Information Notice 2012-02, Potentially Nonconservative Screening Value for Dam Failure Frequency in Probabilistic Risk Assessments: Difference between revisions

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{{#Wiki_filter:ML090510269 UNITED STATES


NUCLEAR REGULATORY COMMISSION
NUCLEAR REGULATORY COMMISSION
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OFFICE OF NEW REACTORS
OFFICE OF NEW REACTORS


WASHINGTON, DC 20555-0001 March 5, 2012 NRC INFORMATION NOTICE 2012-02:                 POTENTIALLY NONCONSERVATIVE
WASHINGTON, DC 20555-0001  
 
March 5, 2012  
 
NRC INFORMATION NOTICE 2012-02:  
POTENTIALLY NONCONSERVATIVE


SCREENING VALUE FOR DAM FAILURE
SCREENING VALUE FOR DAM FAILURE
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approved compliance plan under 10 CFR Part 76, Certification of Gaseous Diffusion Plants.
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,
All holders of and applicants for a specific source material license under 10 CFR Part 40,  
Domestic Licensing of Source Material.
Domestic Licensing of Source Material.


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originated in 1980s reference documents which may have been referenced by licensees in their
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
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
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


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
 
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
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
appropriate, to avoid similar problems. Suggestions contained in this IN are not NRC


requirements; therefore, no specific action or written response is required.
requirements; therefore, no specific action or written response is required.
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frequency contained in Nuclear Safety Analysis Center (NSAC) report NSAC-60, A Probabilistic
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
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
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
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
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 using historical dam failures to estimate the annual failure frequency at the time the
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analysis was performed (1981) and an associated uncertainty range for dams built within
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
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
NSAC-60 PRA analysis determined that the median annual failure frequencies for the Jocassee
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using only the data deemed applicable to the Jocassee Dam, based on characteristics such as
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
(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
acknowledged the challenge in collecting sufficient historical information based on the scarcity


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NUREG/CR-5042, Evaluation of External Hazards to Nuclear Power Plants in the United
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
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
with results of 10-6/year or even smaller for modern well-engineered dams and a range of
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(NUREG/CR-5042, page 5-8, Agencywide Documents Access and Management System
(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
(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
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.
references such as NUREG/CR-5042.
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not the result of a dam-specific analysis but instead utilized a generic dam failure database to
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 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
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
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
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
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.
respect to a generic estimate.
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including or excluding certain failure events and operational years deemed to be applicable to
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
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
integrated manner and instead considered several hazard categories separately, leading to
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certain hazard categories being completely excluded from the calculation of dam failure
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
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
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
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
considered severe precipitation events to be negligible contributors to downstream impacts and
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screened them out from further consideration, although these types of events are responsible
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
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
associated with nonhydrologic, nonseismic phenomena (e.g., internal erosion/degradation) were
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screened out because they were deemed not applicable based on site-specific design
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
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
modes from the historical data and special treatment of failure modes may require additional
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detailed analyses beyond the consideration of the specific dam design features in order to
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
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
hazard categories such as hydrologic events without an integrated assessment could lead to an
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To assess the impact of the NSAC-60 screening assumptions, the NRC staff reviewed currently
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
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
NRC staff used the databases to find (1) the number of historical failures of dams of a particular
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characteristic, such as dam type, and (2) the total number of years of operation for dams of the
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
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.mil1
 
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.


Inventory of Dams maintained by the U.S. Army Corps of Engineers (USACE),
. While these databases contain valuable information and are more
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
complete and accurate than information available when NSAC-60 was prepared, the NRC staff
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still found challenges in using the databases to justify with sufficient technical basis a very low
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
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
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
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
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
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
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potential users also need to take into account when deriving estimates for low-probability
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
events. For example, the NRC staff found that, for historical dam failure events, there is, as indicated by NSAC-60, an inherent challenge in the completeness of failure event accounts
 
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
(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
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
modes from consideration without sufficient technical basis while retaining the population
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To understand the impact of these data challenges, the NRC staff analyzed the ranges of
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
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
staff also reviewed (1) the literature on previously published dam failure rates based on
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publications, which can be obtained from the U.S. Department of the Interiors Bureau of
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
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
Assessment and Historical Data from Bureau of Reclamation Dams, revised in 1998; and Dam
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performance and failure modes for certain dam types, its applicability to site-specific dams has
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
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
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
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
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
justified depending on the quality and quantity of the supporting information beyond certain
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Both NSAC-60 and NUREG/CR-5042 reference dam failure rate estimates in the context of
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
external flooding analyses incorporating a dam failure. However, recent NRC reviews


determined that the generic failure frequency estimate used in NSAC-60 combined generic
determined that the generic failure frequency estimate used in NSAC-60 combined generic
Line 295: Line 300:
available in published literature on latest dam risk assessment methodologies and NRC staffs
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
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
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
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
for this include the fact that generic failure frequency values may not account for site-specific
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information to site-specific dams, which may counteract conservative assumptions in the use of
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
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
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.
NUREG/CR-5042 based on the items discussed in this generic communication.
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Generic failure rate estimates encompass all documented dam failures, irrespective of their
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
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
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
do so, addressees may evaluate their current or planned site-specific estimate of dam failure
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frequency in light of the information contained in the IN and address any resulting implications
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
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
an internal review under the Generic Issues Program, managed by the NRC Office of Nuclear
Line 340: Line 345:


==CONTACT==
==CONTACT==
This IN requires no specific action or written response. Please direct any questions about this
This IN requires no specific action or written response. Please direct any questions about this


matter to the technical contacts listed below.
matter to the technical contacts listed below.


