ML112770237

LOCA Frequencies for STP GSI-191( Final)

ML112770237
Person / Time
Site:	South Texas
Issue date:	09/30/2011
From:	Chrun D, Kreslyon Fleming, Lydell B South Texas, KNF Consulting Services, Risk Management, Scandpower
To:	Balwant Singal Plant Licensing Branch IV
Singal, B K, NRR/DORL, 301-415-301
Shared Package
ML112770162	List: ML112770170 ML112770176 ML112770203 ML112770230 ML112770237 ML112770246 ML112770251
References
TAC ME5358, TAC ME5359, GSI-191
Download: ML112770237 (78)

Text

Development of LOCA Initiating Event Frequencies for South Texas Project GSI-191 Final Report for 2011 Work Scope Developed for South Texas Project Electric Generating Station by Karl N. Fleming KNF Consulting Services LLC Bengt O. Y. Lydell Danielle Chrun September 2011

LOCA Frequencies for STP GSI191 Table of Contents

1. Introduction .......................................................................................................................................... 7 1.1 Background ................................................................................................................................... 7 1.2 Objectives...................................................................................................................................... 8 1.3 Report Guide ................................................................................................................................. 8

2. Technical Approach to LOCA Frequency Quantification....................................................................... 9 2.1 Basic LOCA Frequency Model ....................................................................................................... 9 2.2 StepbyStep Procedure for LOCA Frequency Evaluation ........................................................... 11

3. Failure Rate Development (Step 1) ..................................................................................................... 15 3.1 Definition of Component Types (Step 1.1) ................................................................................. 15 3.2 Evaluation Scope for 2011 .......................................................................................................... 18 3.3 Failure Data Query (Step 1.2)...................................................................................................... 18 3.4 Component Population Exposure (Step 1.3)............................................................................... 20 3.4.1 ReactorYears of Service Experience................................................................................... 20 3.4.2 Component Exposure Estimates for Hot Leg Welds ........................................................... 21 3.4.3 Degradation Mechanism Assessment ................................................................................. 22 3.4.4 Component Exposure for Hot Leg Welds ............................................................................ 24 3.5 Prior Distributions for Hot Leg Weld Failure Rates (Step 1.4) .................................................... 24 3.6 Failure Rate Bayes Updates (Step 1.5) ....................................................................................... 24 3.7 Failure Rate Distribution Synthesis (Steps 1.6 and 1.7) .............................................................. 27 3.8 Failure Rates for Other Calculation Cases (Step 1.7) .................................................................. 28

4. Conditional Rupture Mode Probability Model (Step 2) ...................................................................... 34 4.1 Overview of CRP Model Approach .............................................................................................. 34 4.2 Use of NUREG1829 Data ............................................................................................................ 36 4.3 Model for Deriving Conditional Probabilities from Rupture Frequencies .................................. 38 4.4 Select Components to Define CRP Model Categories (Step 2.1) ................................................ 39 4.5 Use of Data from NUREG1829 Expert Elicitation (Steps 2.2 and 2.3)........................................ 41 4.6 Development of 40Year LOCA Frequency Distributions (Step 2.4) ........................................... 41 4.7 Develop Expert Composite Distributions from NUREG1829 (Step 2.5)..................................... 47 4.8 Benchmark of Lydells Base Case Analysis (Step 2.6) .................................................................. 50 4.9 Select Target LOCA Frequencies from NUREG1829 Data (Step 2.7).......................................... 54 4.10 Develop Conditional Rupture Probabilities from Target LOCA Frequencies (Step 2.8) .............. 58 2 KNF Consulting Services LLC

LOCA Frequencies for STP GSI191 4.11 Bayes Update of the Conditional Probability Distributions (Step 2.9)....................................... 62

5. LOCA Frequencies for STP GSI191 Application (Step 3) .................................................................... 65 5.1 Weld Counts and Pipe Sizes for Each Component (Steps 3.1 and 3.2) ....................................... 65 5.2 Component LOCA Frequency Distributions (Step 3.3) ................................................................ 65 5.3 Application of Markov Model to Address Impact of NDE Program (Step 3.4) ........................... 66 5.4 Total LOCA Frequencies for RISKMAN (Step 3.6) ........................................................................ 71 5.5 LOCA Frequency Summary.......................................................................................................... 76

6. References .......................................................................................................................................... 77 3 KNF Consulting Services LLC

LOCA Frequencies for STP GSI191 Figures Figure 21 StepbyStep Procedure for LOCA Frequency Quantification - Page 1 of 2 .............................. 13 Figure 22 StepbyStep Procedure for LOCA Frequency Quantification - Page 2 of 2 .............................. 14 Figure 31 PIPExp Database and Relationship to Other Databases [20] ..................................................... 19 Figure 32 Damage and Degradation Mechanisms in Commercial Light Water Reactor Plants ................ 23 Figure 33 Event Tree Model to Represent Uncertainty in Hot Leg Weld Exposure for Thermal Fatigue.. 25 Figure 34 Comparison of Mean Failure Rates for Calculation Cases ......................................................... 33 Figure 41 Category 1 LOCA Frequencies for PWR Piping Systems at 25 Years of Plant Operation (Reproduced from Figure L.13 in NUREG1829) ......................................................................................... 38 Figure 42 Comparison of Mixture and Geometric Mean Composite Distributions - RCS Hot Leg ........... 49 Figure 43 Comparison of Mixture and Geometric Mean Composite Distributions - RCS Surge Line ....... 49 Figure 44 Benchmarking of Lognormal Distributions to Lydell Base Case Results - HPI Injection Line .... 51 Figure 45 Benchmarking of Lognormal Distributions to Lydell Base Case Results - RCS Surge Line ........ 51 Figure 46 Benchmarking of Lognormal Distributions to Lydell Base Case Results - RCS Hot Leg ............. 52 Figure 47 Comparison of Experts Geometric Mean and Lydell Base Case - RCS Hot Leg ........................ 55 Figure 48 Comparison of Experts Geometric Mean and Lydell Base Case - RCS Surge Line.................... 55 Figure 49 Comparison of Experts Geometric Mean and Lydell Base Case HPI Line ................................ 56 Figure 410 Comparison of Lydell and STP Models for CRP - RCS Hot Leg................................................. 60 Figure 411 Comparison of Lydell and STP Models for CRP - RCS Surge Line ............................................ 60 Figure 412 Comparison of Lydell and STP Models for CRP - HPI Line ....................................................... 61 Figure 51 LOCA Frequencies vs. Break Size for BF Welds in Hot Leg (Category 1A) ................................ 69 Figure 52 LOCA Frequencies vs. Break Size for BJ Welds in Hot Leg Subject to Thermal Fatigue (Category 1C) ............................................................................................................................................................... 69 Figure 53 Comparison of Mean Frequencies for Hot Leg Welds ............................................................... 70 Figure 54 Comparison of Weld Failure Rates Determined by Markov Model for Different Reliability Integrity Management Approaches ............................................................................................................ 71 Figure 55 Comparison of LOCA Frequencies for Pipes: STP vs. NUREG1829 ........................................... 73 Figure 56 Comparison of Uncertainty Distributions for STP PipeInduced LOCA and NUREG1829 Total LOCA Frequencies ....................................................................................................................................... 74 Figure 57 Contributions to Mean LOCA Category Frequencies by System................................................ 74 Figure 58 System Contributions to Mean LOCA Initiating Event Frequencies .......................................... 75 Figure 59 System Contribution to LOCA Category 6 Frequencies ............................................................. 75 4 KNF Consulting Services LLC

LOCA Frequencies for STP GSI191 Tables Table 21 StepbyStep Approach to LOCA Frequency Development ........................................................ 12 Table 31 Definition of Major Piping System Component Cases ................................................................ 16 Table 32 Definition of Specific Component Categories ............................................................................. 17 Table 33 Results of Class 1 Failure Data Query by System and Component ............................................. 20 Table 34 Service Experience by WestinghouseType PWRs ...................................................................... 21 Table 35 Estimation of Hot Leg Welds per Reactor ................................................................................... 22 Table 36 Damage Mechanism Assessment for Hot Leg Welds.................................................................. 22 Table 37 Prior Distributions for Weld Failure Rates by Damage Mechanism ........................................... 24 Table 38 Parameters of Bayes Updates for Hot Leg Weld Failure Rate Cases ......................................... 26 Table 39 Total Failure Rates for Hot Leg Weld Calculation Cases ............................................................. 27 Table 310 Component Population Exposure Estimates for Pipe Failure Rates ......................................... 28 Table 311 Damage Mechanism Susceptibility Matrix for Failure Rate Development ............................... 30 Table 312 Uncertainty Distributions for Calculation Case Failure Rates ................................................... 32 Table 41 NUREG1829 and STP PRA LOCA Categories............................................................................... 36 Table 42 Assignment of Piping System Categories to CRP Model Categories ........................................... 40 Table 43 NUREG1829 Expert Distributions for Hot Leg LOCA Frequencies ............................................. 44 Table 44 Composite Distributions for NUREG1829 Experts Based on Geometric Mean Method ........... 48 Table 45 Lognormal Distributions for Failure Rates and Conditional Rupture Probabilities (CRPs)

Matching Lydells Base Case Results ........................................................................................................... 52 Table 46 LOCA Frequency Distributions from Benchmarking of Lydell Base Case Results ....................... 53 Table 47 Mixture Distribution of Geometric Mean and Lydell Base Case for Target LOCA Frequencies .. 57 Table 48 Parameters of Target LOCA Frequencies Selected for STP Model .............................................. 59 Table 49 STP CRP Distribution Priors Derived from Target LOCA Frequencies ......................................... 61 Table 410 STP CRP Distributions after Bayes Updating ............................................................................ 63 Table 51 LOCA Frequencies vs. Break Size for Hot Leg, SG Inlet, Cold Leg, and Surge Line Component Categories 1A through 4B ........................................................................................................................... 67 Table 52 LOCA Frequencies vs. Break Size for Pressurizer and Small Bore Component Categories 5A through 6B .................................................................................................................................................. 67 Table 53 LOCA Frequencies vs. Break Size for Safety Injection and Recirculation System Categories 7A through 7L ................................................................................................................................................... 68 Table 54 LOCA Frequencies vs. Break Size for Accumulator Injection and CVCS Categories 7M through 8F

.................................................................................................................................................................... 68 Table 55 Results for Total Pipe BreakInduced LOCA Frequencies ............................................................ 73 5 KNF Consulting Services LLC

Acronyms ASME American Society of Mechanical Engineers BJ ASME Section XI Similar Metal Weld BF ASME Section XI Bimetallic Weld BC Branch Connection Weld CRP Conditional Rupture Probability CVCS Chemical Volume and Control System D&C Design and Construction Defects DEGB Double Ended Guillotine Break DM Damage (Degradation)Mechanism ECCS Emergency Core Cooling System EPRI Electric Power Research Institute GM Geometric Mean GSI Generic Safety Issue HPI High Pressure Injection IGSCC Intergranular Stress Corrosion Cracking LOCA Loss of Coolant Accident NPS Nominal Pipe Size PRA Probabilistic Risk Assessment PWR Pressurized Water Reactor PWSCC Primary Water Stress Corrosion Cracking PZR Pressurizer RCS Reactor Coolant System RIISI Risk Informed Inservice Inspection SB Small Bore SIR Safety Injection and Recirculation Systems TASC Thermal Stratification TF Thermal Fatigue TT Thermal Transients SC Stress Corrosion Cracking TGSCC Transgranular Stress Corrosion Cracking VF Vibration fatigue

LOCA Frequencies for STP GSI191

1. Introduction

1.1 Background

This report documents the analysis of loss of coolant accident (LOCA) frequencies in support of a risk informed evaluation of Generic Safety Issue (GSI) 191 for the South Texas Project Electric Generating Station (STPEGS) Units 1 and 2. The scope of work covered in this report is to develop the location and break sizedependent initiating event frequencies and associated uncertainties, and to provide technical support to interfacing tasks that are necessary to determine the risk significance of debrisinduced failures of core recirculation heat removal during LOCAs.

Historically, probabilistic risk assessments (PRAs) have included a small set of initiating events characterized by the physical sizes and throughwall flow rates associated with breaches in the primary reactor coolant system (RCS) pressure boundary, commonly known as LOCAs. Consideration of the location of the breach has largely been limited to that associated with socalled excessive LOCAs, i.e.

breaches in the reactor pressure vessel that exceed the capabilities of the emergency core cooling systems (ECCSs) to prevent core damage, and interfacing system LOCAs (ISLs). ISLs refer to events where the integrity of the RCS pressure boundary is breached through failure of isolation valves which separate the RCS from safety systems of lower design pressure. The resulting overpressurization could lead to a LOCA with leak flow path bypassing the containment and thereby defeating the recirculation cooling functions of the ECCS. In typical PRAs, the remaining LOCAs inside the containment are differentiated only with respect to size, based on there being different success criteria for preventing core damage for differentsized LOCAs. The differences in success criteria for the different LOCA sizes relate to differences in requirements for secondary side heat removal, high pressure and low pressure safety injection, and for implementing reactor shutdown.

The current STPEGS PRA model has different initiating events for breaches with equivalent break size of 0.5" to 2.0", referred to as Small LOCAs, those with break sizes between 2" and 6", referred to as Medium LOCAs, and those with break sizes from 6" up to and including a doubleended break from the largest pipe in the RCS, known as Large LOCAs. The Very Small LOCAs, with break sizes less than 0.5", are excluded because they would be small enough to be within the makeup of the chemical volume and control system (CVCS), whose operation would be expected to preclude a safety system actuation to mitigate a LOCA.

The STP RiskInformed GSI191 Closure study investigates the size and location of LOCAs more finely in order to assess the risk of debris formation during the LOCAs that could interfere with the operation of the ECCSs during the recirculation phase after an RCS breach. The size and location of the break could influence the amount of debris formation and the timing and need for actions to initiate or terminate containment sprays and recirculation cooling. The purpose of this study is to revise the LOCA initiating event frequency as needed to determine the most risksignificant break sizes and locations for this generic safety issue.

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LOCA Frequencies for STP GSI191 1.2 Objectives The objectives of this study are to:

Define of a sufficient set of RCS piping system failure categories to support each location to be evaluated for debris formation - to be worked out with the integrated team.

Provide failure rates vs. break size for all LOCA locations within the scope of the evaluation. This includes a full quantification of aleatory and epistemic uncertainties that addresses both parameter and modeling uncertainties. The locations shall include pipe welds, nonweld locations within the piping, and nonpipe contributions, e.g., Reactor Coolant Pump (RCP) seals.

Provide revised estimates of the initiating event frequencies for Small, Medium, and Large LOCAs for use as inputs to the PRA model for this GSI191 evaluation.

Include the results of the RIISI (riskinformed inservice inspection) evaluation, including damage mechanism (DM) assessment results and which weld locations are selected for inclusion for nondestructive examinations (NDE).

Support the calculation flow sheet interfaces among the LOCA frequency, debris formation, thermal hydraulics analysis, and risk analysis to ensure proper integration.

Support project meetings and NRC meetings and associated reviews.

Incorporate input from independent reviews that are being done to support the project.

The current report considers LOCAs initiated at or near the location of pipe and nozzle welds. A revision planned for 2012 will address pipe failures at other locations and nonpiperelated failures in the RCS pressure boundary.

1.3 Report Guide The technical approach to determining LOCA frequencies is summarized in Section 2. This approach makes use of a model that expresses LOCA initiating event frequencies as a function of piping system failure rates and conditional probabilities of pipe rupture over a range of break sizes. The models and data used to develop the piping system failure rates are documented in Section 3. Section 4 presents the approach that was selected to derive the conditional rupture probability (CRP) vs. break size, given pipe failure, together with a technical description of the resulting CRP models. The LOCA frequency results are presented in Section 5. These results include those to be used at specific locations within the RCS pressure boundary, as well as the Small, Medium, and Large LOCA frequencies for use in the PRA model.

Comparisons with generic industry estimates of LOCA frequencies are included with these results.

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LOCA Frequencies for STP GSI191

2. Technical Approach to LOCA Frequency Quantification 2.1 Basic LOCA Frequency Model The technical approach to estimating LOCA initiating event frequencies is based on the model expressed by Equations (2.1) and (2.2) for estimating the frequency of a LOCA of a given size. The parameter x is treated as a discrete variable representing different breaksize ranges. Here, x takes on values

{1,2,3,4,5,6} to correspond with the LOCA categories defined in NUREG1829 [1]. We shall use the NUREG1829 categories with the understanding that these may be redefined later if necessary.

F ( LOCAx ) mi ix (2.1) i ix ik P( Rx Fik ) I ik (2.2) k where:

F ( LOCAx ) Frequency of LOCA of size x, per reactor calendaryear, subject to epistemic uncertainty calculated via Monte Carlo mi Number of pipe welds of type i; each type determined by pipe size, weld type, applicable damage mechanisms, and inspection status (leak test and NDE); no significant uncertainty ix Frequency of rupture of component type i with break size x, subject to epistemic uncertainty calculated via Monte Carlo or lognormal formulas ik Failure rate per weldyear for pipe component type i due to failure mechanism k, subject to epistemic uncertainty determined by RIISI Bayes method and Eq. (2.3) below P( Rx Fik ) Conditional probability of rupture of size x given failure of pipe component type i due to damage mechanism k, subject to epistemic uncertainty determined via expert elicitation using NUREG1829 data I ik Integrity management factor for weld type i and failure mechanism k, subject to epistemic uncertainty determined by Monte Carlo and Markov model For a point estimate of the failure rate for type i and failure mechanism k:

nik nik ik (2.3) ik f ik N iTi where:

nik Number of failures in pipe component (i.e., weld) type i due to failure mechanism k; very little epistemic uncertainty 9 KNF Consulting Services LLC

LOCA Frequencies for STP GSI191 ik Component exposure population for welds of type i susceptible to failure mechanism k, subject to epistemic uncertainty determined by expert opinion f ik Estimate of the fraction of the component exposure population for weld type i that is susceptible to failure mechanism k, subject to epistemic uncertainty, estimated from results of RIISI for population of plants and expert opinion Ni Estimate of the average number of pipe welds of type i per reactor in the reactor years exposure for the data query used to determine nik ,

subject to epistemic uncertainty, estimated from results of RIISI for population of plants and expert knowledge of damage mechanisms Ti Total exposure in reactoryears for the data collection for component type i; little or no uncertainty For a Bayes estimate, a prior distribution for the failure rate is updated using nik and ik with a Poisson likelihood function.

The formulation of Equation (2.3) enables the quantification of conditional failure rates, given the known susceptibility to the given damage mechanism. When the parameter fik is applied, the units of the failure rate are failures per welds susceptible to the damage mechanism. This formulation of the failure rate estimate is done because the susceptible damage mechanisms are known from the results of a previously performed riskinformed inservice inspection evaluation for STPEGS. If the parameter fik is set to 1.0, the failure rates become unconditional failure rates, i.e., independent of any knowledge about the susceptibility of damage mechanism, or alternatively that 100% of the components in the population exposure estimate are known to be susceptible.

The key inputs that are needed to provide the pipe failure rate information include:

Identification of which locations will be investigated for debris formation, the groupings of locations that will be performed to support the risk evaluation, and a definition of component categories that are representative of all pipe failure locations within the STPEGS Class 1 pressure boundary.

Counts of pipe failures in applicable nuclear industry piping systems - essentially all the failure data in ASME Class 1 and 2 piping systems in PWRs in U.S. service experience and applicable international plants with similar designs and integrity management programs - from the PIPExp database.[2]

Pipe exposure estimates - quantity of pipe and pipe welds and the reactor years of service experience that produced the failure counts identified above. These estimates are based on information contained in the PIPExp database as well as the information available in risk informed inservice inspection submittals to the NRC, which include an enumeration of weld counts in different categories and the results of damage mechanism evaluations.

