ML14058A058
| ML14058A058 | |
| Person / Time | |
|---|---|
| Site: | Oconee  |
| Issue date: | 02/19/2010 |
| From: | Jeffrey Mitman Office of Nuclear Reactor Regulation |
| To: | Lois James Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML14055A421 | List:
|
| References | |
| FOIA/PA-2012-0325 | |
| Download: ML14058A058 (16) | |
Text
Mitman, Jeffrey 1A) g (/
From: Mitman, Jeffrey Sent: Friday, February 19, 2010 6:05 PM To: James, Lois N/1 P2'-.
Cc: Ferrante, Fernando
Subject:
RE: ACTION: provide BC with latest version of DRA/APOB's documentation of the generic dam failure frequency by COB 2/19/10 Attachments: APOB Failure Rate Evaluation for Jocassee Dam Revised (3).doc Lois, attached is the latest version of the requested analysis. Fernando and I have worked on in to incorporate the concerns regarding peer review (re ADM-504) and suggestions and issues raised by DE and others. I know of no additional changes which are required.
Fernando, I've made only very minor punctuation changes and formatting changes to your most recent version and I've accepted all Word revision mark changes.
To remove the "draft" designation it should be reviewed by Jim Vail as a co-author and the independent review needs to be performed (reperformed). I assume this would be by Steve.
Once this is completed we can formally transmit it again to the front office and put it in Adams as an official agency record.
Jeff From: James, Lois Sent: Friday, February 19, 2010 10:55 AM To: Mitman, Jeffrey
Subject:
ACTION: provide BC with latest version of DRA/APOB's documentation of the generic dam failure frequency by COB 2/19/10
- Jeff, Since you will be out next week and we need to continue work on the dam failure frequency, please email me the latest version of the document in word format. I believe you were making revisions based on reviewing LIC-504.
Lois 1 1.~ Fb
DRAFT
(-I IU.S.NRC UNITED STATES NUCLEAR REGULATORY COMMISSION ProtectingPeople and the Environment Dam Failure Rate Evaluation Probabilistic Risk Assessment (PRA) Analyst: James Vail, Reliability and Risk Analyst, NRR/DRA/APOB Probabilistic Risk Assessment (PRA) Analyst: Fernando Ferrante, Reliability and Risk Analyst, NRR/DRA/APOB Probabilistic Risk Assessment (PRA) Analyst: Jeff Mitman, Senior Reliability and Risk Analyst, NRRJDRA/APOB Peer Reviewer: Steven A. Laur, Senior Technical Advisor NRR/DRA SE!NS1 I iVE! W-0 U,,TJ.*- OFFICIAl & '
APOB Failure Rate Evaluation for Jocassee Dam Revised (3).doc DRAFT 10/5/2012 GENERIC FAILURE RATE EVALUATION FOR JOCASSEE DAM BY DIVISION OF RISK ASSESSMENT'S PRA OPERATIONAL SUPPORT AND MAINTENANCE BRANCH The following documents a generic dam failure rate analysis applicable to the Jocassee Dam performed by the PRA Operational Support and Maintenance Branch (APOB) of the Division of Risk Assessment (DRA) in the Office of Nuclear Reactor Regulation (NRR). The analysis, technical justifications, and databases used in support of the calculations for the derived value are briefly discussed. This evaluation was initially performed in 2007 but was not formally documented at that time.
Approach The approach used in deriving a generic failure rate value applicable to the Jocassee Dam included: (i)an evaluation of the physical characteristics and description of the dam, (ii)an assessment of the overall U.S. dam population for those with similar features to the Jocassee Dam, (iii) a study of U.S. dam performance information for failure events that may be applicable to this subset of the overall population, and (iv)a calculation of a point estimate for the failure rate given the observed failure events and the observed time period (in dam-years). For items (ii)and (iii), two databases were used as sources of information and will be discussed below.
