases. The NRC
staff also reviewed (1) the literature on previously published dam failure rates based on
historical evidence for the U.S. and the international population of dams, and (2) available
information on the latest dam risk assessment methodologies, including the following
publications, which can be obtained from the U.S. Department of the Interiors Bureau of
Reclamation (USBR) (http://www.usbr.gov): Comparison of Failure Modes from Risk
Assessment and Historical Data from Bureau of Reclamation Dams, revised in 1998; and Dam
Safety Risk Analysis Best Practices Training Manual, Version 2.1, issued in 2010.
Although historical dam failure information can provide useful qualitative insights on the general
performance and failure modes for certain dam types, its applicability to site-specific dams has
to be assessed to establish sufficient technical bases. This is due to the variability in site- specific characteristics (i.e., hydrologic, geologic, and operational) and the potential
contributions of site-specific failure modes not covered by databases. The range of estimates
presented in NSAC-60 (i.e., between 2.3x10-5/year and 1.4x10-5/year) is below the range of
estimates found in the available literature for generic dam failure rate estimates. Additionally, frequency extrapolations of severe weather phenomena with insufficient basis may not be fully
justified depending on the quality and quantity of the supporting information beyond certain
values (e.g., see DSO-04-08, Hydrologic Hazard Curve Estimating Procedures, issued
June 2004 by USBR).
DISCUSSION
Both NSAC-60 and NUREG/CR-5042 reference dam failure rate estimates in the context of
external flooding analyses incorporating a dam failure. However, recent NRC reviews
determined that the generic failure frequency estimate used in NSAC-60 combined generic
information with site-specific screening criteria that produced median values lower than those
available in published literature on latest dam risk assessment methodologies and NRC staffs
assessments. Consideration of data sources currently available also indicates that (1) such
significantly lower values may not be justified by historical data alone, and (2) applying the
NSAC-60 estimate to other dams with different characteristics may be inappropriate. Reasons
for this include the fact that generic failure frequency values may not account for site-specific
features and can be highly dependent on the completeness and applicability of available
information to site-specific dams, which may counteract conservative assumptions in the use of
data. Hence, both NSAC-60 and NUREG/CR-5042 provide an insufficient basis for estimating
site-specific dam failure frequency. NRC staff intends to evaluate the need to modify
NUREG/CR-5042 based on the items discussed in this generic communication.
These considerations indicate that data available in these databases are useful in identifying
failure mechanisms and performance insights as well as approximate generic dam failure rate
estimates, but may not provide sufficient basis for site-specific estimates or to screen out the
contribution of external flooding sources or loss of ultimate heat sink to the overall plant risk.
Generic failure rate estimates encompass all documented dam failures, irrespective of their
potential impacts on a downstream site. By including a large population of dams with a wide variety of features, the resulting failure frequency may or may not be appropriate for any one
specific dam or nuclear power plant site. Although there is no specific regulatory requirement to
do so, addressees may evaluate their current or planned site-specific estimate of dam failure
frequency in light of the information contained in the IN and address any resulting implications
on their external event PRA. Based on the information discussed above, NRC staff has initiated
an internal review under the Generic Issues Program, managed by the NRC Office of Nuclear
Regulatory Research, to consider this issue in a generic sense (ADAMS Accession No.
CONTACT
This IN requires no specific action or written response. Please direct any questions about this
matter to the technical contacts listed below.
/RA by EBenner for/ /RA/
Douglas W. Weaver, Acting Director Timothy J. McGinty, Director
Division of Spent Fuel Storage Division of Policy and Rulemaking
and Transportation Office of Nuclear Reactor Regulation
Office of Nuclear Material Safety
and Safeguards
/RA/ /RA/
Laura A. Dudes, Director John D. Kinneman, Director
Division of Construction Inspection Division of Fuel Cycle Safety and Safeguards
and Operational Programs Office of Nuclear Material Safety and Safeguards
Office of New Reactors
/RA by BWatson for/
Larry W. Camper, Director
Division of Waste Management
Office of Federal and State Materials
and Environmental Management Programs
Technical Contacts: Fernando Ferrante, NRR/DRA Jeffrey Mitman, NRR/DRA
301-415-8385 301-415-2843 E-mail: Fernando.Ferrante@nrc.gov E-mail: Jeffrey.Mitman@nrc.gov
Note: NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under NRC Library.
CONTACT
This IN requires no specific action or written response. Please direct any questions about this
matter to the technical contacts listed below.
/RA by EBenner for/ /RA/
Douglas W. Weaver, Acting Director Timothy J. McGinty, Director
Division of Spent Fuel Storage Division of Policy and Rulemaking
and Transportation Office of Nuclear Reactor Regulation
Office of Nuclear Material Safety
and Safeguards
/RA/ /RA/
Laura A. Dudes, Director John D. Kinneman, Director
Division of Construction Inspection Division of Fuel Cycle Safety and Safeguards
and Operational Programs Office of Nuclear Material Safety and Safeguards
Office of New Reactors
/RA by BWatson for/
Larry W. Camper, Director
Division of Waste Management
Office of Federal and State Materials
and Environmental Management Programs
Technical Contacts: Fernando Ferrante, NRR/DRA Jeffrey Mitman, NRR/DRA
301-415-8385 301-415-2843 E-mail: Fernando.Ferrante@nrc.gov E-mail: Jeffrey.Mitman@nrc.gov
Note: NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under NRC Library.
ADAMS Accession Number: ML090510269 OFFICE APOB:NRR APOB:NRR Tech Editor BC:APOB:NRR D:DRA:NRR NRR/DE
NAME F Ferrante J Mitman KKribbs J Circle M Cheok G Wilson
DATE 4/ 11 /2011 4/ 11 /2011 1/17/2011 e-mail 7/14/2011 7/15/2011 05/18/2011 OFFICE BC:ETB:RES BC:PRAB:RES BC:RHEB:NRO BC:SPRA:NRO LA:PGCB:NRR PM:PGCB:NRR
NAME W Ott K Coyne R Raione L Mrowca CHawes DBeaulieu
DATE 4/ 04 /2011 4/ 08 /2011 4/ 05 /2011 4/ 08 /2011 7/25/2011 7/21/2011 OFFICE BC:PGCB:NRR D:DFCSS:NMSS D:DSFST:NMSS D:DWMEP:FSME D:DCIP:NRO D:DPR:NRR
NAME SRosenberg J Kinneman DWeaver LCamper L Dudes T McGinty
OFFICE 7/25/11 1/30/12 2/02/2012 3/02/12 3/05/12 3/05/12