Estimates of the fractions of piping system components in the service data that are susceptible to different damage mechanisms. These estimates are based on NUREG1829 and supporting 10 KNF Consulting Services LLC

LOCA Frequencies for STP GSI191 computer files that provide information on epistemic uncertainty about pipe rupture frequencies vs. break size for different pressure boundary components.

STP RIISI evaluation report and supporting calculations providing information on applicable damage mechanisms for each weld and an identification of which welds are selected for NDE.

Results of inspection reports and other evidence of any pipe failure or degradation at STP that may influence the plantspecific failure rates, as well as the information needed to estimate exposure data.

The integrity management factor Iik of Equation (2.2) is quantified using the Markov model for Piping Reliability that was developed to support the EPRI RIISI projects.

The methodology outlined above and the methods and databases that have been developed to implement this approach were originally developed to support the EPRI RIISI methodology that has been implemented for many of the existing NRClicensed plants and several foreign plants. The part of this methodology that is relevant to estimating LOCA frequencies is described in detail in Reference [3]

and has been recently applied in EPRIsponsored projects to develop piping system failure rates for use in internal flooding and high energy line break PRAs, as documented in References [4] and [5]. The original EPRI study that was responsible for developing the Markov model and Bayes method for estimating pipe failure rates and rupture frequencies was documented in EPRI TR110161 [6], and an early version of the pipe failure rate database for both conditional and unconditional pipe failure rates was published in EPRI TR111880 [7]. An independent review of these reports was carried out by the University of Maryland, which validated the methodology that was developed in these reports. These methods and data were then used as part of the EPRI RIISI technical approach as described in the EPRI RIISI Topical Report [9]. The NRC approved these methods and data for use in applied RIISI evaluations as documented in the Safety Evaluation Report (SER) [10]. The SER was supported by an independent review of the Bayes failure rate method and the Markov model by Los Alamos National Laboratory [11],

which provides a second independent review of the methodology, including a validation of the Markov model solutions.

The application of the Markov model requires the development of rather complex closedform solutions to the differential equations supporting the Markov model, which were originally developed in TR 110161 and are also published in Reference [12]. Using these closedform solutions, it is straightforward to quantify the uncertainties in the resulting inspection factors using Monte Carlo simulation methods via Microsoft Excel' and Oracle Crystal Ball', which is the approach being used in this STP GSI191 evaluation. Bayes update steps in the analysis of LOCA frequency were performed using the RDAT Plus' Version 1.5.8 Program.

2.2 StepbyStep Procedure for LOCA Frequency Evaluation A stepbystep procedure for evaluating the LOCA frequencies for each location as a function of break size and collectively for the determination of Small, Medium, and Large LOCA frequencies for the PRA model is comprised of the steps in Table 21 and depicted in Figures 21 and 22.

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LOCA Frequencies for STP GSI191 Table 21 StepbyStep Approach to LOCA Frequency Development

1. Failure Rate Development 1.1 Determine component and weld types i 1.2 Perform data query for failure counts n 1.3 Estimate component exposure T 1.4 Develop component failure rate prior distributions for each damage mechanism (DM) 1.5 Perform Bayes update for each exposure case (combination of weld count case and DM susceptibility [DMS] case) 1.6 Develop mixture distribution to combine results for different exposure hypotheses to yield conditional failure rate distributions ik given STPspecific applicable DMs 1.7 Calculate total failure rate over all applicable damage mechanisms ik

2. Conditional Rupture Probability (CRP) Development P(RxFik) 2.1 Select components to define conditional rupture probability (CRP) model categories 2.2 Obtain expert reference LOCA distributions from NUREG1829 2.3 Obtain expert multiplier distributions for 40yr LOCA frequencies from NUREG1829 2.4 Determine 40yr LOCA distributions (product of Steps 2.2 and 2.3) for each expert, fit to lognormal 2.5 Determine geometric mean of expert distributions from Step 2.4 (lognormal) 2.6a Benchmark Lydell Base Case Analysis for selected components 2.6b Determine failure rate distribution for Lydell Base Case Analysis in NUREG1829; fit to lognormal 2.6c Apply Lydell CRP model from Base Case Analysis 2.6d Determine LOCA frequency distribution from Lydell Base Case Analysis 2.7 Determine mixture distribution of NUREG1829 GM (from Step 2.5) and Lydell LOCA frequency (from Step 2.6d to obtain Target LOCA frequency distribution for each CRP category component 2.8 Apply formulas to calculate CRP distributions to be used as prior distributions for each valid combination of CRP category and component 2.9 For each component in a given CRP category, perform Bayes' update with evidence of failure and rupture counts from service data

3. STPSpecific LOCA Frequency Development 3.1 Determine weld counts and pipe sizes for each component mi 3.2 Identify which locations are in and out of the NDE program 3.3 Combine the results of Step 1 and Step 2 for component LOCA frequencies 3.4 Apply Markov model to specialize rupture frequencies for NDE or no NDE Iik 3.5 Provide locationbylocation LOCA frequencies vs. break size to CASAGRANDE jx 3.6 Provide Small, Medium, and Large LOCA frequencies to RISKMAN - F(LOCAx)

The application of Steps 1, 2, and 3 is documented in Sections 3, 4, and 5, respectively.

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From 2.9 Bayes Updated 2.8 CRP Model CRP Distribution for Page 2 Category Prior Each Component Distribution for Each with Failure and LOCA Category 1.1 Applicable LOCA Count (Break Size)

Component (Weld) Evidence Types 1.2 Counts of Pipe Failures and LOCAs for Each DM and Component Query PIPExp 1.3 Pipe Population Exposure for 3.3 Component 1.6 Mixture Database for Pipe each DM and Component 1.5 Up to 9 Bayes 1.7 Distribution for LOCA Frequency Distribution to Failures and Pipe Updates, one for the Total Conditional Distribution =

Obtain Conditional Population Exposure each exposure Failure Rate over All Product of Failure 1.3 Uncertainty Distributions for Failure Rate given for each Component hypothesis Applicable DMs Rate x CRP Pipe Population Exposure Inputs STPSpecific DMs Type Distributions 1.4 Prior Distributions for each DM and Component Failure Rate 3.4 Apply Markov Model to Account for Impact of NDE 1.1 Applicable DMs for each Program Component STP Plant Design Information 3.2 Identify Which Locations are Including RIISI Subjected to NDE Evaluation for STP 3.6 Total LOCA Frequencies for 3.5 Component 3.1 Weld Counts and Pipe Sizes Each LOCA LOCA Frequencies Category vs. Break Size for for RISKMAN CASAGRANDE Figure 21 StepbyStep Procedure for LOCA Frequency Quantification - Page 1 of 2

LOCA Frequencies for STP GSI191 Figure 22 StepbyStep Procedure for LOCA Frequency Quantification - Page 2 of 2 14 KNF Consulting Services LLC

LOCA Frequencies for STP GSI191

3. Failure Rate Development (Step 1)

This section documents the failure rate development for STPEGS piping systems, which comprises Step 1 in the procedure outlined in Section 2. As described in the previous section, this step is composed of the following key tasks:

1.1 Determine component and weld types 1.2 Perform data query for failure counts 1.3 Estimate component exposure 1.4 Develop component failure rate prior distributions for each damage mechanism (DM) 1.5 Perform Bayes update for each exposure case (combination of weld count case and DM susceptibility case) 1.6 Develop mixture distribution to combine results for different exposure hypotheses to yield conditional failure rate distributions given STPspecific applicable DMs 1.7 Calculate total failure rate over all applicable DMs 3.1 Definition of Component Types (Step 1.1)

The first three tasks of failure rate development (determine component types, perform data query for failure counts, and estimation of component exposure) are performed as an iterative process. Insights from reviewing failure data are used to formulate criteria for defining homogeneous populations for estimating failure rates. The available data from which to estimate component exposures also influences the characterization of component types in the sense that some groups of components may exhibit unusually high or low incidence of failures compared to other similar components. The following criteria were used to determine homogeneous piping component types:

Pipe materials Pipe size Applicable damage or degradation mechanisms1 (DMs)

Unusual distribution of component failures Inservice inspection program status (within or outside the scope of nondestructive examinations [NDEs])

The first step in defining component categories was to define the eight major piping system cases, described in Table 31 based on the criteria listed above. These cases were then further subdivided to account for specific combinations of damage mechanisms and pipe sizes, as shown in Table 32. This more refined subdivision formed the homogeneous component categories that have distinct failure pipe failure rates and rupture frequency distributions. The 8 system cases give rise to 45 component 1

The terms damage mechanism and degradation mechanism are used interchangeably in this report.

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LOCA Frequencies for STP GSI191 calculation cases. In general, it is assumed that the maximum break size is the equivalent break size of a doubleended guillotine break (DEGB) of the pipe. If D is the inside diameter of the pipe, the DEGB size is 2D .

Table 31 Definition of Major Piping System Component Cases Case Description Weld Type Damage Comment Mechanism (DM) 1 RCS Hot Leg Excl. BF PWSCC, D&C Design basis LOCA location; BF weld has higher SG Inlet failure rate but located inside Rx cavity BJ TF, D&C 2 RCS Cold Leg BF PWSCC, D&C Lower temperatures and different pipe sizes relative to hot leg BJ D&C 3 RCS Hot Leg SG BF PWSCC, D&C This case defined to address S/G Inlet nozzleto Inlet safeend weld that has unusual failure count distribution[1]

4 PZR Surge Line BF PWSCC, TF, D&C Includes surge line from branch connections and nozzles to pressurizer safe end; entire surge line BJ, BC TF, D&C subjected to thermal transients during startup and shutdown 5 PZR Medium Bore BF PWSCC, TF, D&C This includes pressurizer spray, and relief valve Piping piping excluding the pressurizer surge line; BF welds at STP in this category have weld BJ, BC TF, D&C overlays[2]

6 Class 1 Small Bore BJ TF, D&C, TGSCC, VF This is all the Class 1 piping of size 2" and less Piping and inside isolation valves 7 Class 1 Medium BJ TF, D&C, IGSCC Safety injection and residual heat removal (RHR)

Bore SIR Piping systems in standby during normal operation; Class 1 is inside the isolation valves 8 Class 1 Medium BJ, BC TF, D&C, TGSCC, VF CVCS piping with injection and letdown flow Bore CVCS Piping during normal operation BF ASME XI Category BF welds (bimetallic)

BJ ASME XI Category BJ welds (single metal)

BC Branch connection welds, BJ welds used at branch connections CVCS Chemical, Volume, and Control System D&C Design and Construction Defects IGSCC Intergranular Stress Corrosion Cracking PWSCC Primary Water Stress Corrosion Cracking PZR Pressurizer RCS Reactor Coolant System SIR Safety Injection and Recirculation Systems TF Thermal Fatigue, including that due to thermal transients (TT) and thermal stratification (TASC)

TGSCC Transgranular Stress Corrosion Cracking VF Vibration Fatigue Notes :

[1] An unusually high incidence of failures of this component was observed at Japanese plants following Steam Generator replacements. Until it can be ruled out for STP it is included in this study.

[2] NOCAE06002099 (January 30, 2007): Inspection and Mitigation of Alloy 82/182 Pressurizer Butt Welds, South Texas Nuclear Operating Company.

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LOCA Frequencies for STP GSI191 Table 32 Definition of Specific Component Categories STP System Component Weld Total Pipe Size DEGB System Applicable DM Case Case Type No. of (in.) Size (in.)

Welds 1A BF SC, D&C 4 29 41.0 1 RC Hot Leg 1B BJ D&C 11 29 41.0 1C BJ TF, D&C 1 29 41.0 2 RC SG Inlet 2 BF SC, D&C 4 29 41.0 3A BF 4 27.5 38.9 SC, D&C 3B BJ 4 31 43.8 3 RC Cold Leg 3C BJ 12 27.5 38.9 D&C 3D BJ 24 31 43.8 4A BF SC, TF, D&C 1 16 22.6 4B BJ 7 16 22.6 4 RC Surge 4C BC TF, D&C 2 16 22.6 4D BJ 6 2.5 3.5 5A BJ 29 6 8.5 TF, D&C 5B BJ 14 3 4.2 5C BJ 53 4 5.7 5D BJ D&C 4 3 4.2 5E BJ 29 6 8.5 5 PZR 5F BF SC, TF, D&C 0 6 8.5 5G BF SC, D&C 0 6 8.5 5H BF D&C (Weld Overlay) 4 6 8.5 5I BC D&C 2 4 5.7 5J BJ TF, D&C 2 2 2.8 6A BJ 16 2 2.8 6 Small Bore VF, SC, D&C 6B BJ 193 1 1.4 7A BJ 21 12 17.0 TF, D&C 7B BJ 9 8 11.3 7C BJ SC, TF, D&C 3 8 11.31 7D BJ SC, D&C 3 12 17.0 7E BJ, BC 57 12 17.0 SIR Lines Excl. 7F BJ 30 10 14.1 7

Accumulator 7G BJ, BC 42 8 11.3 7H BJ 23 6 8.49 D&C 7I BC 5 4 5.7 7J BC 9 3 4.24 7K BC 10 2 2.8 7L BJ 0 1.5 2.1 17 KNF Consulting Services LLC

LOCA Frequencies for STP GSI191 STP System Component Weld Total Pipe Size DEGB System Applicable DM Case Case Type No. of (in.) Size (in.)

Welds SIR 7M BJ SC, D&C 0 12 17.0 Accumulator 7N BJ TF, D&C 35 12 17.0 Lines 7O BJ, BC D&C 15 12 17.0 8A BJ 10 2 2.8 TF, VF, D&C 8B BJ 19 4 5.7 8 CVCS 8C BJ 47 2 2.8 VF, D&C 8D BJ 6 4 5.7 8E BC TF, D&C 4 4 5.7 8F BC D&C 1 4 5.7 Total 775 3.2 Evaluation Scope for 2011 The evaluation that is documented in this report is limited to the ASME III Class 1 piping system pressure boundary failures; i.e., nonisolable LOCAs. The Class 1 boundary consists of all hot leg, cold leg and crossover leg piping, pressurizer surge, spray, auxiliary spray, relief valve, safety valve and vent lines, and one unit 1 drain line. It also includes branch piping to the Safety Injection System (SIS), Chemical &

Volume Control System (CVCS), and Residual Heat Removal System (RHRS). All piping attached to the RCS loops or pressurizer vessel is considered Class 1 out to the second valve. Class 1 SIS, CVCS and RHRS piping between the first and second valve off the RCS is discussed in later sections.

Isolable LOCAs, seismically induced LOCAs, and LOCAs due to failures of components other than pipes will be considered for the 2012 work scope, as necessary to characterize debrisinduced core damage risks. Also excluded in the current scope are steam line and feedwater line breaks inside the containment that could lead to a need to implement recirculation cooling and/or containment spray actuation as well nonpiping passive component failures. If those break locations are regarded as significant to GSI191, they will also be addressed in 2012.

3.3 Failure Data Query (Step 1.2)

This study uses the term "pipe failure" to include any condition that leads to repair or replacement of the affected piping component. This includes flaws that exceed ASME criteria for repair or replacement, cracks, leaks, and, if they were observed to occur, pipe ruptures. The failure data query found the most severe type of pipe failure to be leak with leak flow rate less than 10 gpm. Nonpipe failures that can produce a LOCA are to be addressed in 2012. Insights from review of service experience clearly show that for failures in ASME Class 1 piping systems, with the exception of leaks from valves and seals, piping system failures occur almost exclusively at or near welds. In fact, the results of our data query show that 100% of the experienced pipe failures occur at or near a weld. Since the welds in a Class 1 pressure boundary are relatively evenly distributed around the piping systems, identifying the failure locations at or near welds also provides for a representative set of pipe failure locations. Hence, all pipe failures that 18 KNF Consulting Services LLC

LOCA Frequencies for STP GSI191 significantly contribute to LOCA frequencies may be assumed to occur at or near welds. This assumption will be replaced by an explicit accounting for nonpipeinduced LOCAs in 2012.

At STP, there are approximately 775 weld locations within the Class 1 RCS pressure boundary including approximately 200 weld locations in small bore pipe ( 2). Hence, modeling pipe failures at these weld locations using the 45 component categories in Table 32 will facilitate the analysis of LOCAs at large number pipe failure locations.

The source of the analyzed pipe failure data is the PIPExp database[2] which is depicted in Figure 31.

The failure data query was performed on Westinghouse, Mitsubishi Heavy Industries (MHI), and Framatome PWR plant operating experience from 1970 through 2010 and included ASME Class 1 piping systems. This generally includes RCS piping and systems that interface with the RCS inside the isolation valves that normally separate the RCS from interfacing ASME Class 2 piping. Interfacing systems include the emergency core cooling, residual heat removal, chemical volume and control system, and various other systems including Reactor Pressure Vessel (RPV) head vents and instrumentation lines. Class 1 piping service experience with Babcox and Wilcox, Combustion Engineering, and KWU/Seimens PWR plants was not considered on the basis of different materials and degradation susceptibilities relative to Westinghouse PWRs and those derived from the Westinghouse design. A contributing factor to this decision is that there is sufficient data from Westinghouse type plants to meet the needs of this study.

SKI R&D Project 1994-1998 SOAR on piping reliability analysis as it relates to PSA (SKI Report 95:58)

Basis for deriving pipe failure parameters from service data (SKI Report 97:26; PIPExp Database Project (1999 - to date) - independent of SKI Active maintenance program (weekly updates);

QA program - extensive data validation; PIPExp-1999 (12-31-1999)

PIPExp-2000 (12-31-2000) OECD/NEA OPDE Project (2002-2011)

Based on SKI-PIPE (1998);

PIPExp-2001 (12-31-2001) Validation of selected records by National Coordinators; Harmonized db-structure; PIPExp-2002 (12-31-2002)

OPDE-2003 (12-31-2003)

PIPExp-2003 (12-31-2003)

OPDE-2011:1 (05-31-2011)

PIPExp-2011 (08-31-2011) 8287 db records (pipe) 566 water hammer records (w/o structural failure)

Figure 31 PIPExp Database and Relationship to Other Databases [20]

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LOCA Frequencies for STP GSI191 The results of the failure data query are shown in Table 33. Because roughly half of the current fleet of operating plants were designed and built prior to the development of ASME nuclear piping codes, much of this pipe was originally designed to B31.1 design codes, and then inspection and ISI requirements for Class 1 piping were retrofitted into these plants. So from a design and materials perspective, the LOCA sensitive piping actually reflects a mixture of B31.1 and Class 1 pipe.

Table 33 Results of Class 1 Failure Data Query by System and Component System Event Nominal Failure Count by DM - Weld Locations System Case Type Pipe Size Totals D&C SC PWSCC TF V-F RCS Hot Leg Crack 32" 5 5 1

RCS Hot Leg Leak 32" 1 1 2 RCS Cold Leg Crack 32" 3 3 3 S/G Inlet Crack 32" 19 1 18 4 PZR-Surge Crack 16" 3 3 PZR-PORV Crack 4" ø 10" 2 2 PZR-SPRAY Crack 4" ø 10" 2 2 5 PZR-SPRAY Leak 4" ø 10" 1 1 PZR-SRV Crack 4" ø 10" 6 1 5 PZR-SRV Leak 4" ø 10" 1 1 CVCS Crack 1" 1 1 CVCS Leak 1" 6 1 5 Safety Injection Leak 1" 2 2 PZR-Sample/Instr. Crack 2" 5 1 2 2 PZR-SPRAY Crack 1" 1 1 6 PZR-SPRAY Leak 1" 3 1 1 1 RCS Crack 2" 14 1 3 2 1 7 RCS Leak 2" 62 12 10 2 2 36 RHR Leak 1" 6 1 5 S/G System Crack 1" 2 1 1 S/G System Leak 1" 4 2 2 Safety Injection Crack 4" ø 12" 3 1 2 7 Safety Injection Leak 4" ø 12" 3 3 RHR Crack 4" ø 12" 1 1 CVCS Crack 2" ø 4" 1 1 8

CVCS Leak 2" ø 4" 6 1 5 Total 163 23 21 46 9 64 3.4 Component Population Exposure (Step 1.3) 3.4.1 ReactorYears of Service Experience Pipe component exposure is evaluated in the current analysis in terms of pipe welds in the data query.