Jocassee Dam Description The Jocassee Dam is located in northwest South Carolina, forming a reservoir (Lake Jocassee) with a 7565-acre surface area, a water volume of 1,160,298 acre-feet, and a total drainage area of 147 sq-miles at full pond (1,110 feet elevation above mean sea level). The reservoir was created in 1973 with the construction of the dam. The Jocassee Dam is an embankment dam with an earthen core and rockfilled and random rockfilled zones (see Figure 1). The dam is 385 feet in height (1,125 crest elevation above mean sea level) and 1,825 feet in length and, along with two homogeneous earthfill dikes and a reinforced concrete spillway, is part of a hydroelectric station and pumped storage project. The underground powerhouse generating units receive water from two cylindrical intake towers through eight openings. The water is channeled from the intake towers to four hydro turbines by two bifurcated power tunnels which are constructed through the bedrock of the east abutment. Two gates 33 feet in height and 38 feet in width control the outflow of the spillway.
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APOB Failure Rate Evaluation for Jocassee Dam Revised (3).doc DRAFT 10/5/2012 Figure 1: Jocassee Dam Cross-Section (b)(7)(F)
Databases The staff used two databases to obtain information about the population of dams in the US: the National Inventory of Dams (NID), maintained by the US Army Corps of Engineers, and the National Performance of Dams Program (NPDP), developed by the Department of Civil and Environmental Engineering at Stanford University. The NID database contains data describing multiple attributes such as dimensions, type, impoundment characteristics, etc. The NPDP database contains a collection of dam incident reports searchable by various parameters including dam type, incident type, and consequences.
Failure Events Table 1 lists the applicable dam failures initially derived from the NPDP database. To choose these 13 failures, the analysts used criteria based on the previously discussed dam characteristics (i.e., dam type and height). However, due to the ambiguity in the classification of the dam type (i.e., based on material composition) between and within the NID and NPDP databases, as well as the lack of information to establish an exact link with the Jocassee Dam characteristics for every data point, the staff determined that both rockfill dams and mixed-rockfill dams would be considered (i.e., those classified exclusively as rockfill dams as well as mixed dam types that include rockfill in their categorization). It should be noted that the NPDP database does not list any failures post-2006 and at least two well-known large dam failures in the U.S. are not included: the Big Bay Dam in Mississippi (March 2004) and the Taum Sauk Reservoir (December 2005) in Missouri. While the Big Bay Dam was an earthen dam (i.e.,
excluded based on dam type), the Taum Sauk Reservoir consisted of a concrete-faced rockfill dam approximately 100 feet in height and is, therefore, included in the current analysis.
Additionally, the list was screened using two criteria for failure events observed prior to (i) 1900, and (ii) 1940, under the assumption that events prior to these construction periods could 2
APOB Failure Rate Evaluation for Jocassee Dam Revised (3).doc DRAFT 10/5/2012 produce different results representative of distinct design practices. In part, this choice is due to the lack of information on the exact construction date of several dams in the database. The staff expended an extensive effort to determine the construction completion date for several dams for which the information is missing in the NPDP database (this information is included in Table 1).
Several failures listed in Table 1 have (or are assumed to have) occurred within a few years of either the start or completion of construction (e.g., the Lower Hell Hole Dam and the Frenchman Dam failures). Based on the information available and the estimated completion dates, the staff screened out such failures since the occurrence of the events is assumed to be related to the construction phase and, therefore, not applicable to a mature dam such as Jocassee.
Finally, the analysts chose to include the Dresser No. 4 Dam failure, because they deemed this dam to be similar to the Jocassee Dam in composition (i.e., a large mixed earthfill-rockfill dam),
despite the fact that it is listed as a tailings dam (i.e., a dam theoretically built under lower standards of quality and maintenance).
Therefore, the final list of failures of dams similar to, and therefore applicable to, the Jocassee Dam includes 6 failures occurring between 1900 and 2005. These six failures are highlighted in Table 1. The staff included these failures based on the following criteria: (i) rockfill or mixed-rockfill dam type, (ii) dam height above 50 feet, (iii) failure occurring after 1900, and (iv) no failures during or within a few years of completion of construction. Note that if failures occurring prior to 1940 are screened, then only 4 events remain: (1) Taum Sauk, (2) Dresser No.4 Dam, (3) Skagway, and (4) Kern Brothers Reservoir.