/RA by EBenner for/                                     /RA/
/RA by EBenner for/
Douglas W. Weaver, Acting Director             Timothy J. McGinty, Director
 
/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 Spent Fuel Storage                  Division of Policy and Rulemaking
Division of Construction Inspection


and Transportation                            Office of Nuclear Reactor Regulation
Division of Fuel Cycle Safety and Safeguards


===Office of Nuclear Material Safety===
and Operational Programs
  and Safeguards


/RA/                                            /RA/
Office of Nuclear Material Safety and Safeguards
Laura A. Dudes, Director                        John D. Kinneman, Director


Division of Construction Inspection            Division of Fuel Cycle Safety and Safeguards
Office of New Reactors


and Operational Programs                    Office of Nuclear Material Safety and Safeguards
/RA by BWatson for/


===Office of New Reactors===
Larry W. Camper, Director
/RA by BWatson for/


===Larry W. Camper, Director===
Division of Waste Management
Division of Waste Management


===Office of Federal and State Materials===
Office of Federal and State Materials
  and Environmental Management Programs
 
and Environmental Management Programs


Technical Contacts:     Fernando Ferrante, NRR/DRA                Jeffrey Mitman, NRR/DRA
Technical Contacts: Fernando Ferrante, NRR/DRA


301-415-8385                              301-415-2843 E-mail: Fernando.Ferrante@nrc.gov E-mail: Jeffrey.Mitman@nrc.gov
Jeffrey Mitman, NRR/DRA


Note: NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under NRC Library.
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.


ML102210339).
ML102210339).


==CONTACT==
==CONTACT==
This IN requires no specific action or written response. Please direct any questions about this
This IN requires no specific action or written response. Please direct any questions about this


matter to the technical contacts listed below.
matter to the technical contacts listed below.


/RA by EBenner for/                               /RA/
/RA by EBenner for/
Douglas W. Weaver, Acting Director                Timothy J. McGinty, Director


Division of Spent Fuel Storage                    Division of Policy and Rulemaking
/RA/
Douglas W. Weaver, Acting Director


and Transportation                              Office of Nuclear Reactor Regulation
Timothy J. McGinty, Director


===Office of Nuclear Material Safety===
Division of Spent Fuel Storage
  and Safeguards


/RA/                                              /RA/
Division of Policy and Rulemaking
Laura A. Dudes, Director                          John D. Kinneman, Director


Division of Construction Inspection              Division of Fuel Cycle Safety and Safeguards
and Transportation


and Operational Programs                      Office of Nuclear Material Safety and Safeguards
Office of Nuclear Reactor Regulation


===Office of New Reactors===
Office of Nuclear Material Safety
/RA by BWatson for/
 
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


===Larry W. Camper, Director===
Division of Waste Management
Division of Waste Management


===Office of Federal and State Materials===
Office of Federal and State Materials
  and Environmental Management Programs
 
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


Technical Contacts:     Fernando Ferrante, NRR/DRA                    Jeffrey Mitman, NRR/DRA
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


301-415-8385                                  301-415-2843 E-mail: Fernando.Ferrante@nrc.gov E-mail: Jeffrey.Mitman@nrc.gov
D:DPR:NRR


Note: NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under NRC Library.
NAME


ADAMS Accession Number: ML090510269 OFFICE  APOB:NRR        APOB:NRR        Tech Editor      BC:APOB:NRR      D:DRA:NRR    NRR/DE
SRosenberg


NAME    F Ferrante      J Mitman        KKribbs          J Circle        M Cheok      G Wilson
J Kinneman


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
DWeaver


NAME    W Ott          K Coyne        R Raione          L Mrowca        CHawes      DBeaulieu
LCamper


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
L Dudes


NAME    SRosenberg      J Kinneman      DWeaver          LCamper          L Dudes      T McGinty
T McGinty


OFFICE   7/25/11         1/30/12         2/02/2012         3/02/12         3/05/12     3/05/12}}
OFFICE 7/25/11  
1/30/12  
2/02/2012  
3/02/12  
3/05/12  
3/05/12}}


{{Information notice-Nav}}
{{Information notice-Nav}}

Latest revision as of 13:07, 14 January 2025

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)
v • d • e


ML090510269 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.mil1

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.

. 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 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.

ML102210339).

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.

ML102210339).

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


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