This is estimated from a combination of reactoryears of service experience and an estimate of the total number of welds per plant. In principle, the number of welds per plant is known but is seldom found in public domain references. In addition, there is usually significant planttoplant variability in the number of welds for different components. To address this, the component exposure, i.e., total weldyears of 20 KNF Consulting Services LLC

LOCA Frequencies for STP GSI191 experience responsible for the identified failures, is treated as an uncertain parameter in failure rate development. In addition, to support the estimation of failure rates from different damage mechanisms, it is necessary to estimate the fraction of the exposure that is susceptible to a given damage mechanism, which is also uncertain. Results of published reports on RIISI evaluations provided the basis for both weld count and fractionsusceptible estimates.

The reactoryears of service experience by reactor type responsible for the failures in Table 23 are listed in Table 34.

Table 34 Service Experience by WestinghouseType PWRs Reactor-Calendar Years WE Type Rx Initial Grid Initial Connection Criticality 2-Loop 570.1 581.4 3-Loop 2052.6 2096.1 4-Loop 1193.9 1236.5 Total 3816.6 3914.0 For the purposes of failure rate estimation, reactorcalendar years was based on the initial grid connection.

3.4.2 Component Exposure Estimates for Hot Leg Welds To illustrate the detailed approach to failure rate development, the piping components for the hot leg are examined. As shown in Table 32, the hot leg has two types of welds: BF (bimetallic) welds and BJ (single metal welds). To estimate failure rates requires estimating the number of welds in the reactor population that corresponds to the reactoryears in the data query. For this purpose, the authors of this report from Scandpower reviewed isometric drawings for a selected sample of PWR plants and determined from this sample a best estimate, upper bound, and lower bound of weld counts per reactor in the database. These three estimates were used to define a threepoint discrete distribution to characterize the uncertainty in the total reactor year population that was queried for failure counts. This approach was developed to support pipe failure rate development for the EPRI RIISI program [7] as part of the overall Bayes method for pipe failure rate estimation. This was reviewed by LANL (Los Alamos National Laboratory) for the NRC [11] and approved for use in RIISI evaluations by the NRC [10].

As shown in Table 34, the reactoryear population is distributed among 2loop, 3loop, and 4loop PWR plants. For components like the hotleg and coldleg welds, the number of welds per plant may be reasonably assumed to be proportional to the number of coolant loops. The review of isometric drawings at 10 PWR reactors produced the hot leg weld estimates that are presented in Table 35. This sample was used to determine the average number of welds per loop, the minimum, and the maximum.

This information is used to characterize the uncertainty in the exposure terms as described in the sections below.

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LOCA Frequencies for STP GSI191 Table 35 Estimation of Hot Leg Welds per Reactor NPS29 Weld Population PWR Plant BF BJ Type BF Welds BJ Welds Welds/loop Welds/loop Braidwood1 4Loop 8 12 2 3 Braidwood2 4Loop 8 12 2 3 Byron1 4Loop 8 12 2 3 Byron2 4Loop 8 11 2 2.75 Kewaunee 2Loop 4 6 2 3 Koeberg1 3Loop 3 9 1 3 Koeberg2 3Loop 3 9 1 3 STP1 4Loop 8 8 2 2 STP2 4Loop 8 8 2 2 V.C. Summer 3Loop 6 6 2 2 Average 1.8 2.68 Min 1 2 Max 2 3 3.4.3 Degradation Mechanism Assessment As was determined in the development of failure rates for the EPRI RIISI evaluations, it is assumed that all welds are subject to design and construction defects. Insights from service experience indicate that certain weld types are always susceptible to a specific DM, whereas in some cases a DM can be ruled out generically for a given weld type. In other cases, there is uncertainty on how many welds in the reactoryear population that was queried for the failure counts are susceptible to a given DM. The evaluation of DM susceptibility for the hot leg welds is shown in Table 36.

Table 36 Damage Mechanism Assessment for Hot Leg Welds Calc. Confidence Weld Susceptibility Fractions System Location Case Level CF D&C ECSCC Fretting IGSCC PWSCC TF TGSCC VF Low N/A 1 N/A N/A N/A 1 N/A N/A N/A BF (Un 1A Medium N/A 1 N/A N/A N/A 1 N/A N/A N/A mitigated)

High N/A 1 N/A N/A N/A 1 N/A N/A N/A RC Hot Leg Low N/A 1 N/A N/A N/A N/A 0.01 N/A N/A 1B, 1C BJ Medium N/A 1 N/A N/A N/A N/A 0.02 N/A N/A High N/A 1 N/A N/A N/A N/A 0.08 N/A N/A The damage mechanism assessment is based on insights from service experience, results of completed RIISI evaluations for Westinghousetype PWRs, and understanding of the DM criteria that were developed for the EPRI RIISI evaluation [9]. Other sources of information that were available to assess damage mechanisms include the Expert Panel Report on Proactive Materials Degradation Assessment (NUREG/CR6923) [16], SCAPSCC Working Group [17][18], OECD Nuclear Energy Agency topical report on thermal fatigue [19]. Dissimilar metal welds (Category BF welds) are known to be susceptible to primary water stress corrosion cracking (PWSCC). Only a small, albeit uncertain fraction of the BJ welds 22 KNF Consulting Services LLC

LOCA Frequencies for STP GSI191 are susceptible to thermal fatigue and both are susceptible to design and construction defects. For the Phase I scope weld population and because of the unique conjoint conditions for degradation that are associated with the PWR primary system operating environment, all other identified damage mechanisms identified in Figure 32 can be ruled out. Serviceinduced degradation of reactor components results from synergies among material characteristics, stress and environment conditions.

Illustrated in Figure 32 are four categories of degradation mechanisms and their failure potential. The four categories are:

1) stress corrosion cracking (SCC) mechanisms,

2) flowassisted mechanisms,

3) corrosion mechanisms, and

4) fatigue mechanisms.

In order for SSC or fatigue mechanisms to develop into structural degradation or failure a crack initiation must occur and then grow into a surface connected crack and beyond. Design and construction (D&C) defects oftentimes provide for crack/flaw initiation, but not for crack/flaw growth.

In addition to the above generic industry inputs to assessment of degradation mechanisms, the study also benefitted from a plant specific riskinformed inservice inspection (RIISI) evaluation of Class 1 piping that was performed for STP in 2001 [21]. This evaluation included a deterministic engineering evaluation of all the Class 1 pipe welds against screening criteria that were developed by EPRI for use in RIISI evaluations. These screening criteria and their technical bases are documented in the EPRI RIISI Topical Report [9] which was approved by the NRC for use in applied RIISI evaluations [10]. The RIISI application at STP was also approved by the NRC.

PIPE DAMAGE & DEGRADATION / FAILURE MANIFESTATIONS PSI / ISI ISI / Visual Inspection / Walkdown Inspection / Leak Detection / CR Indication Crack - Part Through- Structural Failure Recordable / Crack - Through-Wall Active Leakage (< TS Active Leakage ( TS Wall (Surface ("Significant" Through-Rejectable Flaw (No Active Leakage) Limit) Limit)

Connected) Wall Flow Rate)

FLAW INITIATION FLAW GROWTH FAILURE Construction / Fabrication Defect D&C (Damage Design Error Maintenance / Repair Error State)

Programmatic / Procedural Error Welding Error Corrosion Fatigue FATIGUE High-Cycle Fatigue Low Cycle Fatigue Thermal Fatigue (TT, TASCS)

Crevice/Pitting Corrosion CORROSION Galvanic Corrosion General Corrosion MIC - Microbiologically Influenced Corrosion Steam Jet Impingement Erosion FLOW-Erosion-Corrosion ASSISTED Erosion-Cavitation DEGR.

FAC - Flow-Accelerated Corrosion ECSCC - Cl Induced SCC (ID/OD)

STRESS CORROSION TGSCC PWSCC - Inconel SICC - Strain Induced Corrosion Cracking CRACKING IGSCC - Stabilized Austenitic SS IGSCC-PWR - Unstabilized Austenitic SS IGSCC-BWR - Unstabilized Austenitic SS Observed Failures Figure 32 Damage and Degradation Mechanisms in Commercial Light Water Reactor Plants 23 KNF Consulting Services LLC

LOCA Frequencies for STP GSI191 3.4.4 Component Exposure for Hot Leg Welds The uncertainty in component exposure is determined by the uncertainty in the component's welds per reactor and by the fraction of those welds that are susceptible to the various DMs. Table 35 showed that for the hot leg welds, there is uncertainty in the components per reactor for both BF and BJ welds.

In addition there is uncertainty in BJ weld susceptibility for thermal fatigue. Based on the methodology developed in the EPRI RIISI program, each of these sources of uncertainty are characterized by three point discrete distributions. For the BJ welds prone to thermal fatigue, there are nine combinations of exposure derived from the two threepoint distributions, as shown in Figure 33.

3.5 Prior Distributions for Hot Leg Weld Failure Rates (Step 1.4)

Prior distributions for each DM were developed for the failure rate development in the EPRI RIISI program in Reference [7], based on early estimates of pipe failure rates, engineering judgment, and insights from review of service data. This is part of the methodology that was reviewed by LANL [11] and approved by the NRC [10] for RIISI evaluations using the EPRI RIISI methodology [9]. The applicable prior distributions for the hot leg welds and other Class 1 welds subject to these same DMs are presented in Table 37. These are very broad distributions, all lognormal with range factors of 100. As such, they only weakly influence the posterior distributions during the Bayes updating process.

Table 37 Prior Distributions for Weld Failure Rates by Damage Mechanism Prior Distribution Damage Mechanism Distribution Failure Rate per WeldYr Range Type Mean Median Factor Stress Corrosion Cracking Lognormal 4.27E05 8.48E07 100 Design and Construction Errors Lognormal 2.75E06 5.46E08 100 Thermal Fatigue Lognormal 1.34E05 2.66E07 100 3.6 Failure Rate Bayes Updates (Step 1.5)

The next step in the LOCA frequency quantification procedure is to perform Bayes updates for each component/DM/populationexposure estimate that supports the calculation. The prior distributions used in this assessment are based on those that were developed in Reference [7] for use in the EPRI RI ISI evaluations that followed the methodology in the EPRI RIISI Topical Report [9], which was reviewed by the NRC and LANL as documented in References [10] and [11]. The evidence for the updates is based on three failures of BF surge line welds due to PWSCC, and zero failures for both the branch connection and BJ welds for the surge line. The parameters of the prior and updated distributions for all the cases that were needed to support the surge line welds are listed in Table 27. The failure data query yielded a total of six pipe failures for hot leg welds, all of which failures were at BF welds and caused by primary water stress corrosion cracking.

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LOCA Frequencies for STP GSI191 Welds/Loop Number Number/Average Welds/Loop Loops Rx-yrs Weld-yrs Average 2.675 1 2.675 2 570 3,050 Minimum 2 0.75 2.675 3 2,053 16,472 Maximum 3 1.12 2.675 4 1,194 12,775 Base Exposure 32,297 Fraction of B-J Welds Exposure Weld Count Exposure Susceptible to Thermal Case Exposure Uncertainty Multiplier Fatigue Probability p=.25 0.0625 0.08972 2,898 weld-yrs High (.08 x Base) p=.25 p=.50 0.125 0.02243 724 weld-yrs High (1.12 x Base) Medium (.02 x Base) p=.25 0.0625 0.011215 362 weld-yrs Low (.01 x Base) p=.25 0.125 0.08 2,584 weld-yrs High (.08 x Base) p=.50 p=.50 0.25 0.02 646 weld-yrs Medium (1.0 x Base) Medium (.02 x Base) p=.25 0.125 0.01 323 weld-yrs Low (.01 x Base) p=.25 0.0625 0.059813 1,932 weld-yrs High (.08 x Base) p=.25 p=.50 0.125 0.014953 483 weld-yrs Low (0.75 x Base) Medium (.02 x Base) p=.25 0.0625 0.007477 241 weld-yrs Low (.01 x Base)

Figure 33 Event Tree Model to Represent Uncertainty in Hot Leg Weld Exposure for Thermal Fatigue 25 KNF Consulting Services LLC

LOCA Frequencies for STP GSI191 Table 38 Parameters of Bayes Updates for Hot Leg Weld Failure Rate Cases (1) (2) (1)

DM Prior Distribution Evidence Bayes Posterior Distribution Weld Type and Weld Count (3) Susceptibility (4)

DM Case Type Median RF Failures Exposure Mean 5%tile 50%tile 95%tile RF Case Low Base Lognormal 8.48E07 100 6 12,074 4.32E04 1.78E04 4.05E04 7.78E04 2.1 Hot Leg BF SC Medium Base Lognormal 8.48E07 100 6 21,732 2.43E04 1.01E04 2.29E04 4.37E04 2.1 High Base Lognormal 8.48E07 100 6 24,147 2.20E04 9.10E05 2.06E04 3.94E04 2.1 Low Base Lognormal 5.46E08 100 0 12,074 1.02E06 5.34E10 5.16E08 4.05E06 87.1 Hot Leg BF DC Medium Base Lognormal 5.46E08 100 0 21,732 8.31E07 5.28E10 5.01E08 3.54E06 81.9 High Base Lognormal 5.46E08 100 0 24,147 8.31E07 5.28E10 5.01E08 3.54E06 81.9 Low Low Lognormal 2.66E07 100 0 241 8.88E06 2.65E09 2.64E07 2.53E05 97.6 Medium Low Lognormal 2.66E07 100 0 323 8.41E06 2.65E09 2.63E07 2.49E05 97.0 High Low Lognormal 2.66E07 100 0 362 8.22E06 2.65E09 2.63E07 2.47E05 96.7 Low Medium Lognormal 2.66E07 100 0 483 7.74E06 2.64E09 2.62E07 2.43E05 95.8 Hot Leg BJ TF Medium Medium Lognormal 2.66E07 100 0 646 7.25E06 2.64E09 2.61E07 2.37E05 94.8 High Medium Lognormal 2.66E07 100 0 724 7.05E06 2.64E09 2.60E07 2.35E05 94.3 Low High Lognormal 2.66E07 100 0 1,932 5.38E06 2.61E09 2.54E07 2.06E05 88.9 Medium High Lognormal 2.66E07 100 0 2,584 4.90E06 2.60E09 2.51E07 1.96E05 86.7 High High Lognormal 2.66E07 100 0 2,898 4.72E06 2.59E09 2.50E07 1.91E05 85.8 Low Base Lognormal 5.46E08 100 0 24,147 7.99E07 5.26E10 4.98E08 3.45E06 80.9 Hot Leg BJ DC Medium Base Lognormal 5.46E08 100 0 32,297 7.14E07 5.22E10 4.87E08 3.17E06 77.9 High Base Lognormal 5.46E08 100 0 36,221 6.82E07 5.20E10 4.83E08 3.06E06 76.7 Notes:

(1) Failure rates expressed in failures per weldyear.

(2) Exposure expressed in weldyears.

(3) DM = Damage Mechanism; SC = stress corrosion cracking; TF = thermal fatigue; DC = design and construction defects.

(4) RF = Range Factor = SQRT (95%tile/5%tile).

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LOCA Frequencies for STP GSI191 3.7 Failure Rate Distribution Synthesis (Steps 1.6 and 1.7)

The total weld failure rates are calculated using a Monte Carlo posterior weighting technique to combine the distributions from the different weldcount and DM susceptibility hypotheses and then summing the contributions from applicable DMs. The result is often referred to as a mixture distribution. For BJ welds, the failure rate for thermal fatigue was developed by Monte Carlo sampling from a discrete distribution defined by probabilities and exposure cases in Figure 33 to determine which of the lognormal distributions for BJ TF from Table 38 to sample for that trial. Repeating this process over many trials (100,000 trials used for all Monte Carlo calculations in this report) yields a single distribution for the BJ weld failure rate due to thermal fatigue. This single failure rate incorporates a probabilistically weighted contribution from each supporting weldcount and DM susceptibility hypothesis. For the BJ weld failure rate due to design and construction (D&C) defects, only three cases are required to model uncertainty in the weld counts because all welds are assumed to be susceptible to D&C. Then the total failure rate for BJ welds due to thermal fatigue (Case 1C) is calculated by summing the contributions from TF and D&C. For BJ welds that are not susceptible to thermal fatigue (as determined at STP in the RIISI evaluation), only the D&C contribution applies. For the BF welds, there is no significant uncertainty for DM susceptibility. Hence only the three cases for weld count uncertainty need to be combined for the SC contribution to the BF failure rate, as well as three cases for the D&C failure rate, and then the SC and D&C contributions are summed to obtain the total BF weld failure rate for Case 1A. It is significant that the mean failure rates for the three calculation cases span more than two orders of magnitude, from BJ welds that are not subject to thermal fatigue on the low side, to the BF welds on the high side. The uncertainty in the BF weld failure rate (Case 1A) as measured by the range factor is relatively small due to the significant number of pipe failures in these welds. The range factors are much higher for the BJ weld cases (1B and 1C) because there were zero failures. In these situations, the large range factor used in the prior distribution has a much larger influence on the posterior distribution parameters. These results are expected due to the properties of Bayes updating.

Table 39 Total Failure Rates for Hot Leg Weld Calculation Cases Calculation Weld Failure Rate Distribution (failures per weldyear)

DM Case Type Mean 5%tile 50%tile 95%tile RF 1A BF SC + D&C 2.73E04 1.04E04 2.33E04 5.78E04 2.4 1B D&C 1.44E06 5.27E10 4.12E08 3.19E06 77.8 BJ 1C TF + D&C 1.07E05 1.79E08 5.79E07 2.83E05 39.8 27 KNF Consulting Services LLC

LOCA Frequencies for STP GSI191 3.8 Failure Rates for Other Calculation Cases (Step 1.7)

This section presents information that can be used to derive the results for the failure rate distributions for the remaining Calculation Cases in Table 32. The process outlined in the previous section was repeated for all the cases in Table 32. Actual weld counts from the 10 PWR units in Table 35 were used to characterize the uncertainty in the weld population per plant using the average value from the sample as the best estimate, the minimum in the sample as the lower bound, and the maximum in the sample as the upper bound. Table 310 provides a summary of the best estimate, upper bound, and lower bound weldyears estimated for each component type. As seen in this table, the population exposure accounted for in this failure rate development is several million componentyears of experience.

Table 310 Component Population Exposure Estimates for Pipe Failure Rates System Component Weld Best Upper Lower System Case Case Type Estimate Bound Bound 1A BF 21,732 24,147 12,074 1 RCS Hot Leg 1B, 1C BJ 32,297 36,221 24,147 2 RCS SG Inlet 2 BF 12,074 12,074 12,074 3A BF 22,315 24,794 12,397 3 RCS Cold Leg 3B BJ 123,764 177,279 99,177 4A BF 3,914 3,914 3,914 4 RCS Surge 4B BJ 27,007 54,013 13,503 4C BC 7,828 7,828 7,828 5A-5D BJ 351,127 496,158 286,245 5 PZR 5E-5G BF 19,083 19,083 19,083 6 SB 6A-6B BJ 744,237 1,144,980 366,394 SIR Lines Excl. Accumulator 7A-7L BJ 590,797 637,190 507,518 7

SIR Accumulator Lines 7M-7O BJ 175,067 277,693 132,810 8A-8D BJ 562,348 627,324 403,018 8 CVCS 8E, 8F BC 81,393 90,797 58,332 Total Estimated WeldYrs 2,774,983 3,633,494 1,958,513 Table 311 presents the DM susceptibility matrix for the development of fraction of the component population susceptible to each DM. This is based on the results of the failure data query, EPRI criteria for screening for DM susceptibility, and insights from review of pipe failure data in this and previous pipe failure rate studies performed by the authors.