Total Dam-years Calculation To calculate the dam failure rate, the staff needed to also obtain the total number of dam-years of both failed and non-failed dams. The analysts extracted a subset of dams from the NID database based on a set of parameters to narrow the US population of dams to those reflecting the characteristics of the Jocassee Dam discussed above: rockfill dams at least 50 feet in height (where NID categorizes rockfill dams under the 'ER' label for dam type, which is the label used for the Jocassee Dam in NID). They assumed that dams above 50 feet in height appropriately reflect design practices and structural characteristics of larger dams such as Jocassee. This height criterion is consistent with the large dam definition established by the International Commission on Large Dams (ICOLD) which "defines a large dam as a dam with a height of 15m or more from the foundation." If dams are between 5-15m high and have a reservoir volume of more than 3 million M3 , the World Commission on Dams ("Dams and Development," November 2000) also classified such dams as large. Hence, the staff used a height of 15 meters (or approx. 50 feet) as a screening criterion.
The staff included the dam-year contributions from Skagway and the replacement for the failed Frenchman Dam, while those from Kern Brothers Reservoir, Dresser No. 4 Dam, Penn Forest, and the failed Frenchman Dam were not; the staff does not anticipate the contribution from these omissions to significantly impact the resulting dam-year total. The staff calculated the final result using the difference between the last year (2005) and either 1900 or 1940. For the 1900-2005 period, the staff obtained a total of 21,490 dam-years; while for 1940-2005 the result is 13,889 dam-years. See Table 2 for a tabulation of the dams and the associated dam-years.
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APOB Failure Rate Evaluation for Jocassee Dam Revised (3).doc DRAFT 10/5/2012 Generic Point Estimate of the Dam Failure Rate The staff calculated the point estimate by dividing the number of applicable dam failures by the total applicable dam-years; as described above this includes large, rockfill or mixed rockfill dams. Assuming a 1900-2005 range for the year of occurrence of the failure events and the dam-year estimation (based on completion year), the analysts obtained a failure rate of 2.8E-4 per dam-year. When considering a 1940-2005 range, the staff obtained a result of 2.9E-4 per dam-year. Based on the criteria discussed in the previous sections, the staff performed a simple sensitivity study in order to evaluate the changes due to screening failure events between 1900-2005 and 1940-2005 periods.
Because the NID database does not give information regarding the quality of design, construction and/or maintenance, and the NPDP database does not supply information on the dam health (i.e., is it well maintained?) at time of failure, the staff could not derive failure rates for above or below average built and maintained dams. This lack of information precluded the staff from making any judgment as to whether Jocassee is or is not an above average designed, constructed and maintained dam deserving of a failure frequency different than an average failure frequency.
Additionally, the staff recognizes that ambiguity and lack of complete information with respect to dam type, construction completion data, and dam incident reporting, may result in variations in the failure rate estimation. Therefore, the staff performed a simple sensitivity study in order to evaluate the changes due to screening failure events and cut-off year criteria. The results are shown in Table 3 and clearly indicated that the results exhibit small variations for the period cut-off selected (1900-2005 and 1940-2005) and the number of failures considered (6 and 4, respectively). Additionally, the extent of the variation in the point estimate is shown for other number of failures and cut-off years based on the subset of dams selected.
Bayesian Estimate of the Dam Failure Rate To evaluate the dam failure rate uncertainty, the staff conducted a Bayesian analysis of the failure rate for the 1900-2005 period via an Empirical Bayes approach (Atwood, 2003). In this approach, an empirical prior distribution is derived from the number of failures and dam-years for each dam category identified in the NID and NPDP databases. The number of dam failure events is modeled as a Poisson distribution and the conjugate prior is assumed to follow a Gamma distribution (i.e., the conjugate prior in a Gamma-Poisson model). Following the Empirical Bayes approach, the prior distribution derived from the data is a Gamma distribution with parameters a = 2.4026 and P3 = 10095 (i.e., mean equal to 2.4E-4/dam-years). By updating the corresponding subset of the analysis (e.g., rockfill dams and failure events with characteristics deemed to be applicable to the Jocassee Dam) with this prior, the resulting posterior distribution has parameters a = 12.40 and 13= 29255, which corresponds to a Gamma distribution with mean equal to 4.2E-4/dam-years, a 5 th percentile of 2.5E-4/dam-years, and a 9 5 th percentile of 6.4E-4/dam-years. Figure 2 indicates both the generic dam prior and the posterior specific to rockfill dams.