Table 312 and Figure 34 present the results of the failure rate uncertainty analysis that supports all the calculation cases. These exhibits show that the mean failure rates span more than three orders of magnitude. They also indicate that bimetallic welds (BF) tend to have much higher failure rates than BJ or Branch Connection (BC) welds. This is due to the susceptibility of BF welds to PWSCC, a higher incidence of failures for this DM in the service data, and a smaller component exposure population than 28 KNF Consulting Services LLC

LOCA Frequencies for STP GSI191 that of the BJ and BC welds. At STP, there has been some cracking observed in the Pressurizer BF welds, which has been mitigated through application of weld overlays. The failure rates for these welds (Case 5G) is estimated by using the failure rate for D&C defects that is assumed to apply to these weld overlays. This is viewed as a conservative assumption because no credit is taken for the capability of the underlying cracked material to inhibit a rupture if the overlay weld would fail. There are fewer distinct failure rates than calculation cases because the additional calculation cases are differentiated only by pipe size. For example, this applies to Cases 3A and 3B. When we develop the LOCA frequencies vs.

break size in the following sections, each of these cases will have a different maximum break size, but they otherwise will exhibit an identical frequency vs. break size curve.

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LOCA Frequencies for STP GSI191 Table 311 Damage Mechanism Susceptibility Matrix for Failure Rate Development Confidence Weld Susceptibility Fractions System Location Level CF D&C ECSCC Fretting IGSCC PWSCC TF TGSCC VF Low N/A 1 N/A N/A N/A 1 N/A N/A N/A BF (Un Medium N/A 1 N/A N/A N/A 1 N/A N/A N/A mitigated)

High N/A 1 N/A N/A N/A 1 N/A N/A N/A RC Hot Leg Low N/A 1 N/A N/A N/A N/A 0.01 N/A N/A BJ Medium N/A 1 N/A N/A N/A N/A 0.02 N/A N/A High N/A 1 N/A N/A N/A N/A 0.08 N/A N/A Low N/A 1 N/A N/A N/A N/A N/A N/A N/A BF Medium N/A 1 N/A N/A N/A N/A N/A N/A N/A High N/A 1 N/A N/A N/A N/A N/A N/A N/A RC Cold Leg Low N/A 1 N/A N/A N/A N/A N/A N/A N/A BJ Medium N/A 1 N/A N/A N/A N/A N/A N/A N/A High N/A 1 N/A N/A N/A N/A N/A N/A N/A Low N/A 1 N/A N/A N/A 1 N/A N/A N/A RC Hot Leg / SG BF Medium N/A 1 N/A N/A N/A 1 N/A N/A N/A Inlet High N/A 1 N/A N/A N/A 1 N/A N/A N/A Low N/A 1 N/A N/A N/A 1 1 N/A N/A BF Medium N/A 1 N/A N/A N/A 1 1 N/A N/A High N/A 1 N/A N/A N/A 1 1 N/A N/A Low N/A 1 N/A N/A N/A N/A 1 N/A N/A Pressurizer Surge BJ Medium N/A 1 N/A N/A N/A N/A 1 N/A N/A Line High N/A 1 N/A N/A N/A N/A 1 N/A N/A Low N/A 1 N/A N/A N/A N/A 1 N/A N/A RCHL Branch Medium N/A 1 N/A N/A N/A N/A 1 N/A N/A Connection High N/A 1 N/A N/A N/A N/A 1 N/A N/A Low N/A 1 N/A N/A N/A 1 0.01 N/A N/A BF Medium N/A 1 N/A N/A N/A 1 0.04 N/A N/A Pressurizer (Unmitigated)

High N/A 1 N/A N/A N/A 1 0.20 N/A N/A PRV/SRV & Spray Low N/A 1 N/A N/A 0.01 N/A 0.01 N/A N/A Lines BJ Medium N/A 1 N/A N/A 0.02 N/A 0.04 N/A N/A High N/A 1 N/A N/A 0.08 N/A 0.20 N/A N/A 30 KNF Consulting Services LLC

LOCA Frequencies for STP GSI191 Confidence Weld Susceptibility Fractions System Location Level CF D&C ECSCC Fretting IGSCC PWSCC TF TGSCC VF Low N/A 1 N/A N/A 1 N/A N/A N/A 1 Small Bore BJ Medium N/A 1 N/A N/A 1 N/A N/A N/A 1 High N/A 1 N/A N/A 1 N/A N/A N/A 1 Low N/A 1 N/A N/A 0.01 N/A 0.01 N/A N/A BJ Medium N/A 1 N/A N/A 0.05 N/A 0.04 N/A N/A SIR - Medium High N/A 1 N/A N/A 0.25 N/A 0.20 N/A N/A Bore Low N/A 1 N/A N/A 0.01 N/A 0.01 N/A N/A CF1 Medium N/A 1 N/A N/A 0.05 N/A 0.04 N/A N/A High N/A 1 N/A N/A 0.25 N/A 0.20 N/A N/A SIR - Large Bore Low N/A 1 N/A N/A N/A N/A 0.01 N/A N/A (Accumulator BJ Medium N/A 1 N/A N/A N/A N/A 0.04 N/A N/A lines) High N/A 1 N/A N/A N/A N/A 0.20 N/A N/A Low N/A 1 N/A N/A N/A N/A 0.01 N/A 1 BJ Medium N/A 1 N/A N/A N/A N/A 0.04 N/A 1 High N/A 1 N/A N/A N/A N/A 0.20 N/A 1 CV Low N/A 1 N/A N/A N/A N/A 1 N/A N/A CF1 Medium N/A 1 N/A N/A N/A N/A 1 N/A N/A High N/A 1 N/A N/A N/A N/A 1 N/A N/A CF CorrosionFatigue D&C Design & Construction Flaws ECSCC External Chlorideinduced SCC IGSCC Intergranular SCC PWSCC Primary Water SCC SCC Stress Corrosion Cracking TF Thermal Fatigue TGSCC Transgranular SCC VF Vibration Fatigue 31 KNF Consulting Services LLC

LOCA Frequencies for STP GSI191 Table 312 Uncertainty Distributions for Calculation Case Failure Rates Failure Rate Distribution System Calculation Weld Applicable Damage (Failures per WeldYear)

System RF Case Case Type Mechanisms Mean 5%tile 50%tile 95%tile 1A BF SC, D&C 2.73E04 1.04E04 2.33E04 5.78E04 2.4 1 RC Hot Leg 1B BJ D&C 1.44E06 5.27E10 4.12E08 3.19E06 77.8 1C BJ TF, D&C 1.07E05 1.79E08 5.79E07 2.83E05 39.8 2 RC SG Inlet 2 BF SC, D&C 1.42E03 9.22E04 1.37E03 2.06E03 1.5 3A, 3B BF SC, D&C 1.25E04 2.99E05 9.34E05 3.17E04 3.3 3 RC Cold Leg 3C, 3D BJ D&C 2.39E06 2.14E08 4.35E07 8.84E06 20.3 4A BF SC, TF, D&C 5.19E04 1.26E04 4.04E04 1.28E03 3.2 4 RC Surge 4B BC TF, D&C 8.06E06 1.80E08 5.39E07 2.24E05 35.3 4C BJ TF, D&C 4.52E06 1.51E08 4.04E07 1.40E05 30.4 5A, 5B BJ TF, D&C 6.29E06 1.63E07 1.32E06 1.61E05 10.0 5C, 5D BJ D&C 1.61E06 7.31E08 6.59E07 5.93E06 9.0 5 Pressurizer 5E BF SC, TF, D&C 4.80E04 2.59E04 4.49E04 7.83E04 1.7 5F BF SC, D&C 4.69E04 2.56E04 4.43E04 7.68E04 1.7 5G BF D&C (Weld Overlay) 8.72E07 5.29E10 5.05E08 3.66E06 83.2 6 Small Bore 6A, 6B BJ VF, SC, D&C 1.23E04 7.03E05 1.11E04 2.02E04 1.7 7A, 7B BJ TF, D&C 2.59E04 2.17E05 1.50E04 8.81E04 6.4 SIR Excl. 7C BJ SC, TF, D&C 2.91E04 3.06E05 1.78E04 9.37E04 5.5 Accumulator 7D BJ SC, D&C 3.32E05 1.24E06 9.38E06 1.25E04 10.1 7 7E-7L BJ, BC D&C 1.07E06 5.61E08 4.64E07 3.89E06 8.3 SIR 7M BJ SC, D&C 3.32E05 1.24E06 9.38E06 1.25E04 10.1 Accumulator 7N BJ TF, D&C 6.72E06 1.42E08 3.90E07 1.66E05 34.2 Lines 7O BJ, BC D&C 5.45E07 4.61E10 2.72E08 1.60E06 59.0 8A, 8B BJ TF, VF, D&C 5.33E06 2.43E07 1.49E06 1.43E05 7.7 8C, 8D BJ VF, D&C 1.76E06 1.37E07 8.61E07 6.01E06 6.6 8 CVCS 8E BC TF, D&C 7.75E06 3.03E07 2.76E06 2.80E05 9.6 8F BC D&C 1.07E06 5.61E08 4.64E07 3.89E06 8.3 32 KNF Consulting Services LLC

LOCA Frequencies for STP GSI191 Figure 34 Comparison of Mean Failure Rates for Calculation Cases 33 KNF Consulting Services LLC

LOCA Frequencies for STP GSI191

4. Conditional Rupture Mode Probability Model (Step 2) 4.1 Overview of CRP Model Approach This section documents the development of the conditional rupture probabilities (CRPs) given pipe failure for an appropriate range of pipe break sizes for each component. The components to be covered by this analysis are determined by the Component Categories in Table 23. In accordance with the step bystep approach to LOCA frequency determination presented in Section 2, this section covers the following key tasks of Step 2, Conditional Rupture Probability (CRP) Development [P(RxFik) in Equation (2.2)]:

2.1 Select components to define conditional rupture probability (CRP) model categories 2.2 Obtain expert reference LOCA distributions from NUREG1829 2.3 Obtain expert multiplier distributions for 40yr LOCA frequencies from NUREG1829 2.4 Determine 40yr LOCA distributions (product of Steps 2.2 and 2.3) for each expert, fit to lognormal 2.5 Determine geometric mean of expert distributions from Step 2.4 (lognormal) 2.6a Benchmark Lydell Base Case Analysis for selected components 2.6b Determine failure rate distribution for Lydell Base Case Analysis in NUREG1829; fit to lognormal 2.6c Apply Lydell CRP model from Base Case Analysis 2.6d Determine LOCA frequency distribution from Lydell Base Case Analysis 2.7 Determine mixture distribution of NUREG1829 GM (from Step 2.5) and Lydell LOCA frequency (from Step 2.8) to obtain Target LOCA Frequency Distribution for each CRP category component 2.8 Apply formulas to calculate CRP distributions to be used as prior distributions for each component assigned to each CRP category 2.9 For each component in a given CRP category, perform Bayes' update with evidence of failure and rupture counts from service data.

The goal of this section is to establish a set of CRPs vs. break size for each Component Category in Table

32. For each Component Category, the break sizes to be considered range from an equivalent break size of 0.5" to the break size corresponding to a doubleended guillotine break (DEGB) of the pipe. The lower bound is based on the lower bound of the Small LOCA initiating event in the STP PRA model, which covers breaks in the range of 0.5" to 2.0". The break size for a DEGB is assumed to be 2 D , where D is the inside diameter of the pipe. (This comes from the fact that an offset rupture effectively doubles the flow area, which would be equivalent to increasing the break size diameter of a single break by the factor 2 .) This model of maximum break size, as is that for a DEGB for all Class 1 pipes, is conservative for pipe locations with a closed end on one side of the break, which would be the case for most branch connection welds in the safety injection and recirculation system piping.

Results for LOCA frequencies at each location can generally be depicted as a curve of CRP vs. break size.

However, this study generates a family of curves due to the epistemic uncertainty in estimating the CRP for each component. This yields both a mean curve and various curves representing different percentiles 34 KNF Consulting Services LLC

LOCA Frequencies for STP GSI191 of the LOCA frequency uncertainty distributions. In this report, results are presented in terms of means, 5%tiles, 50%tiles (medians), and 95%tiles.

The technical approach to CRP development used here has been structured to capture the current state of knowledge of LOCA frequencies. The steps to deriving CRPs for each component are based on the following strategy for LOCA frequency estimation. It was a study objective to make use of information on LOCA frequencies in NUREG1829 [1], which reasonably captures the current state of knowledge among piping system reliability experts on LOCA frequencies. The expert elicitation that is documented in this report captured inputs from experts representing two schools of thought on how to best quantify pipe break frequencies: one based on statistical analysis of service data and simple models, and another based on probabilistic fracture mechanics approaches. The 12 experts that participated in this expert elicitation provided a balanced perspective on these two approaches and produced estimates of the LOCA frequencies vs. break size for use in riskinformed evaluations. NUREG1829 included some base case analyses that were performed on selected components to inform the expert elicitation. One set of these base case analyses was performed by Bengt Lydell, who is a coauthor of this report. Lydell performed his base case analysis using a methodology that is very similar to that used in this study and produced a set of LOCA results with a quantification of epistemic results for a set of PWR cases, namely the hot leg, the surge line, and a high pressure injection system line. In addition to these base case analyses, nine of the participating experts provided individual distributions for LOCA frequencies for a range of components, including the components covered in the base case analyses. The technical approach to CRP model development was designed to make use of both sets of information developed in NUREG1829, namely, the base case analyses and the inputs provided by the nine experts and documented in Reference [14].

Our approach to developing CRPs is to establish a set of target LOCA frequencies that captures the epistemic uncertainties developed for NUREG1829. Inputs from the nine experts who provided inputs at the component level are collected in Steps 2.2 and 2.3 and used to recreate their respective LOCA frequency distributions in Step 2.4. A composite distribution of these nine expert distributions is developed using a geometric mean method similar to that used in NUREG1829 in Step 2.5. In parallel with these steps, the Lydell Base Case Analysis for these same components is benchmarked and deconstructed in Step 2.6 and is used to provide an alternative model to the target LOCA frequencies for these components. In Step 2.7, the LOCA frequency distributions provided by Lydell and the geometric mean composite distributions from Step 2.5 are combined to produce the target LOCA frequency distribution. In Step 2.8, formulas are used to derive the equivalent CRP distributions. These CRP distributions serve as prior distributions for the final step in the CRP model development, Step 2.9, in which Bayes updates of the CRP distributions are performed for each component category.

To appropriately apply to this study the information from NUREG1829 and the supporting inputs and analyses, the following differences between that study and this study need to be understood.

NUREG1829 was meant to develop estimates of PWR and BWR total LOCA frequencies for generic application to U.S. nuclear power plants. In contrast, this study is intended to develop plantspecific estimates of LOCA frequencies not only for a plant as a whole but for numerous locations within a 35 KNF Consulting Services LLC

LOCA Frequencies for STP GSI191 plant. This is made possible by the modeling assumption that LOCA frequencies can be estimated as the product of a pipe failure rate and a CRP.

The PWR LOCA frequencies address planttoplant variability in design characteristics such as number of coolant loops, piping system designs, and configurations, whereas this study focuses on a specific 4loop PWR with three trains of emergency core cooling system interface piping that is unique to STP.

This study has benefitted from several hundred reactoryears of service data on PWR piping systems that were not available when the technical inputs to NUREG1829 were created.

4.2 Use of NUREG1829 Data The expert elicitation that was performed and documented in NUREG1829 [1] provided estimates of the frequencies for LOCAs based on a set of LOCA categories selected to span the break sizes and leak rates that are normally modeled in PWR and BWR PRAs. The estimates provided in NUREG1829 included both pipe failures and nonpipe failures. However, only pipe failures are within the scope of this study. LOCAs caused by nonpipe failures will be addressed in 2012. The LOCA categories for PWRs used in NUREG1829 are summarized in Table 41. Since the largest pipes in a PWR reactor coolant system, which correspond to the cold leg piping, are on the order of 31 nominal pipe size (NPS), the NUREG1829 LOCA categories do not differentiate a DEGB from a single break of the largest pipe in the system. The effective DEGB size of a cold leg pipe of 31 NPS would be about 44.

Table 41 NUREG1829 and STP PRA LOCA Categories Effective LOCA Flow Rate STP PRA Category Break Size Category (gpm)

(in.)

1 Small LOCA(1) 0.5 100 2 Medium LOCA(1) 1.5 1,500 3 3 5,000 4 Large LOCA 6.75 25,000 5 14 100,000 6 31.5 500,000 Note:

(1) In the STP PRA, the breakpoint between Small and Medium LOCAs is actually 2, and the breakpoint between Medium and Large LOCAs is 6.

The approach to using information in NUREG1829 to develop estimates of the conditional probability of pipe ruptures is based on the following observations and information presented in that document.

Base case results are presented in the report for three welldefined piping components for PWRs, namely, hot leg piping, pressurizer surge line piping, and high pressure injection piping, which comprise part of the ASME Class 1 pressure boundary. For each component, four independent estimates were provided for each applicable LOCA category: two estimates based on a statistical analysis of service data and simple models similar to those that will be used in the STP GSI191 evaluation, and two estimates based on probabilistic fracture mechanics 36 KNF Consulting Services LLC

LOCA Frequencies for STP GSI191 analyses. These base case results were provided as input to the experts, and some experts chose to use them as anchors for their respective inputs. The base case results are summarized in Section 4 of NUREG1829 as well as in the supporting appendices of that document.

As part of the elicitation, most of the experts provided input to the estimation of LOCA frequencies for specific components on the RCS pressure boundary, including the components that were evaluated in the base case results as well as essentially all the major components on the Class 1 pressure boundary. Selected componentlevel results of this elicitation are found in Appendix L of NUREG1829. For PWRs, these results are presented for LOCA Categories 1, 3, and

5. An example of the form of this information for LOCA Category 1 is shown in Figure 41. There is also componentlevel expert elicitation information presented in that appendix for hot leg piping for LOCA Category 6. The NUREG1829 supporting information that was just recently released has additional information on componentlevel LOCA estimates for LOCA frequencies that covers all applicable LOCA categories for each component [14].

In the evaluation of service data that was performed in support of NUREG1829, which includes the base case analyses performed by Bill Gallean and Bengt Lydell, none of the reviewed service data involved the occurrence of any LOCAs. The service data we have collected in Table 33 for these systems, encompassing a total of 166 pipe failures, include flaws, cracks, and rather small leaks, but no leaks that would constitute even a Small LOCA, which corresponds to LOCA Category 1. The pipe rupture models used in the base case studies of Gallean and Lydell , as well as the one used in this study, assume that each pipe failure is a precursor to a LOCA. Each of these models starts with an estimate of the failure rate, which includes all pipe failures requiring repair or replacement. The model integrates the failure rates, which are estimated using service data, with the more significant pipe failures that produce LOCAs. This is accomplished by defining the conditional probability of a break of a given size given a pipe failure. Another way to look at this model is that pipe failures are assumed to represent challenges to the system and that upon each challenge, there is a probability of experiencing a break of a given size. By considering the full range of different break sizes, all the LOCA frequency categories can be quantified.

The pipe break frequency model described in Section 2 and Equations (2.1) and (2.2) provide the capability to estimate failure rates at each location in the Class 1 piping system pressure boundary, which is needed for this GSI191 riskinformed evaluation. Conversion of the LOCA frequency inputs in NUREG1829 from a LOCA frequency basis to a conditional probability of LOCA basis was necessitated by this model. This required establishing target LOCA frequencies for key components and then deriving the equivalent CRP model that when combined with the failure rate model will produce the same target LOCA frequencies.

Based on the above information and insights, we will use information from NUREG1829 to convert information that was presented in the form of LOCA frequencies vs. LOCA category, to conditional probabilities vs. break size. This approach is applied to the four PWR components that were included in the base case results as well as in the expert elicitation: the RCS hot leg, the RCS cold leg, the RCS surge line, and the HPI injection line. These span a representative range of nominal pipe sizes on the PWR Class 1 pressure boundary of 30", 30, 14, and 3.75, respectively.

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LOCA Frequencies for STP GSI191 Figure 41 Category 1 LOCA Frequencies for PWR Piping Systems at 25 Years of Plant Operation (Reproduced from Figure L.13 in NUREG1829) 4.3 Model for Deriving Conditional Probabilities from Rupture Frequencies The model used to convert information on unconditional rupture frequencies to conditional failure probabilities makes use of the base case results of Lydell for each of the four selected PWR components (hot leg, cold leg, surge line, HP injection line) and the following equation:

F ( LOCAj ) ml l P( R j F ) (4.1) l Where:

F ( LOCA j ) Unconditional frequency of LOCA Category j due to pipe failures in selected component, per reactor calendaryear ml Number of pipe welds of type l in selected component having the same failure rate l Failure rate per weldyear for pipe weld type l within the selected component in Lydells Base Case Analysis from Appendix D in NUREG 1829 P( R j F ) Conditional rupture probability (CRP) in LOCA Category j given failure in selected component Each term in this model is subject to epistemic uncertainty, which is to be estimated. Therefore, this model and the base case analysis of the failure rates from Lydell are used to derive epistemic uncertainties for the CRPs in each LOCA category. This produces a set of target LOCA frequency distribution parameters that have been selected to incorporate the epistemic uncertainties developed in NUREG1829. This approach makes use of there being a technical basis for the failure rate estimates 38 KNF Consulting Services LLC

LOCA Frequencies for STP GSI191 from service data and a wellreviewed and extensively applied Bayes uncertainty analysis method.

These estimates were part of the information that was available to each NUREG1829 expert to anchor his inputs. Since there have been no Category 1, 2, 3, 4, 5, or 6 LOCAs, the expert elicitation results of all the experts constitute an extrapolation from the existing service data. Therefore, our approach simply assumes that the variability in the expert elicitation inputs for LOCA frequency represents the epistemic uncertainty in the LOCA frequency for each component. This epistemic uncertainty is then assumed to result from the combination of the epistemic uncertainty in the failure rate and the epistemic uncertainty in the conditional probability of each LOCA category.

This model is somewhat simplified from the Lydell Base Case Analysis in Appendix D of NUREG1829.

Lydells Base Case Analysis uses different conditional LOCA category probabilities for different loading conditions and then combines them to produce his base case results. So, as part of Step 2.6, we shall derive an equivalent conditional probability model using equations described in the following in order to benchmark this model against the slightly different model of Lydell. Then we shall adjust the epistemic uncertainties in the conditional probability of a LOCA in a manner that matches target LOCA frequencies that are set to incorporate the variability among experts estimates in NUREG1829.

4.4 Select Components to Define CRP Model Categories (Step 2.1)

As shown in the previous section, 45 failure rate categories were used to characterize the pipe failure rates for 775 distinct weld locations for all the Class 1 piping systems at STP. The failure rate categories cover all combinations of systems, weld types, damage mechanisms, and pipe sizes that are defined by the component categories. In order to estimate CRP, the following CRP Model categories were selected:

Hot Leg CRP model Cold Leg CRP model Surge Line CRP model High Pressure Injection CRP model This selection was based on the following considerations:

There are sufficient data in NUREG1829 and supporting input data to support estimation of the CRPs and the associated epistemic uncertainties using the technical approach adopted in this study.

The above categories provide a unique model for all the categories with large pipe sizes, i.e., those with pipe diameters at least 12, which are expected to be the most prone to debris generation.

Further detail in the treatment of smaller pipes is not warranted for this application, nor is it supported by sufficient pipe failure data.

The SG Inlet categories are a special case of the welds in the hot leg and constitute a separate category solely to capture any outliers in the failure rate data. The conditional probability of pipe rupture for the SG Inlet is not expected to differ from that for the other welds in the hot leg.

The High Pressure Injection CRP category is representative of the medium and small bore pipe with pipe diameter up to 12. They are all stainless steel lines connected to the larger pipe sizes and are subject to a similar range of DMs. This category includes both bimetallic (BF) and similar metal (BJ and BC) welds and covers a full range of DMs that are found in Class 1 piping.

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LOCA Frequencies for STP GSI191 In combination with the 45 unique failure rate cases developed in Section 3, the above CRP model categories provide a reasonably complete set to characterize the LOCA frequencies in all 775 weld locations in Table 33.

The above four models will be used to develop prior distributions for the CRP epistemic uncertainties. Applying them to specific components will entail the Bayes' update in Step 2.9, in which the number of failures and ruptures for each failure rate case will be used as the evidence for updating these base priors. Hence, the four CRP models will actually produce eight different sets of CRPs, one for each system. These will be expanded further to apply to different pipe sizes, where the maximum break size for each pipe size is set to the DEGB size.

A summary of the mapping of CRP model categories to the piping system categories is shown in Table 4 2

Table 42 Assignment of Piping System Categories to CRP Model Categories Damage Case Description Weld Type CRP Model and Bayes Update Evidence Mechanism (DM)

RCS Hot Leg Excl. BF PWSCC, D&C Hot Leg CRP Model, 1

SG Inlet updated with 0 ruptures in 6 failures BJ TF, D&C BF PWSCC, D&C Cold Leg CRP Model, 2 RCS Cold Leg updated with 0 ruptures in 3 failures BJ D&C RCS Hot Leg SG Hot Leg CRP Model, 3 BF PWSCC, D&C Inlet updated with 0 ruptures in 19 failures BF PWSCC, TF, D&C Surge Line CRP Model, 4 PZR Surge Line updated with 0 ruptures in 3 failures BJ, BC TF, D&C PZR Medium Bore BF PWSCC, TF, D&C HPI CRP Model, 5

Piping updated with 0 ruptures in 12 failures BJ, BC TF, D&C Class 1 Small Bore HPI CRP Model, 6 BJ TF, D&C, TGSCC, VF Piping updated with 0 ruptures in 106 failures Class 1 Medium HPI CRP Model, 7 BJ TF, D&C, IGSCC Bore SIR Piping updated with 0 ruptures in 14 failures Class 1 Medium HPI CRP Model 8 BJ, BC TF, D&C, TGSCC, VF Bore CVCS Piping Updated with 0 ruptures in 14 failures BF ASME Category BF welds (bimetallic)

BJ ASME Category BJ welds (single metal)

BC Branch Connection Weld, BJ welds used at branch connections CVCS Chemical, Volume, and Control System D&C Design and Construction Defects IGSCC Intergranular Stress Corrosion Cracking PWSCC Primary Water Stress Corrosion Cracking PZR Pressurizer 40 KNF Consulting Services LLC

LOCA Frequencies for STP GSI191 Damage Case Description Weld Type CRP Model and Bayes Update Evidence Mechanism (DM)

RCS Reactor Coolant System SIR Safety Injection and Recirculation Systems TF Thermal Fatigue TGSCC Transgranular Stress Corrosion Cracking VF Vibration Fatigue 4.5 Use of Data from NUREG1829 Expert Elicitation (Steps 2.2 and 2.3)

The expert elicitation that was performed for NUREG1829 included a request for estimates of LOCA frequencies for specific pipe and nonpiperelated components [14]. Nine experts provided input at this level, and Steps 2.1 through 2.5 involve analysis of these data for selected components, namely the hot leg, cold leg, surge line, and HPI line components in PWRs. One set of numbers provided by the experts was LOCA frequencies by LOCA category in terms of a midvalue (Mid), an upper bound (UB), and a lower bound (LB), with the understanding that those would be interpreted as medians, 95%tiles, and 5%tiles of a lognormal uncertainty distribution. For symmetric inputs (i.e., when UB/Mid = Mid/LB),

which were provided in most cases, these distributions were assumed to be lognormal distributions. For asymmetric inputs provided by the experts, a specific split lognormal distribution was assumed.

The first set of LOCA frequencies was for the existing fleet of plants, which involves a mixture of plant ages and an average plant age of about 25 years at the time the elicitation was performed. The experts provided multipliers for normalizing these LOCA frequencies to plant ages of 25 years, 40 years, and 60 years prior to the occurrence of a LOCA. These multipliers enabled the experts to express whether LOCA frequencies could be affected by aging effects and whether such effects might be mitigated. This study is intended to develop LOCA frequencies that will be valid over the 40 years of the current plant license.

Therefore, only the 40year values are used here.

The expert elicitation inputs for the hot leg pipes are provided in Table 43. The nine experts are labeled A through L, with D, F, and K unassigned. The data highlighted in yellow are copied directly from the questionnaire sheets in Reference [14]. Similar tables were developed for the cold leg, surge line, and HPI line (the variant of the HPI line with volume injection was selected for consistency with the Lydell HPI Base Case Analysis in Appendix D of NUREG1829). This completes Steps 2.2 and 2.3 for each CRP model component.

4.6 Development of 40Year LOCA Frequency Distributions (Step 2.4)

In Step 2.4, the LOCA Frequencies for System distributions are multiplied by the 40year multiplier distributions, to obtain the 40year LOCA frequency distributions. This is straightforward because the product of two lognormal distributions is also a lognormal distribution.

When the two input distributions are lognormal, the parameters of the lognormal distribution for the 40year LOCA frequencies can be directly computed using the following formulas.

median40YLF medianBase median40YM (4.2) 41 KNF Consulting Services LLC

LOCA Frequencies for STP GSI191 RF40YLF e1.645 40 YLF (4.3)

Where:

2 2 ln( RFBase ) ln( RF40YM )

40YLF (4.4) 1.645 1.645 median40YLF Median of the lognormal distribution for the 40year LOCA frequency, evaluated for each combination of expert and LOCA Category median Base Median of the lognormal distribution for the base LOCA frequency (LOCA Frequency for System provided by each expert for each LOCA Category )

median 40YM Median of the lognormal distribution for the 40year multiplier provided by each expert for each LOCA Category RF40YLF Range factor of the lognormal distribution for the 40year LOCA frequency, equal to SQRT(95%tile/5%tile) of the lognormal distribution 40YLF Logarithmic standard deviation for the lognormal distribution for the 40year LOCA frequency, evaluated for each combination of expert and LOCA Category RFBase Range factor of the lognormal distribution for the base LOCA frequency provided by each expert for each LOCA Category RF40YM Range factor of the lognormal distribution for the 40year multiplier provided by each expert for each LOCA Category When the experts provided asymmetric inputs, NUREG1829 utilized a split lognormal formulation for calculating the 40year LOCA frequency distributions. In this study, the inputs provided by the experts were fit to lognormals by preserving the medians and the 95%tiles of the input distributions, while ignoring the asymmetries on the left side of the distributions. An alternative procedure was also tested, in which the median and the range factor defined as the square root of the ratio of the 95%tile to the 5%tile were preserved in the input distributions, which were again assumed to be lognormal. In his independent review of an earlier draft of this report, Dr. Ali Mosleh recommended the former procedure, which was adopted in this report. The adopted approach retains the simplicity of using lognormals in lieu of the more complicated split lognormals, retains the identification of the best estimates with the medians of the distributions, and by preserving the 50th and higher percentiles is more effective in preserving the means of the underlying input distributions.

In Table 43, the first procedure is applied as indicated in the blueshaded cells. The RF95 values were calculated based on UB/Mid for the base LOCA frequencies and the 40year multipliers, and the RF95 values for the 40year LOCA frequencies were calculated from Equation (4.4).

As a result of the above procedure, there is a single lognormal distribution defined for each LOCA category frequency at 40 years of operation. This distribution is applicable to each component provided 42 KNF Consulting Services LLC

LOCA Frequencies for STP GSI191 by each of the 9 experts who provided component level inputs in Reference [14]. In some cases, experts provided fixed values for one parameter (base LOCA frequency or multiplier) and a distribution for the other, in which case the distribution for the 40year LOCA frequency was found simply by scaling the provided distribution parameters with the supplied fixed values. The results in the last column of Table 43 reflect the execution of Step 2.4 for the hot leg.

43 KNF Consulting Services LLC

LOCA Frequencies for STP GSI191 Table 43 NUREG1829 Expert Distributions for Hot Leg LOCA Frequencies LOCA Frequency for System[1] [1] 40Yr LOCA Frequency[1]

40Yr Multiplier Expert ID LOCA Category (Per ReactorCalendar Year) (Per ReactorCalendar Year)

LB Mid UB RF95=UB/Mid[2] LB Mid UB RF95=UB/Mid[2] Mid[3] RF95[4]

1 (> 100) 5.33E08 1.60E07 4.80E07 3.00E+00 1.00E+00 1.00E+00 1.00E+00 1.00E+00 1.60E07 3.00E+00 2 (> 1,500) 5.33E08 1.60E07 4.80E07 3.00E+00 1.50E02 3.00E01 5.85E01 1.95E+00 4.80E08 3.62E+00 3 (> 5,000) 5.33E08 1.60E07 4.80E07 3.00E+00 5.00E03 1.00E01 1.95E01 1.95E+00 1.60E08 3.62E+00 A

4 (> 25,000) 5.33E08 1.60E07 4.80E07 3.00E+00 1.50E03 3.00E02 5.85E02 1.95E+00 4.80E09 3.62E+00 5 (> 100,000) 5.33E08 1.60E07 4.80E07 3.00E+00 5.00E04 1.00E02 1.95E02 1.95E+00 1.60E09 3.62E+00 6 (> 500,000) 5.33E08 1.60E07 4.80E07 3.00E+00 1.50E04 3.00E03 5.85E03 1.95E+00 4.80E10 3.62E+00 1 (> 100) 3.00E07 3.00E07 3.00E07 1.00E+00 1.00E01 1.00E+00 1.00E+01 1.00E+01 3.00E07 1.00E+01 2 (> 1,500) 1.20E07 1.20E07 1.20E07 1.00E+00 1.00E01 1.00E+00 1.00E+01 1.00E+01 1.20E07 1.00E+01 3 (> 5,000) 4.80E08 4.80E08 4.80E08 1.00E+00 1.00E01 1.00E+00 1.00E+01 1.00E+01 4.80E08 1.00E+01 B

4 (> 25,000) 1.92E08 1.92E08 1.92E08 1.00E+00 1.00E01 1.00E+00 1.00E+01 1.00E+01 1.92E08 1.00E+01 5 (> 100,000) 7.68E09 7.68E09 7.68E09 1.00E+00 1.00E01 1.00E+00 1.00E+01 1.00E+01 7.68E09 1.00E+01 6 (> 500,000) 3.07E09 3.07E09 3.07E09 1.00E+00 1.00E01 1.00E+00 1.00E+01 1.00E+01 3.07E09 1.00E+01 1 (> 100) 6.00E07 6.00E07 6.00E07 1.00E+00 3.00E02 1.00E+00 3.00E+01 3.00E+01 6.00E07 3.00E+01 2 (> 1,500) 5.00E08 5.00E08 5.00E08 1.00E+00 3.00E02 1.00E+00 3.00E+01 3.00E+01 5.00E08 3.00E+01 3 (> 5,000) 2.00E08 2.00E08 2.00E08 1.00E+00 3.00E02 1.00E+00 3.00E+01 3.00E+01 2.00E08 3.00E+01 C

4 (> 25,000) 3.00E09 3.00E09 3.00E09 1.00E+00 5.00E02 1.67E+00 1.67E+02 1.00E+02 5.01E09 1.00E+02 5 (> 100,000) 1.00E09 1.00E09 1.00E09 1.00E+00 6.00E02 2.00E+00 2.00E+03 1.00E+03 2.00E09 1.00E+03 6 (> 500,000) 2.00E10 2.00E10 2.00E10 1.00E+00 6.00E02 2.00E+00 2.00E+03 1.00E+03 4.00E10 1.00E+03 1 (> 100) 3.07E07 9.22E07 2.77E06 3.00E+00 3.33E04 2.83E02 3.33E01 1.18E+01 2.61E08 1.49E+01 2 (> 1,500) 3.07E07 9.22E07 2.77E06 3.00E+00 3.33E04 2.83E02 3.33E01 1.18E+01 2.61E08 1.49E+01 E 3 (> 5,000) 3.07E07 9.22E07 2.77E06 3.00E+00 3.33E04 2.83E02 3.33E01 1.18E+01 2.61E08 1.49E+01 4 (> 25,000) 3.67E09 1.10E08 3.30E08 3.00E+00 1.00E03 1.00E01 1.50E+00 1.50E+01 1.10E09 1.86E+01 5 (> 100,000) 1.27E09 3.80E09 1.14E08 3.00E+00 1.00E04 5.00E02 1.00E+00 2.00E+01 1.90E10 2.43E+01 44 KNF Consulting Services LLC

LOCA Frequencies for STP GSI191

[1]

LOCA Frequency for System 40Yr LOCA Frequency[1]

40Yr Multiplier[1]

Expert ID LOCA Category (Per ReactorCalendar Year) (Per ReactorCalendar Year)

LB Mid UB RF95=UB/Mid[2] LB Mid UB RF95=UB/Mid[2] Mid[3] RF95[4]

6 (> 500,000) 4.33E10 1.30E09 3.90E09 3.00E+00 1.00E04 3.00E02 3.00E+00 1.00E+02 3.90E11 1.14E+02 1 (> 100) 5.13E08 1.54E07 4.62E07 3.00E+00 1.00E01 1.14E+00 1.00E+01 8.77E+00 1.76E07 1.14E+01 2 (> 1,500) 7.50E09 2.25E08 6.75E08 3.00E+00 1.00E01 1.14E+00 1.00E+01 8.77E+00 2.57E08 1.14E+01 3 (> 5,000) 2.78E09 8.33E09 2.50E08 3.00E+00 1.00E01 1.14E+00 1.00E+01 8.77E+00 9.50E09 1.14E+01 G

4 (> 25,000) 9.50E10 2.85E09 8.55E09 3.00E+00 1.00E01 1.14E+00 1.00E+01 8.77E+00 3.25E09 1.14E+01 5 (> 100,000) 1.71E10 8.53E10 4.27E09 5.01E+00 1.00E01 1.14E+00 1.00E+01 8.77E+00 9.72E10 1.49E+01 6 (> 500,000) 1.58E11 1.58E10 1.58E09 1.00E+01 1.00E01 1.14E+00 1.00E+01 8.77E+00 1.80E10 2.37E+01 1 (> 100) 1.48E07 4.45E07 1.34E06 3.01E+00 2.50E+00 2.50E+01 2.50E+02 1.00E+01 1.11E05 1.28E+01 2 (> 1,500) 2.03E08 6.10E08 1.83E07 3.00E+00 1.00E+00 1.00E+01 1.00E+02 1.00E+01 6.10E07 1.28E+01 3 (> 5,000) 7.33E09 2.20E08 6.60E08 3.00E+00 5.00E01 5.00E+00 5.00E+01 1.00E+01 1.10E07 1.28E+01 H

4 (> 25,000) 2.60E09 7.80E09 2.34E08 3.00E+00 5.00E01 5.00E+00 5.00E+01 1.00E+01 3.90E08 1.28E+01 5 (> 100,000) 8.83E10 2.65E09 7.95E09 3.00E+00 5.00E01 5.00E+00 5.00E+01 1.00E+01 1.33E08 1.28E+01 6 (> 500,000) 2.93E10 8.80E10 2.64E09 3.00E+00 5.00E01 5.00E+00 5.00E+01 1.00E+01 4.40E09 1.28E+01 1 (> 100) 4.00E11 2.00E09 1.00E07 5.00E+01 5.00E01 5.00E01 5.00E01 1.00E+00 1.00E09 5.00E+01 2 (> 1,500) 4.00E11 2.00E09 1.00E07 5.00E+01 5.00E01 5.00E01 5.00E01 1.00E+00 1.00E09 5.00E+01 3 (> 5,000) 4.00E11 2.00E09 1.00E07 5.00E+01 5.00E01 5.00E01 5.00E01 1.00E+00 1.00E09 5.00E+01 I