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APOB Failure Rate Evaluation for Jocassee Dam Revised (3).doc DRAFT 10/5/2012 Figure 2: Failure Rate Probability Distributions Used in Bayesian Updating Z*UU 62000 2oo 0-15--
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1 2 3 4 5 6 7 8 9 Failure Rate (per dam-years) X 10-Conclusions The staff estimated generic dam failure rates which itconsiders applicable to the Jocassee Dam. The point estimate fluctuates around a 2E-4/dam-year value. Given the nature of the data and the assumptions involved in narrowing the applicable failure events and subset of the U.S. dam population comparable to this specific dam, the staff performed a simple sensitivity analysis for the failure rate and included an estimate using a Bayesian approach. This yielded a range between 1.4E-4 per dam-year and 4.3E-4 per dam-year ( 5th - 9 5 th percentile) around a mean of 2.7E-4 per dam-year. In other words, while a value between 1E-4/dam-year and 5E-4/dam-year could be possible under the set of assumptions and criteria used above, the staff concludes that results lower than 1E-4 per dam-year are not presently justifiable based on available statistical information.
A literature review performed by the authors for statistical studies of dam failures appears to corroborate this conclusion. Such studies were found in Baecher et al (1980), Donnelly (1994),
ICOLD (1995), Foster (2000a), and Foster et al (2000b). Additionally, the ANS/ANSI External Events PRA standard mentions the use of databases for the development of dam failure rates, where these "databases must be used with care, depending on how closely the specific dam fits into the database. The mean failure rate for all U.S. dams is in the range between about 104 and 10S/year," and that "for some modern dams with extensive engineering, values below 105/year have been quoted," finally concluding that an "accurate and useful probabilistic analysis of any specific dam would require detailed engineering evaluations."
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APOB Failure Rate Evaluation for Jocassee Dam Revised (3).doc DRAFT 10/5/2012 References ANS (2007), "ANSI/ANS-58.21-2007: External-events PRA methodology," American Nuclear Society, March 1, 2007 C.L. Atwood, J.L. LaChance, H.F. Martz, D.J. Anderson, M. Englehardt, D. Whitehead, and T.
Wheeler (2003), "Handbook of Parameter Estimation for Probabilistic Risk Assessment,"
NUREG/CR-6823, US NRC.
Foster M, Fell R, Spannagle M (2000a), "The statistics of embankment dam failures and accidents." Canadian Geotechnical Journal 37: 1000-1024 Foster M, Fell R, Spannagle M (2000b) "A method for assessing the relative likelihood of failure of embankment dams by piping. Canadian Geotechnical Journal 37: 1025-1061 "Dam Failures Statistical Analysis," Bulletin 99, International Commission on Large Dams, 1995 ICOLD (1995), "Dam Failures Statistical Analysis," Bulletin 99, International Commission on Large Dams.
Donnely, R. (1994), "Issues in Dam Safety, ACRES International Innovations Autumn Edition":
http://www. hatch, com.cn/Hatchenerpqy/Innovations/autumn2004/featu re. html Baecher, G. B., M. E. Patk, and R. De Neufville (1980), Risk of Dam Failure in Benefit-Cost Analysis, Water Resour. Res., 16(3), 449-456.
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APOB Failure Rate Evaluation for Jocassee Dam Revised (3).doc DRAFT 10/5/2012 du9 to overWp nof ri",Independent analysis Taum Sauk 2005 . 1963 Ro"k 94 ir"c -
MOtMM1dfot auss(tg, iifatorin aphamy).
DreserNo,4 D1975 Unknn / l . 10, Ca, p Mui that ad abreach 300feet wide Inte levee.
Ifo Flood.-
Skaga 1965 192 HrokroEe RAl 79 Thean faid during aflood in965.
Hell Hole e 194 1964 Not Known Rockfill 41 Dam failed dunng construction. Overtopped by 100 feet- washing out most of the fill.
Penn Forest 1960 1960 Piping Concrete Rockfll Earth 151 Partial failure. Sinkhole occurred inupstream slope of dam.