4 (> 25,000) 4.00E11 2.00E09 1.00E07 5.00E+01 5.00E01 5.00E01 5.00E01 1.00E+00 1.00E09 5.00E+01 5 (> 100,000) 4.00E11 2.00E09 1.00E07 5.00E+01 5.00E01 5.00E01 5.00E01 1.00E+00 1.00E09 5.00E+01 6 (> 500,000) 4.00E11 2.00E09 1.00E07 5.00E+01 5.00E01 5.00E01 5.00E01 1.00E+00 1.00E09 5.00E+01 1 (> 100) 9.25E12 9.80E11 2.88E09 2.94E+01 3.19E+01 3.19E+01 3.19E+01 1.00E+00 3.13E09 2.94E+01 2 (> 1,500) 5.78E13 1.03E11 7.61E10 7.39E+01 5.24E+01 5.24E+01 5.24E+01 1.00E+00 5.40E10 7.39E+01 3 (> 5,000) 1.40E13 3.21E12 3.38E10 1.05E+02 6.04E+01 6.04E+01 6.04E+01 1.00E+00 1.94E10 1.05E+02 J

4 (> 25,000) 1.53E14 4.82E13 9.75E11 2.02E+02 7.50E+01 7.50E+01 7.50E+01 1.00E+00 3.62E11 2.02E+02 5 (> 100,000) 2.42E15 6.99E14 1.93E11 2.76E+02 9.81E+01 9.81E+01 9.81E+01 1.00E+00 6.86E12 2.76E+02 6 (> 500,000) 1.44E17 6.28E16 7.56E13 1.20E+03 1.14E+02 1.14E+02 1.14E+02 1.00E+00 7.16E14 1.20E+03 45 KNF Consulting Services LLC

LOCA Frequencies for STP GSI191

[1]

LOCA Frequency for System 40Yr LOCA Frequency[1]

40Yr Multiplier[1]

Expert ID LOCA Category (Per ReactorCalendar Year) (Per ReactorCalendar Year)

LB Mid UB RF95=UB/Mid[2] LB Mid UB RF95=UB/Mid[2] Mid[3] RF95[4]

1 (> 100) 2.62E06 9.60E06 3.52E05 3.67E+00 1.27E01 1.27E01 1.27E01 1.00E+00 1.22E06 3.67E+00 2 (> 1,500) 1.58E06 6.34E06 2.53E05 3.99E+00 1.27E01 1.27E01 1.27E01 1.00E+00 8.05E07 3.99E+00 3 (> 5,000) 3.84E07 1.92E06 9.60E06 5.00E+00 4.19E01 4.19E01 4.19E01 1.00E+00 8.04E07 5.00E+00 L

4 (> 25,000) 1.54E07 7.68E07 3.84E06 5.00E+00 1.01E+00 1.01E+00 1.01E+00 1.00E+00 7.76E07 5.00E+00 5 (> 100,000) 6.40E08 3.20E07 1.60E06 5.00E+00 2.41E+00 2.41E+00 2.41E+00 1.00E+00 7.71E07 5.00E+00 6 (> 500,000) 3.20E11 3.20E10 3.20E09 1.00E+01 2.61E+00 2.61E+00 2.61E+00 1.00E+00 8.35E10 1.00E+01 Notes:

[1] Data shaded in yellow are taken from NUREG1829 expert questionnaires in Reference [14]. Data shaded in blue were calculated in this study per Notes [2] through [4].

[2] RF = Range Factor of a lognormal distribution defined by the Mid value as the median and by the UB value as the 95%tile.

[3] Median of a lognormal distribution for the 40year LOCA frequency created by the product of two lognormal distributions: the medians of the lognormal distributions for LOCA frequency for system and the 40year multiplier (see Equation [4.1]).

[4] Range Factor of the 40year LOCA frequency lognormal distribution (see Equation [4.2]).

46 KNF Consulting Services LLC

LOCA Frequencies for STP GSI191 4.7 Develop Expert Composite Distributions from NUREG1829 (Step 2.5)

In this step, the nine expert distributions for 40year LOCA frequencies obtained in Step 2.4 are combined into a single composite distribution.

NUREG1829 discussed two approaches for developing expert composite distributions: the Mixture Distribution Method and the Geometric Mean Method. NUREG1829 adopted the latter approach, whereas this study evaluated both approaches, briefly described below.

Mixture Distribution Method The expert elicitation input sheets that are found in Reference [14] furnished each expert with input on the LOCA frequencies in each of the six applicable LOCA categories for each component in the RCS pressure boundary, the same components as shown in Figure 41.

A single mixture distribution was developed for each combination of component and LOCA category by combining the 40year LOCA frequency distributions provided by each expert. A single mixture distribution was developed by sampling a discrete distribution on each Monte Carlo trial to determine which experts lognormal distribution for the 40year LOCA frequency to be sampled for that trial. The discrete distribution has a value for each expert, with each value's being assigned the same probability in order to give all experts equal weight. In the several cases where experts did not provide inputs for each LOCA category, the mixture distribution was developed only for those experts providing inputs for that category. In all cases, a minimum of seven experts provided input, and the vast majority of cases had nine. This method is discussed in NUREG1829 but was rejected in favor of the Geometric Mean method.

Geometric Mean Method When this method was used in NUREG1829, it was oriented toward the calculation of the total LOCA frequency rather than the LOCA frequency for many locations. Another contrast was the use in NUREG 1829 of split lognormals, whereas this study used lognormal fitting based on preserving medians and 95%tiles. In this study, a single lognormal distribution for each component and each LOCA category was defined by taking the geometric mean of the medians of the experts lognormal distributions as the composite distribution median, and the geometric means of the range factors of the experts lognormal distributions for the 40year LOCA frequencies as the composite distribution range factor. As with the Mixture Method, in this study the input lognormal distributions provided by the experts were fit to lognormal distribution by matching the 50th and 95th percentiles. A summary of the derived composite distribution parameters is provided in Table 44, which completes Step 2.5 of our LOCA frequency procedure.

A comparison of the resulting composite distributions using both methods is provided in Figures 42 and 43, for the RCS hot leg and RCS surge line, respectively. As seen in these figures, the composite distributions generated by the Mixture Distribution method produce much broader ranges of uncertainty than those obtained by the Geometric Mean method. As discussed in NUREG1829 and 47 KNF Consulting Services LLC

LOCA Frequencies for STP GSI191 confirmed by our study, the upper and lower bounds of the mixture distributions are heavily influenced by the experts' extreme highside and lowside inputs, respectively, whereas the distribution percentiles from the Geometric Mean method more fairly represent the experts inputs. However, in the selection of target LOCA frequencies, this study departs from NUREG1829 by employing both the Geometric Mean Model of LOCA frequencies and a recreation of the Lydell Base Case Analysis in NUREG1829 Appendix D for the same group of components.

Table 44 Composite Distributions for NUREG1829 Experts Based on Geometric Mean Method Break Geometric Mean Distribution Parameters LOCA Events per ReactorCalendar Year Component Size Cat.

(Inches) Mean 5%tile 50%tile 95%tile RF 1 0.5 4.08E07 9.32E09 1.21E07 1.57E06 13.0 2 1.5 1.28E07 2.25E09 3.34E08 4.95E07 14.8 3 3 6.51E08 1.01E09 1.59E08 2.52E07 15.8 Hot Leg 4 6.75 2.59E08 2.49E10 4.96E09 9.88E08 19.9 5 14 1.50E08 6.70E11 1.90E09 5.37E08 28.3 6 31.5 3.16E09 4.84E12 2.18E10 9.78E09 45.0 1 0.5 1.47E07 3.27E09 4.30E08 5.66E07 13.2 2 1.5 5.20E08 9.07E10 1.35E08 2.01E07 14.9 3 3 2.19E08 3.33E10 5.31E09 8.48E08 16.0 Cold Leg 4 6.75 7.85E09 7.41E11 1.49E09 2.99E08 20.1 5 14 4.54E09 1.94E11 5.60E10 1.62E08 28.9 6 31.5 1.10E09 1.56E12 7.23E11 3.36E09 46.4 1 0.5 3.60E07 1.33E08 1.34E07 1.35E06 10.1 2 1.5 1.26E07 3.46E09 4.09E08 4.83E07 11.8 Surge Line 3 3 6.45E08 1.29E09 1.79E08 2.49E07 13.9 4 6.75 1.92E08 2.47E10 4.28E09 7.41E08 17.3 5 14 2.72E09 4.22E11 6.66E10 1.05E08 15.8 1 0.5 1.27E05 6.40E07 5.45E06 4.65E05 8.5 2 1.5 4.58E06 1.51E07 1.62E06 1.74E05 10.7 HPI Line 3 3 7.21E07 1.53E08 2.06E07 2.78E06 13.5 4 6.75 1.29E07 1.41E09 2.64E08 4.95E07 18.8 5 14 3.03E08 3.30E10 6.20E09 1.16E07 18.8 48 KNF Consulting Services LLC

LOCA Frequencies for STP GSI191 Figure 42 Comparison of Mixture and Geometric Mean Composite Distributions - RCS Hot Leg Figure 43 Comparison of Mixture and Geometric Mean Composite Distributions - RCS Surge Line 49 KNF Consulting Services LLC

LOCA Frequencies for STP GSI191 4.8 Benchmark of Lydells Base Case Analysis (Step 2.6)

This step establishes inputs to the selection of target LOCA frequencies from the Lydell Base Case Analysis. A secondary purpose is to establish the corresponding failure rate and CRP distributions that are responsible for the Base Case results. The failure rate distribution parameters will be used in Step 2.8 to convert the target LOCA frequency distributions to CRP distributions.

Using the same Microsoft Excel' and Oracle Crystal Ball' files that Lydell used to develop his Base Case results, the simplified model of Equation (4.1) was applied to the same failure rate estimates that Lydell derived and documented in Appendix D of NUREG1829, assuming a lognormal distribution for the conditional LOCA category probability for each component. This resulted in lognormal parameters that essentially reproduce Lydells Appendix D results, as shown in Figures 44, 45, and 46, for the HPI injection line, RCS surge line, and RCS hot leg, respectively. The CRP distribution parameters were obtained by first developing the LOCA frequencies and then calculating the CRP distribution parameters using formulas for calculating the parameters for the product of two lognormal distributions - similar to Equations (4.2) and (4.3). The figures comparing the Base Case results from Appendix D in NUREG1829 with the results obtained using the equivalent lognormal distributions indicate excellent agreement. The underlying lognormal distribution parameters for the conditional LOCA probabilities in Table 45 appear to the authors to be reasonable, i.e. they are neither very large nor very small. The conditional probability of a given break size is indicated to be inversely proportional to pipe size which is in agreement with previous estimates of LOCA frequencies.

The uncertainty distribution parameters for the LOCA frequencies from this reconstruction of the Lydell Base Case results are shown in Table 46. This completes Step 2.6 of the LOCA frequency procedure.

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LOCA Frequencies for STP GSI191 Figure 44 Benchmarking of Lognormal Distributions to Lydell Base Case Results - HPI Injection Line Figure 45 Benchmarking of Lognormal Distributions to Lydell Base Case Results - RCS Surge Line 51 KNF Consulting Services LLC

LOCA Frequencies for STP GSI191 Figure 46 Benchmarking of Lognormal Distributions to Lydell Base Case Results - RCS Hot Leg Table 45 Lognormal Distributions for Failure Rates and Conditional Rupture Probabilities (CRPs)

Matching Lydells Base Case Results LOCA Break Size Range Component Mean 5%tile Median 95%tile Category (in.) Factor Failure Rate 3.46E04 1.01E05 1.15E04 1.32E03 11.4 1 .5 1.67E03 9.49E05 7.55E04 6.01E03 8.0 2 1.5 1.18E04 5.38E06 4.85E05 4.37E04 9.0 RCS -

3 3 4.73E05 2.13E06 1.93E05 1.75E04 9.1 Hot Leg 4 6.75 1.76E05 7.71E07 7.09E06 6.52E05 9.2 5 14 6.59E06 2.97E07 2.69E06 2.43E05 9.1 6 31.5 3.23E06 1.38E07 1.28E06 1.20E05 9.3 Failure Rate 1.73E04 5.04E06 5.77E05 6.61E04 11.4 1 .5 1.67E03 9.49E05 7.55E04 6.01E03 8.0 2 1.5 1.18E04 5.38E06 4.85E05 4.37E04 9.0 RCS -

3 3 4.73E05 2.13E06 1.93E05 1.75E04 9.1 Cold Leg 4 6.75 1.76E05 7.71E07 7.09E06 6.52E05 9.2 5 14 6.59E06 2.97E07 2.69E06 2.43E05 9.1 6 31.5 3.23E06 1.38E07 1.28E06 1.20E05 9.3 52 KNF Consulting Services LLC

LOCA Frequencies for STP GSI191 LOCA Break Size Range Component Mean 5%tile Median 95%tile Category (in.) Factor Failure Rate 1.33E05 5.55E07 5.25E06 4.96E05 9.5 1 .5 7.65E03 1.46E03 5.52E03 2.08E02 3.8 RCS - Surge 2 1.5 6.70E04 9.19E05 4.31E04 2.02E03 4.7 Line 3 3 2.62E04 3.59E05 1.68E04 7.89E04 4.7 4 6.75 9.81E05 1.21E05 6.08E05 3.04E04 5.0 5 14 3.62E05 5.17E06 2.36E05 1.08E04 4.6 Failure Rate 1.33E03 4.27E05 4.65E04 5.07E03 10.9 1 .5 1.18E02 2.32E03 8.59E03 3.18E02 3.7 HPI 2 1.5 1.75E03 2.69E04 1.17E03 5.11E03 4.4 3 3 6.97E04 1.03E04 4.61E04 2.06E03 4.5 Table 46 LOCA Frequency Distributions from Benchmarking of Lydell Base Case Results Lydell Base Case Distribution Parameters LOCA Break Events per ReactorCalendar Year Component Cat. Size (in.)

Mean 5%tile 50%tile 95%tile RF 1 0.5 6.65E07 3.55E09 9.39E08 2.14E06 24.6 2 1.5 4.87E08 2.10E10 6.15E09 1.49E07 26.6 3 3 1.83E08 8.33E11 2.42E09 5.95E08 26.7 Hot Leg 4 6.75 6.99E09 3.03E11 8.93E10 2.21E08 27.0 5 14 2.55E09 1.16E11 3.29E10 8.29E09 26.7 6 31.5 1.26E09 5.44E12 1.58E10 4.04E09 27.3 1 0.5 3.33E07 1.78E09 4.70E08 1.07E06 24.6 2 1.5 2.44E08 1.05E10 3.08E09 7.45E08 26.6 3 3 9.15E09 4.17E11 1.21E09 2.98E08 26.7 Cold Leg 4 6.75 3.50E09 1.52E11 4.47E10 1.11E08 27.0 5 14 1.28E09 5.80E12 1.65E10 4.15E09 26.7 6 31.5 6.30E10 2.72E12 7.90E11 2.02E09 27.3 1 0.5 1.14E07 2.13E09 2.36E08 3.94E07 13.6 2 1.5 9.60E09 1.48E10 1.88E09 3.46E08 15.3 Surge Line 3 3 3.84E09 5.78E11 8.50E10 1.35E08 15.3 4 6.75 1.44E09 2.01E11 2.77E10 5.06E09 15.9 5 14 5.31E10 8.23E12 1.01E10 1.87E09 15.1 1 0.5 1.60E05 2.62E07 3.93E06 6.09E05 15.2 HPI Line 2 1.5 2.33E06 3.30E08 5.40E07 9.02E06 16.5 3 3 9.22E07 1.28E08 2.14E07 3.59E06 16.7 53 KNF Consulting Services LLC

LOCA Frequencies for STP GSI191 4.9 Select Target LOCA Frequencies from NUREG1829 Data (Step 2.7)

In selecting the target LOCA frequencies, four options were considered.

Option 1: use only the Lydell Base Case results Option 2: use only the Experts' Mixture Distribution results Option 3: use only the Experts Geometric Mean results Option 4: use a hybrid of the Experts' Geometric Mean and Lydell Base Case results Option 1 would be consistent with the STP approach to LOCA frequency assessment but would not be making full use of the expert elicitation results of NUREG1829. Option 2 would be making use of the expert elicitation but would produce unreasonably large spreads between the upper and lower percentiles, which would overemphasize the most extreme expert inputs. Option 3 would be superior to Options 1 and 2 in that it would better represent the diverse inputs of the expert panel and would include the input of Lydell. The option selected, Option 4, is a hybrid of Options 1 and 3 and is comprised of a mixture distribution of the LOCA frequencies produced by those options.

Option 4 places equal weight on the Lydell Base Case results and the Expert Geometric Mean results.

This option's mixture distribution was developed by Monte Carlo simulation, which involved a binary variable to select either Lydell Base Case results or Expert Geometric Mean results, after which a random sample was obtained from that selected distribution. The use of the mixture distribution method to provide a composite target LOCA distribution was recommended by Dr. Ali Mosleh, who performed an independent review of an earlier draft where a different method was used to develop the hybrid of the two LOCA frequency models. In the earlier approach, a hybrid distribution was constructed using the worst case 95%tiles and 5%tiles of the distributions from Options 1 and 3, and the 95%tile and 5%tile were then selected from that hybrid distribution.

Option 4 is preferred over Option 3 as it exhibits a larger degree of epistemic uncertainty while providing mean values that are very close to those of Option 3. These target LOCA frequencies are used in the next step to derive CRPs for LOCAs in each of the LOCA break size categories given a pipe failure.

The parameters of the target LOCA frequency distributions for the Hot Leg, Cold Leg, Surge Line, and HPI Line selected using this method were shown in Table 211. Figures 47, 48 and 49 compare the resulting target LOCA frequencies and those for Option 3, for the RCS Hot Leg, Surge Line, and HPI Line, respectively. The net effect is to increase the uncertainty with slight reductions in the mean and 95%tile and larger reductions for the 5%tiles compared to Option 3 for the Hot Leg and Surge Line. In the case of the HPI line, the Lydell Base Case was for a small pipe size so only Categories 1, 2, and 3 were included.

Hence the target LOCA frequencies for Categories 4 and 5 are the same as those for the GM method and the impact of incorporating the Lydell Base inputs to the mixture distribution is much smaller for this case when compared to the hot leg and surge line. The cold leg results are very similar to the hot leg results except that the frequencies are scaled down somewhat. This completes Step 2.7.

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LOCA Frequencies for STP GSI191 Figure 47 Comparison of Experts Geometric Mean and STP Target LOCA ModelHot Leg Figure 48 Comparison of Experts Geometric Mean and STP Target LOCA Model - Surge Line 55 KNF Consulting Services LLC

LOCA Frequencies for STP GSI191 Figure 49 Comparison of Experts Geometric Mean and STP Target LOCA Model HPI Line 56 KNF Consulting Services LLC

LOCA Frequencies for STP GSI191 Table 47 Mixture Distribution of Geometric Mean and Lydell Base Case for STP Target LOCA Frequencies Target LOCA Frequency Distribution Parameters LOCA Break Events per ReactorCalendar Year Component Cat. Size (in.)