Frenchman 1952 1951 Inflow Flood- Rockfll 63 Runoff from melting snow. Adike section was overtopped eardy Dam Hydrologic Event morning Apri 15, 1952. Later that day, dam breached, Kern rothei 1949 Unknown Settlement Earth Rockflll 54 Failure due to excessive settlement of fil.
Resetvoir ____
Blowout failure under concrete spillway weir structure during period Lake Francis 1899 1899 Piping Earth Rockfill 79 of heavy spillway flow. Spillway failure thought to be due to piping in soft saturated foundation.
Lafayette 1928 1928 Embankment Slide Earth Rockfill 132 Foundation slide during construction (at 120 feet). Height raised to 170 feet in1932. Not sure ifthis isconsidered a failure.
Manitou 1924 1917 Seepage Earth Rockfill 123 Partial failure was disintegrating and converted into gravel fill.
Failure by piping through abutment; undermined by passage of water Lyman 1915 1912 Piping Earth Rockfdl 76.4 under cap of lava rock which flanked dam and extended beneath spillway. Main part of dam uninjured.
Lower Otay 1916 1897 Spillway Earth Rocill 15 Foundation slide during con*tudion (at 120 feet). Height raised to 170 feet in1932. Not re ifthis isconsidered afailure.
Failure by piping through abutment; undermined by passage of water Black Rock 1909 1908 Piping Earth Roclill 70 under cap of lava rock which flanked dam and extended beneath spillway. Portion of spillway dropped 7feet; some fill at south end washed out. Main part of dam uninjured.
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APOB Failure Rate Evaluation for Jocassee Dam Revised (3).doc DRAFT 10/5/2012 n-Year Tabulation S,.9.S', WFAM, ,-- 8IAL-0-F PFFIC 8
APOB Failure Rate Evaluation for Jocassee Dam Revised (3).doc DRAFT 10/5/2012 9
APOB Failure Rate Evaluation for Jocassee Dam Revised (3).doc DRAFT 10/5/2012 10
APOB Failure Rate Evaluation for Jocassee Dam Revised (3).doc DRAFT 10/5/2012 SERVOIR I (NP&LFERC) 11
. 0 APOB Failure Rate Evaluation for Jocassee Dam Revised (3).doc DRAFT 10/5/2012 12
APOB Failure Rate Evaluation for Jocassee Dam Revised (3).doc DRAFT 10/5/2012 83 1 075 63 65 427 61 117 50 175 51 132 53 8,101 1135 A 57 45 114 52 455 139 87 73 143 48955D 107 19.027 1255 118 205 103 53 630 5I 120 1 4 66 3r700 47 263 155s 225
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APOB Failure Rate Evaluation for Jocassee Dam Revised (3).doc DRAFT 10/5/2012 Table 3: Failure Rate Sensitivity Analysis
- NUMBER OF FAILURES CUT-OFF DAM-YEARS DAMS J1 2 [ 3 4 5 6 ALL 25137 484 4,OE-5 8.OE-5 I1.2E-4 I 1.6E.4 - 2.OE4 2.4E-4 1900 21490 466 4.7E-5 9.3E-5 1,4E4 1,9E4 2,3E4 2,8E-4 1910 19778 449 5.1E-5 1.1E4 1.5E-4 20.E4 2.5E4 3.OE..4 1920 18389 434 5.4E-5 1.1E4 1.6E4 2.2E4 2.7E-4 3.3E-4 1930 16475 410 6.1E-5 142E-4 1.8E-4 2.4E-4 3,0E-4 3.6E,4 1940 13889 373 7.2E-5 1,4E4 2.2E-4 2.9E4 3.6E4 4'Z4 1950 12269 346 8.2 E-5 1.6E.4 8.2E-5 1,6E4 2.5E.4 2.5E4 3.3E..4 3.3E4 I 4.IE-4 I 4.9E.4 4JE 7JE4 4.9E-4 1960 7154.. 5,14 8453 270 1.2E-4 2.464 HE4 Vt 4 5.
1970 3242 143 3.1 E-4 624 9.3E-4 1980 1339 82 1990 381 36 I FAILURE RATE GIVEN NUMBER OF FAILURES &CUTOFF YEAR 14