Mean 5%tile 50%tile 95%tile RF 1 0.5 5.07E07 5.39E09 1.05E07 1.83E06 18.4 2 1.5 8.22E08 4.29E10 1.49E08 3.30E07 27.7 3 3 4.10E08 1.68E10 6.47E09 1.60E07 30.9 Hot Leg 4 6.75 1.57E08 5.65E11 2.09E09 6.07E08 32.8 5 14 8.69E09 2.09E11 7.64E10 2.93E08 37.4 6 31.5 2.11E09 5.01E12 1.79E10 6.63E09 36.4

[1]

6D 44.5 1.05E09 2.72E12 9.80E11 3.52E09 36.0 1 0.5 2.28E07 2.36E09 4.32E08 8.08E07 18.5 2 1.5 3.71E08 2.09E10 6.63E09 1.43E07 26.1 3 3 1.53E08 8.09E11 2.62E09 5.92E08 27.1 Cold Leg 4 6.75 5.38E09 2.68E11 8.13E10 2.03E08 27.5 5 14 2.72E09 8.97E12 2.94E10 9.45E09 32.5 6 31.5 8.03E10 2.05E12 7.27E11 2.64E09 35.8 6D 44.5 4.63E10 1.10E12 4.10E11 1.53E09 37.4 1 0.5 2.34E07 3.55E09 6.60E08 9.35E07 16.2 2 1.5 6.78E08 2.72E10 1.04E08 2.83E07 32.3 3 3 3.33E08 1.05E10 4.04E09 1.38E07 36.2 Surge Line 4 6.75 1.05E08 3.52E11 1.14E09 4.06E08 34.0 5 14 1.61E09 1.35E11 2.84E10 6.17E09 21.4

[2]

5D 19.8 6.53E10 8.56E12 1.47E10 2.52E09 17.2 1 0.5 1.39E05 3.88E07 4.73E06 5.26E05 11.6 2 1.5 3.51E06 5.50E08 9.78E07 1.37E05 15.8 HPI Line 3 3 8.11E07 1.41E08 2.11E07 3.11E06 14.9 4 6.75 1.29E07 1.41E09 2.64E08 4.95E07 18.8 5 14 3.03E08 3.30E10 6.20E09 1.16E07 18.8 Notes:

[1] LOCA Category 6D is introduced in this study to denote a double ended break of a 31.5 in. pipe

[2] LOCA Category 5D is introduced in this study to denote a double ended break of a 14 in. pipe 57 KNF Consulting Services LLC

LOCA Frequencies for STP GSI191 4.10 Develop Conditional Rupture Probabilities from Target LOCA Frequencies (Step 2.8)

This step uses the target LOCA frequencies from Step 2.7 and information from the Lydell Base Case results on the underlying failure rates for each component, to derive a CRP model that when linked with the Lydell Base Case failure rate model, will reproduce the target LOCA frequencies that were developed in Step 2.7. The Lydell failure rate analysis that was performed in Appendix D of NUREG1829 used the same methodology for failure rate development as used here, using an earlier set of PIPExp failure data.

The results for his Base Case failure rates for the components associated with the target LOCA frequencies are shown in Table 48. This includes the three PWR components analyzed in NUREG1829 as well as the RCS cold leg, whose results have been developed using a set of assumptions that are comparable to that used for the RCS hot leg. In order to derive the model for conditional probability of rupture, the Lydell failure rates were fit to lognormal distributions by matching the 5th and 95th percentiles and the range factor calculated from these percentiles.

Since the use of lognormal distributions enables the LOCA frequency to be expressed as the product of a lognormally distributed failure rate and a lognormally distributed CRP, the parameters of the CRP distributions may be calculated directly. Using the same methodology as used in Equations (4.2, (4.3),

and (4.4), the following relations are established.

medianTLFk medianCRPk (4.5) medianFR 1.645 CRPk RFCRPk e (4.6) where 2

ln( RFTLFk ) ln( RFFR )

2 CRP (4.7) 1.645 1.645 k

medianCRPk Median of the lognormal distribution for the conditional probability of pipe rupture in LOCA Category k given pipe failure medianTLFk Median of the lognormal distribution for the target LOCA frequency for LOCA Category k medianFR Median of the lognormal distribution for the pipe failure rate RFCRPk Range factor of the lognormal distribution for the conditional probability of pipe rupture in LOCA Category k given pipe failure, equal to SQRT(95%tile/5%tile) of the lognormal distribution CRP k

Logarithmic standard deviation for the lognormal distribution for the conditional probability of pipe rupture in LOCA Category k given pipe failure 58 KNF Consulting Services LLC

LOCA Frequencies for STP GSI191 RFTLFk Range factor of the lognormal distribution for the target LOCA frequency for LOCA Category k RFFR Range factor of the lognormal distribution for the pipe failure rate The medians and range factors of the CRP distributions were computed from the medians and range factors of the target LOCA frequency distributions using the above formulas. Then, using the properties of the lognormal distribution, the remaining parameters of the distributions may be directly computed.

Table 48 Parameters of Target LOCA Frequencies Selected for STP Model Cumulative LOCA Frequency[1],

LOCA Break Component per ReactorCalendarYear RF[2]

Category Size (in.)

Mean 5%tile 50%tile 95%tile 1 0.5 4.45E07 3.55E09 7.72E08 1.68E06 21.7 2 1.5 1.95E07 2.10E10 1.09E08 5.68E07 52.0 3 3 1.05E07 8.33E11 4.89E09 2.87E07 58.7 Hot Leg 4 6.75 3.75E08 3.03E11 1.77E09 1.03E07 58.3 5 14 2.02E08 1.16E11 7.75E10 5.17E08 66.8 6 31.5 2.41E09 5.44E12 2.08E10 7.94E09 38.2 1 0.5 1.52E07 1.78E09 3.22E08 5.85E07 18.2 2 1.5 7.47E08 1.05E10 4.89E09 2.28E07 46.6 3 3 3.17E08 4.17E11 1.99E09 9.55E08 47.9 Cold Leg 4 6.75 1.00E08 1.52E11 6.85E10 3.09E08 45.2 5 14 5.27E09 5.80E12 2.99E10 1.54E08 51.5 6 31.5 7.60E10 2.72E12 8.51E11 2.66E09 31.3 1 0.5 3.85E07 2.13E09 5.48E08 1.41E06 25.7 2 1.5 1.94E07 1.48E10 8.84E09 5.27E07 59.7 Surge Line 3 3 1.10E07 5.78E11 4.00E09 2.77E07 69.2 4 6.75 2.86E08 2.01E11 1.24E09 7.64E08 61.7 5 14 3.01E09 8.23E12 2.89E10 1.02E08 35.2 1 0.5 1.57E05 2.62E07 3.99E06 6.09E05 15.2 2 1.5 5.31E06 3.30E08 8.05E07 1.96E05 24.4 HPI Line 3 3 9.30E07 1.28E08 2.14E07 3.59E06 16.7 4 6.75 1.36E07 1.34E09 2.64E08 5.17E07 19.6 5 14 3.19E08 3.16E10 6.20E09 1.22E07 19.6 Notes:

[1] Frequency of LOCA with break size greater than or equal to the indicated value.

[2] RF = SQRT(95%tile/5%tile).

Comparisons of the STP Model CRP distributions with those used in the Lydell Base Case are shown in Figures 411, 412, and 413. The net result of the procedure is to produce somewhat more pessimistic CRP values with larger epistemic uncertainties than those used in the Lydell Base Case Analysis. This completes Step 2.8 of the LOCA frequency procedure.

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LOCA Frequencies for STP GSI191 Figure 410 Comparison of Lydell and STP Models for CRP - RCS Hot Leg Figure 411 Comparison of Lydell and STP Models for CRP - RCS Surge Line 60 KNF Consulting Services LLC

LOCA Frequencies for STP GSI191 Figure 412 Comparison of Lydell and STP Models for CRP - HPI Line Table 49 STP CRP Distribution Priors Derived from Target LOCA Frequencies Conditional Rupture Probability Distribution Parameters LOCA Break Component 5th 95th Range Category Size (in.) Median Mean Percentile Percentile Factor[1]

1 0.5 1.46E03 1.84E04 9.10E04 4.50E03 4.9 2 1.5 3.31E04 1.35E05 1.29E04 1.23E03 9.6 Hot Leg 3 3 1.65E04 5.01E06 5.61E05 6.28E04 11.2 4 6.75 5.74E05 1.49E06 1.81E05 2.20E04 12.2 5 14 2.49E05 4.54E07 6.62E06 9.65E05 14.6 6 31.5 5.84E06 1.06E07 1.55E06 2.26E05 14.6[4]

6D[2] 44.5 3.20E06 5.82E08 8.49E07 1.24E05 14.6[4]

1 0.5 1.20E03 1.50E04 7.48E04 3.72E03 5.0 2 1.5 2.74E04 1.31E05 1.15E04 1.00E03 8.7 3 3 1.13E04 4.92E06 4.54E05 4.18E04 9.2 Cold Leg 4 6.75 3.58E05 1.49E06 1.41E05 1.33E04 9.5 5 14 1.59E05 4.25E07 5.09E06 6.10E05 12.0 6 31.5 4.48E06 9.17E08 1.26E06 1.73E05 13.7 6D[2] 44.5 2.67E06 4.88E08 7.10E07 1.03E05 14.6 61 KNF Consulting Services LLC

LOCA Frequencies for STP GSI191 Conditional Rupture Probability Distribution Parameters LOCA Break Component 5th 95th Range Category Size (in.) Median Mean Percentile Percentile Factor[1]

1 0.5 2.08E02 2.42E03 1.26E02 6.53E02 5.2 2 1.5 7.24E03 1.40E04 1.98E03 2.80E02 14.1 3 3 3.28E03 4.68E05 7.70E04 1.27E02 16.4 Surge Line 4 6.75 9.24E04 1.32E05 2.17E04 3.57E03 16.4[4]

5 14 2.30E04 3.29E06 5.41E05 8.90E04 16.4[4]

5D[3] 19.8 1.19E04 1.70E06 2.80E05 4.60E04 16.4[4]

1 0.5 1.08E02 5.77E03 1.02E02 1.80E02 1.8 HPI Line 2 1.5 3.00E03 5.27E04 2.10E03 8.39E03 4.0[4]

3 3 6.45E04 1.13E04 4.53E04 1.81E03 4.0 4 6.75 9.67E05 1.03E05 5.67E05 3.11E04 5.5 5 14 2.27E05 2.43E06 1.33E05 7.30E05 5.5[4]

Notes:

[1] Range Factor = SQRT(95%tile/5%tile).

[2] 6D corresponds to a doubleended break of a 31.5 pipe.

[3] 5D corresponds to a doubleended break of a 14 pipe.

[4] Range factors adjusted upwards to ensure no RF decrease with decreasing LOCA frequency.

4.11 Bayes Update of the Conditional Probability Distributions (Step 2.9)

The conditional probability models developed in the previous section are used as the basis for a prior distribution, which we then update with the evidence from the service data on the number of experienced pipe failures with no LOCAs for each system. During the Bayes updating, the lognormal distributions developed in Step 2.8 as the prior distributions, were truncated to avoid CRP values greater than 1.0. However, this truncation only impacts the extreme righthand tails of the distribution and therefore does not significantly affect the major quoted parameters (mean, median, 5%tile, and 95%tile). The Bayes updates were performed using RDAT Plus' Version 1.5.8 (Build 1691) software.

The truncated lognormal distributions described in Table 49 were used as prior distributions and then updated with 0 LOCAs in each LOCA category out of the number of observed failures for each system.

The results are summarized in Table 410. The final Bayes updated distributions for the CRP distributions in Table 410 show a small decrease relative to the values in Table 49. This completes Step 2.9 and all the steps associated with developing the CRP model for the STP LOCA frequencies.

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LOCA Frequencies for STP GSI191 Table 410 STP CRP Distributions after Bayes Updating Bayes Conditional Rupture Probability Distribution Parameters LOCA Break Component Update Category Size (in.) Mean 5%tile Median 95%tile RF[1]

Evidence 1 0.5 1.43E03 1.85E04 9.04E04 4.39E03 4.9 2 1.5 3.28E04 1.34E05 1.29E04 1.23E03 9.6 0 Ruptures/ 3 3 1.64E04 5.01E06 5.60E05 6.25E04 11.2 6 Failures; Hot Leg 4 6.75 5.74E05 1.48E06 1.81E05 2.20E04 12.2 Hot Leg CRP Model 5 14 2.49E05 4.53E07 6.62E06 9.66E05 14.6 6 31.5 5.85E06 1.06E07 1.55E06 2.26E05 14.6

[2]

6D 44.5 3.20E06 5.82E08 8.49E07 1.24E05 14.6 1 0.5 1.39E03 1.84E04 8.91E04 4.25E03 4.8 2 1.5 3.22E04 1.34E05 1.28E04 1.20E03 9.5 0 Ruptures/ 3 3 1.61E04 5.00E06 5.58E05 6.18E04 11.1 Hot Leg at SG 19 Failures; 4 6.75 5.70E05 1.48E06 1.81E05 2.19E04 12.2 Inlet Hot Leg CRP Model 5 14 2.35E05 4.29E07 6.26E06 9.11E05 14.6 6 31.5 5.84E06 1.06E07 1.55E06 2.26E05 14.6

[2]

6D 44.5 3.20E06 5.82E08 8.49E07 1.24E05 14.6 1 0.5 1.20E03 1.49E04 7.46E04 3.71E03 5.0 2 1.5 2.72E04 1.32E05 1.15E04 9.97E04 8.7 0 Ruptures/ 3 3 1.13E04 4.93E06 4.54E05 4.17E04 9.2 3 Failures; Cold Leg 4 6.75 3.60E05 1.48E06 1.41E05 1.34E04 9.5 Cold Leg CRP Model 5 14 1.59E05 4.24E07 5.09E06 6.11E05 12.0 6 31.5 4.47E06 9.20E08 1.26E06 1.73E05 13.7

[2]

6D 44.5 2.68E06 4.86E08 7.10E07 1.04E05 14.6 1 0.5 1.89E02 2.36E03 1.20E02 5.81E02 5.0 2 1.5 6.09E03 1.38E04 1.91E03 2.46E02 13.3 0 Ruptures/

3 Failures; 3 3 2.92E03 4.66E05 7.56E04 1.18E02 15.9 Surge Line Surge Line 4 6.75 8.86E04 1.32E05 2.16E04 3.49E03 16.2 CRP Model 5 14 2.27E04 3.30E06 5.40E05 8.83E04 16.4

[3]

5D 19.8 1.18E04 1.71E06 2.80E05 4.58E04 16.4 1 0.5 1.07E02 5.59E03 1.00E02 1.79E02 1.8 0 Ruptures/ 2 1.5 2.88E03 5.17E04 2.05E03 7.97E03 3.9 14 Failures; CVCS Line 3 3 6.40E04 1.13E04 4.50E04 1.79E03 4.0 HPI CRP Model 4 6.75 9.68E05 1.03E05 5.66E05 3.11E04 5.5 5 14 2.27E05 2.42E06 1.33E05 7.31E05 5.5 63 KNF Consulting Services LLC

LOCA Frequencies for STP GSI191 Bayes Conditional Rupture Probability Distribution Parameters LOCA Break Component Update Category Size (in.) Mean 5%tile Median 95%tile RF[1]

Evidence 1 0.5 1.07E02 5.59E03 1.00E02 1.79E02 1.8 0 Ruptures/ 2 1.5 2.88E03 5.17E04 2.05E03 7.97E03 3.9 Safety Injection 14 Failures; Recirculation 3 3 6.40E04 1.13E04 4.50E04 1.79E03 4.0 HPI CRP (SIR) Lines Model 4 6.75 9.68E05 1.03E05 5.66E05 3.11E04 5.5 5 14 2.27E05 2.42E06 1.33E05 7.31E05 5.5 1 0.5 1.07E02 5.60E03 1.00E02 1.80E02 1.8 0 Ruptures/ 2 1.5 2.89E03 5.18E04 2.05E03 8.03E03 3.9 Pressurizer 12 Failures; 3 3 6.41E04 1.13E04 4.51E04 1.79E03 4.0 Lines HPI CRP Model 4 6.75 9.68E05 1.03E05 5.66E05 3.11E04 5.5 5 14 2.27E05 2.42E06 1.33E05 7.31E05 5.5 0 Ruptures/ 1 0.5 8.21E03 1.10E02 2.26E03 2.91E02 3.6 79 Failures; Small Bore 2 1.5 1.67E03 3.60E03 2.05E04 1.30E02 8 HPI CRP Model 3 3 4.57E04 1.02E03 5.53E05 3.72E03 8.2 Notes:

[1] Range Factor = SQRT(95%tile/5%tile).

[2] 6D corresponds to a doubleended break of a 31.5 pipe.

[3] 5D corresponds to a doubleended break of a 16 pipe.

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LOCA Frequencies for STP GSI191

5. LOCA Frequencies for STP GSI191 Application (Step 3)

This section documents the quantification of LOCA frequencies for input to the CASAGRANDE model for evaluation of debrisinduced failures of the recirculation cooling function and to the RISKMAN model for evaluation of the changes to core damage frequency and large earlyrelease frequency for the GSI191 application. In accordance with the stepbystep approach to LOCA frequency determination presented in Section 2, this section covers the following key tasks:

3. STPSpecific LOCA Frequency Development 3.1 Determine weld counts and pipe sizes for each component mi 3.2 Identify which locations are in and out of the NDE program 3.3 Combine the results of Step 1 and Step 2 for component LOCA frequencies 3.4 Apply Markov Model to specialize rupture frequencies (Iik) for NDE or no NDE 3.5 Provide locationbylocation LOCA frequencies vs. break size to CASAGRANDE - jx 3.6 Provide Small, Medium, and Large LOCA frequencies (F(LOCAx)) to RISKMAN 5.1 Weld Counts and Pipe Sizes for Each Component (Steps 3.1 and 3.2)

A detailed review of the piping system isometric diagrams was performed to establish the pipe sizes and weld counts for each of the component categories listed in Table 32. This review was done independently by the group at Alion that developed the CAD model of the STP LOCA sensitive piping systems and containment, and by another group at Scandpower that prepared a database of STP piping system components and supporting design information [15]. This database identifies which welds are being inspected in the NDE program both before and after the implementation of riskinformed in service inspection at STP.

5.2 Component LOCA Frequency Distributions (Step 3.3)

The LOCA frequencies for each component category were developed by combining the results for the failure rate uncertainty distributions developed in Step 1 and documented in Section 3, with the results for the conditional rupture probability distributions developed in Step 2 and documented in Section 4.

This was done using two methods: Method 1 is Monte Carlo simulation via Equation (2.2), and Method 2 is the use of formulas for computing the parameters of the arithmetic product of two lognormal distributions. The results of Method 2 are regarded as the official results, as these are not influenced by any Monte Carlo sampling uncertainty and are exact under the assumption that both the failure rate and CRP distributions are lognormal. In general, the results of Method 1 and 2 were in excellent agreement, which facilitated checking the results and debugging the spreadsheets (small differences in the second significant figure). The uncertainties in the frequency of LOCA vs. break size reflect the uncertainties in the failure rate estimation as well as in the CRP model estimates.

The component LOCA frequency vs. break size distributions for each of the 41 component categories are found in Tables 51 through 54. These tables have been customized to fit the various pipe sizes that are reflected in the component category definitions. The last entry in the table is the estimated frequency of a doubleended break of the pipe. The LOCA frequencies for the other entries are cumulative frequencies, i.e., frequencies of a break equal to or greater than the indicated break size.

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LOCA Frequencies for STP GSI191 In converting from LOCA category to break size in the CRP model, the frequencies for break sizes other than those indicated in Table 41 were developed using linear interpolation and extrapolation on a log frequency vs. log breaksize curve. This approach is justified by the trends of the frequencies vs. break size curves on a loglog plot being well behaved and showing limited curvature. The shape of these curves is driven by the assumptions underlying the CRP model.

The Monte Carlo calculations were carried out using Crystal Ball' Version 11.1.2.1.000 (32 bit) and Microsoft Excel Office Professional 2010 Version 14.0.6106.5005. Straight Monte Carlo rather than Latin Hypercube was used, with 100,000 trials. The CRP distributions derived from each of the CRP component categories (hot leg, cold leg, surge line, HPI line) were assumed to correlate fully, i.e., to have a correlation coefficient of +1.0. The Monte Carlo analysis for the failure rate development and LOCA frequency analysis were fully integrated rather than done in stages.

Plots of the LOCA frequencies vs. break size for hot leg components are shown in Figures 51, 52, and 5

3. The first two figures show the epistemic uncertainties for component categories 1A (BF welds in hot leg subject to stress corrosion cracking and design & construction defects) and 1C (BJ welds in hot leg susceptible to thermal fatigue and design & construction defects). As seen in these figures, the ratios between the 95th and 5th percentiles are two to three orders of magnitude, indicating great uncertainty.

In Figure 53, the mean LOCA frequencies for the three types of hot leg welds (BF, and BJ with and without thermal fatigue) are compared. There is significant variability in LOCA frequencies across these categories. The results for these cases are parallel because they use the same CRP model. Hence the variability is sourced to the variability in the failure rates, whose details were presented in Section 4.

The four BF welds in the pressurizer at STP (excluding the BF weld in the surge line at the pressurizer) have been repaired using weld overlays to address observed cracking. Prior to application of these weld overlays, these welds were in Categories 5F and 5G in Table 32. They are now assigned to Category 5H.

The LOCA frequency model used for these welds is to apply the pressurizer failure rate for design &

construction defects to the weld overlay itself, under the assumption that the underlying cracks and associated damage mechanisms have been adequately mitigated by the overlay.

5.3 Application of Markov Model to Address Impact of NDE Program (Step 3.4)

All the results presented to this point have included the effects of piping inspections and integrity management programs only implicitly. This is because the failure rate data and inputs from NUREG1829 that form the basis for our conditional probability of the LOCA model have been based on an analysis that has implicitly reflected the effects of the industry reliability integrity management (RIM) programs.

Such programs include testing and monitoring for leaks as well as nondestructive examinations that are performed in the various ISI programs on a periodic basis. Hence, the LOCA frequencies developed for STP component categories in Step 3.3 reflect an averaging of the effects of these RIM programs. For Class 1 welds, there is a variability in RIM because only a relatively small fraction of the weld population is subjected to NDE (approximately 10%), whereas all the Class 1 welds benefit from the same 100%

coverage of leak testing.

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LOCA Frequencies for STP GSI191 Table 51 LOCA Frequencies vs. Break Size for Hot Leg, SG Inlet, Cold Leg, and Surge Line Component Categories 1A through 4B Calc. Case 1A 1B 1C 2 3A 3B 3C 3D 4A 4B 4C 4D System Hot Leg Hot Leg Hot Leg SG Inlet Cold Leg Cold Leg Cold Leg Cold Leg Surge Line Surge Line Surge Line Surge Line Size Case (in.) 29 29 29 29 27.5 31 27.5 31 16 16 16 2.5 DEGB (in.) 41.01 41.01 41.01 41.01 38.89 43.84 38.89 43.84 22.63 22.63 22.63 3.54 Weld Type BF BJ BJ BF BF BF BJ BJ BF BJ BC BJ DM SC, D&C D&C TF, D&C SC, D&C SC, D&C SC, D&C D&C D&C SC, TF, D&C TF, D&C TF, D&C TF, D&C No. Welds 4 11 1 4 4 4 12 24 1 7 2 6 X, Break Size X, Break Size X, Break Size X, Break Size X, Break Size X, Break Size X, Break X, Break X, Break X, Break X, Break X, Break (in.) F(LOCA X) (in.) F(LOCA X) (in.) F(LOCA X) (in.) F(LOCA X) (in.) F(LOCA X) (in.) F(LOCA X) Size (in.) F(LOCA X) Size (in.) F(LOCA X) Size (in.) F(LOCA X) Size (in.) F(LOCA X) Size (in.) F(LOCA X) Size (in.)

0.50 4.02E07 0.50 1.95E09 0.50 1.25E08 0.50 1.98E06 0.50 1.51E07 0.50 1.51E07 0.50 2.79E09 0.50 2.79E09 0.50 9.75E06 0.50 7.44E08 0.50 1.21E07 0.50 7.44E08 1.50 9.25E08 1.50 4.49E10 1.50 2.87E09 1.50 4.59E07 1.50 3.43E08 1.50 3.43E08 1.50 6.33E10 1.50 6.33E10 1.50 3.30E06 1.50 2.52E08 1.50 4.11E08 1.50 2.52E08 2.00 6.92E08 2.00 3.36E10 2.00 2.15E09 2.00 3.45E07 2.00 2.38E08 2.00 2.38E08 2.00 4.39E10 2.00 4.39E10 2.00 2.43E06 2.00 1.85E08 2.00 3.02E08 2.00 1.85E08 3.00 4.61E08 3.00 2.24E10 3.00 1.43E09 3.00 2.31E07 3.00 1.42E08 3.00 1.42E08 3.00 2.62E10 3.00 2.62E10 3.00 1.58E06 3.00 1.20E08 3.00 1.97E08 3.00 1.20E08 4.00 3.19E08 4.00 1.55E10 4.00 9.90E10 4.00 1.60E07 4.00 9.49E09 4.00 9.49E09 4.00 1.75E10 4.00 1.75E10 4.00 1.03E06 4.00 7.82E09 4.00 1.28E08 3.54 9.42E09 6.00 1.89E08 6.00 9.19E11 6.00 5.89E10 6.00 9.52E08 6.00 5.39E09 6.00 5.39E09 6.00 9.95E11 6.00 9.95E11 6.00 5.58E07 6.00 4.26E09 6.00 6.94E09 6.75 1.61E08 6.75 7.83E11 6.75 5.01E10 6.75 8.12E08 6.75 4.53E09 6.75 4.53E09 6.75 8.36E11 6.75 8.36E11 6.75 4.68E07 6.75 3.57E09 6.75 5.82E09 14.00 7.01E09 14.00 3.40E11 14.00 2.18E10 14.00 3.35E08 14.00 2.01E09 14.00 2.01E09 14.00 3.70E11 14.00 3.70E11 14.00 1.18E07 14.00 9.03E10 14.00 1.47E09 20.00 3.70E09 20.00 1.80E11 20.00 1.15E10 20.00 1.81E08 20.00 1.15E09 20.00 1.15E09 20.00 2.11E11 20.00 2.11E11 16.00 9.19E08 16.00 7.02E10 16.00 1.15E09 29.00 1.90E09 29.00 9.24E12 29.00 5.92E11 29.00 9.57E09 27.50 6.96E10 27.50 6.96E10 27.50 1.28E11 27.50 1.28E11 20.00 6.14E08 20.00 4.69E10 20.00 7.65E10 31.50 1.64E09 31.50 7.97E12 31.50 5.11E11 31.50 8.30E09 31.50 5.63E10 31.50 5.63E10 31.50 1.04E11 31.50 1.04E11 22.63 4.77E08 22.63 3.64E10 22.63 5.93E10 41.01 1.04E09 41.01 5.03E12 41.01 3.22E11 41.01 5.24E09 38.89 4.12E10 43.80 3.38E10 38.89 7.60E12 43.80 6.23E12 Table 52 LOCA Frequencies vs. Break Size for Pressurizer and Small Bore Component Categories 5A through 6B Calc. Case 5A 5B 5C 5D 5E 5F 5G 5H 5I 5J 6A 6B System Pressurizer Pressurizer Pressurizer Pressurizer Pressurizer Pressurizer Pressurizer Pressurizer Pressurizer Pressurizer Small Bore Small Bore Size Case (in.) 6 3 4 3 6 6 6 6 4 2 2 1 DEGB (in.) 8.49 4.24 5.66 4.24 8.49 8.49 8.49 8.49 5.66 2.83 2.83 1.41 Weld Type BJ BJ BJ BJ BJ BF BF BF BC BJ BJ BJ DM TF, D&C TF, D&C D&C D&C D&C SC, TF, D&C SC, D&C D&C (Weld Overlay) D&C TF, D&C VF, SC, D&C VF, SC, D&C No. Welds 29 14 53 4 29 0 0 4 2 2 16 193 X, Break X, Break X, Break X, Break X, Break X, Break X, Break X, Break X, Break X, Break X, Break Size (in.) F(LOCA X) Size (in.) F(LOCA X) Size (in.) F(LOCA X) Size (in.) F(LOCA X) Size (in.) F(LOCA X) Size (in.) F(LOCA X) Size (in.) F(LOCA X) Size (in.) F(LOCA X) Size (in.) F(LOCA X) Size (in.) F(LOCA X) Size (in.) F(LOCA X) 0.50 4.59E08 0.50 4.59E08 0.50 1.72E08 0.50 1.72E08 0.50 1.72E08 0.50 5.09E06 0.50 5.01E06 0.50 1.74E08 0.50 1.72E08 0.50 4.59E08 0.5 1.22E06 0.5 1.22E06 0.75 2.76E08 0.75 2.76E08 0.75 1.03E08 0.75 1.03E08 0.75 1.03E08 0.75 3.06E06 0.75 3.01E06 0.75 1.05E08 0.75 1.03E08 0.75 2.76E08 0.75 7.18E07 0.75 7.18E07 1.00 1.96E08 1.00 1.96E08 1.00 7.33E09 1.00 7.33E09 1.00 7.33E09 1.00 2.17E06 1.00 2.13E06 1.00 7.42E09 1.00 7.33E09 1.00 1.96E08 1 5.00E07 1 5.00E07 1.50 1.24E08 1.50 1.24E08 1.50 4.64E09 1.50 4.64E09 1.50 4.64E09 1.50 1.38E06 1.50 1.35E06 1.50 4.70E09 1.50 4.64E09 1.50 1.24E08 1.4 3.30E07 1.4 3.30E07 2.00 6.64E09 2.00 6.64E09 2.00 2.49E09 2.00 2.49E09 2.00 2.49E09 2.00 7.36E07 2.00 7.24E07 2.00 2.52E09 2.00 2.49E09 2.00 6.64E09 1.5 3.08E07 3.00 2.75E09 3.00 2.75E09 3.00 1.03E09 3.00 1.03E09 3.00 1.03E09 3.00 3.05E07 3.00 3.00E07 3.00 1.04E09 3.00 1.03E09 2.83 3.13E09 1.99 1.75E07 4.24 1.30E09 4.24 1.30E09 4.24 4.87E10 4.24 4.87E10 4.24 4.87E10 4.24 1.44E07 4.24 1.42E07 4.24 4.94E10 4.24 4.87E10 2.0 1.73E07 5.66 6.26E10 5.66 2.34E10 5.66 2.34E10 5.66 6.94E08 5.66 6.83E08 5.66 2.37E10 5.66 2.34E10 2.8 8.66E08 6.00 5.47E10 6.00 2.05E10 6.00 6.06E08 6.00 5.96E08 6.00 2.07E10 6.75 4.16E10 6.75 1.56E10 6.75 4.61E08 6.75 4.54E08 6.75 1.58E10 8.49 2.64E10 8.49 9.89E11 8.49 2.93E08 8.49 2.88E08 8.49 1.00E10 67 KNF Consulting Services LLC

LOCA Frequencies for STP GSI191 Table 53 LOCA Frequencies vs. Break Size for Safety Injection and Recirculation System Categories 7A through 7L Calc. Case 7A 7B 7C 7D 7E 7F 7G 7H 7I 7J 7K 7L System SIR SIR SIR SIR SIR SIR SIR SIR SIR SIR SIR SIR Size Case (in.) 12 8 8 12 12 10 8 6 4 3 2 1.5 DEGB (in.) 16.97 11.31 11.31 16.97 16.97 14.14 11.31 8.49 5.66 4.24 2.83 2.12 Weld Type BJ BJ BJ BJ BC, BJ BJ BC, BJ BJ BC BC BC BJ DM TF, D&C TF, D&C SC, TF, D&C SC, D&C D&C D&C D&C D&C D&C D&C D&C D&C No. Welds 21 9 3 3 57 30 42 23 5 9 10 0 X, Break X, Break X, Break X, Break X, Break X, Break X, Break X, Break X, Break X, Break X, Break X, Break Size (in.) F(LOCA X) Size (in.) F(LOCA X) Size (in.) F(LOCA X) Size (in.) Size (in.) F(LOCA X) Size (in.) F(LOCA X) Size (in.) F(LOCA X) Size (in.) F(LOCA X) Size (in.) F(LOCA X) Size (in.) F(LOCA X) Size (in.) F(LOCA X) Size (in.) F(LOCA X) 0.50 2.78E06 0.50 2.78E06 0.50 3.10E06 0.50 3.54E07 0.50 1.14E08 0.50 1.14E08 0.50 1.14E08 0.50 1.14E08 0.50 1.14E08 0.50 1.14E08 0.50 1.14E08 0.50 1.14E08 0.75 1.67E06 0.75 1.67E06 0.75 1.86E06 0.75 2.12E07 0.75 6.84E09 0.75 6.84E09 0.75 6.84E09 0.75 6.84E09 0.75 6.84E09 0.75 6.84E09 0.75 6.84E09 0.75 6.84E09 1.00 1.18E06 1.00 1.18E06 1.00 1.32E06 1.00 1.51E07 1.00 4.85E09 1.00 4.85E09 1.00 4.85E09 1.00 4.85E09 1.00 4.85E09 1.00 4.85E09 1.00 4.85E09 1.00 4.85E09 1.50 7.48E07 1.50 7.48E07 1.50 8.34E07 1.50 9.54E08 1.50 3.07E09 1.50 3.07E09 1.50 3.07E09 1.50 3.07E09 1.50 3.07E09 1.50 3.07E09 1.50 3.07E09 1.50 3.07E09 2.00 4.01E07 2.00 4.01E07 2.00 4.48E07 2.00 5.12E08 2.00 1.65E09 2.00 1.65E09 2.00 1.65E09 2.00 1.65E09 2.00 1.65E09 2.00 1.65E09 2.00 1.65E09 2.00 1.65E09 2.83 1.67E07 2.83 1.67E07 2.83 1.86E07 2.83 2.13E08 2.83 6.85E10 2.83 6.85E10 2.83 6.85E10 2.83 6.85E10 2.83 6.85E10 2.83 6.85E10 2.83 6.85E10 4.00 8.50E08 4.00 8.50E08 4.00 9.48E08 4.00 1.08E08 4.00 3.49E10 4.00 3.49E10 4.00 3.49E10 4.00 3.49E10 4.00 3.49E10 4.00 3.49E10 4.24 7.41E08 4.24 7.41E08 4.24 8.26E08 4.24 9.45E09 4.24 3.04E10 4.24 3.04E10 4.24 3.04E10 4.24 3.04E10 4.24 3.04E10 4.24 3.04E10 5.66 3.79E08 5.66 3.79E08 5.66 4.23E08 5.66 4.84E09 5.66 1.56E10 5.66 1.56E10 5.66 1.56E10 5.66 1.56E10 5.66 1.56E10 6.00 3.31E08 6.00 3.31E08 6.00 3.70E08 6.00 4.23E09 6.00 1.36E10 6.00 1.36E10 6.00 1.36E10 6.00 1.36E10 6.75 2.52E08 6.75 2.52E08 6.75 2.81E08 6.75 3.22E09 6.75 1.04E10 6.75 1.04E10 6.75 1.04E10 6.75 1.04E10 7.20 2.22E08 7.20 2.22E08 7.20 2.48E08 7.20 2.83E09 7.20 9.12E11 7.20 9.12E11 7.20 9.12E11 7.20 9.12E11 8.49 1.60E08 8.49 1.60E08 8.49 1.79E08 8.49 2.04E09 8.49 6.58E11 8.49 6.58E11 8.49 6.58E11 8.49 6.58E11 10.00 1.16E08 10.00 1.16E08 10.00 1.29E08 10.00 1.47E09 10.00 4.75E11 10.00 4.75E11 10.00 4.75E11 11.31 9.11E09 11.31 9.11E09 11.31 1.02E08 11.31 1.16E09 11.31 3.74E11 11.31 3.74E11 11.31 3.74E11 14.14 5.93E09 14.14 7.56E10 14.14 2.44E11 14.14 2.44E11 16.97 4.05E09 16.97 5.16E10 16.97 1.66E11 Table 54 LOCA Frequencies vs. Break Size for Accumulator Injection and CVCS Categories 7M through 8F Calc. Case 7M 7N 7O 8A 8B 8C 8D 8E 8F System ACC ACC ACC CVCS CVCS CVCS CVCS CVCS CVCS Size Case (in.) 12 12 12 2 4 2 4 4 4 DEGB (in.) 16.97 16.97 16.97 2.83 5.66 2.83 5.66 5.66 5.66 Weld Type BJ BJ BC, BJ BJ BJ BJ BJ BC BC DM SC, D&C TF, D&C D&C TF, VF, D&C TF, VF, D&C VF, D&C VF, D&C TF, D&C D&C No. Welds 0 35 15 10 19 47 6 4 1 X, Break X, Break X, Break X, Break X, Break X, Break X, Break X, Break X, Break Size (in.) F(LOCA X) Size (in.) F(LOCA X) Size (in.) F(LOCA X) Size (in.) F(LOCA X) Size (in.) F(LOCA X) Size (in.) F(LOCA X) Size (in.) F(LOCA X) Size (in.) F(LOCA X) Size (in.) F(LOCA X) 0.50 3.54E07 0.50 5.18E08 0.50 6.26E09 0.50 4.28E08 0.50 4.28E08 0.50 1.87E08 0.50 1.87E08 0.50 7.98E08 0.50 1.87E08 0.75 2.12E07 0.75 3.11E08 0.75 3.75E09 0.75 2.57E08 0.75 2.57E08 0.75 1.12E08 0.75 1.12E08 0.75 4.79E08 0.75 1.12E08 1.00 1.51E07 1.00 2.21E08 1.00 2.66E09 1.00 1.82E08 1.00 1.82E08 1.00 7.97E09 1.00 7.97E09 1.00 3.40E08 1.00 7.97E09 1.50 9.54E08 1.50 1.40E08 1.50 1.69E09 1.50 1.15E08 1.50 1.15E08 1.50 5.04E09 1.50 5.04E09 1.50 2.15E08 1.50 5.04E09 2.00 5.12E08 2.00 7.49E09 2.00 9.04E10 2.00 6.03E09 2.00 6.03E09 2.00 2.64E09 2.00 2.64E09 2.00 1.12E08 2.00 2.64E09 2.83 2.13E08 2.83 3.12E09 2.83 3.76E10 3.00 2.42E09 3.00 2.42E09 3.00 1.06E09 3.00 1.06E09 3.00 4.51E09 3.00 1.06E09 4.00 1.08E08 4.00 1.67E09 4.00 2.02E10 4.00 1.26E09 4.00 5.49E10 4.00 2.34E09 4.00 5.49E10 4.24 9.45E09 5.66 7.09E10 5.66 8.55E11 5.66 5.77E10 5.66 2.52E10 5.66 1.08E09 5.66 2.52E10 5.66 4.84E09 6.00 6.19E10 6.00 7.47E11 6.00 4.23E09 6.80 4.71E10 6.80 5.69E11 6.75 3.22E09 7.20 4.14E10 7.20 5.00E11 7.20 2.83E09 10.00 2.16E10 10.00 2.61E11 8.49 2.04E09 14.14 1.11E10 14.14 1.34E11 10.00 1.47E09 16.97 7.56E11 16.97 9.12E12 11.31 1.16E09 14.14 7.56E10 16.97 5.16E10 68 KNF Consulting Services LLC

LOCA Frequencies for STP GSI191 Figure 51 LOCA Frequencies vs. Break Size for BF Welds in Hot Leg (Category 1A)

Figure 52 LOCA Frequencies vs. Break Size for BJ Welds in Hot Leg Subject to Thermal Fatigue (Category 1C) 69 KNF Consulting Services LLC

LOCA Frequencies for STP GSI191 Figure 53 Comparison of Mean Frequencies for Hot Leg Welds An example of the kind of change in LOCA frequencies that can result from locationbylocation changes in the pipe inspection and leak monitoring program is shown in Figure 54 for an RCS weld subject to stress corrosion cracking [13]. As seen in this figure, the frequency of a pipe break may vary by more than an order of magnitude based on the reliability integrity management program, all other factors being equal.

The application of the Markov model to STP components is deferred until more information is available to identify which locations are risksignificant with respect to debris formation. Because the number of input parameters needed to quantify the Markov model is significant, it is impractical to apply that model to all 41 unique component categories at STP. The analysis presented in Figure 54 would be representative of BF welds in the large bore pipes, such as Categories 1A, 2, 3A, and 3B. When the Markov model is applied to STP components, the LOCA frequencies for those welds not subjected to NDE will be increased by a small and not significant amount, and the LOCA frequencies for those subjected to NDE will be decreased by factors ranging from 3 to 10. This will also provide an opportunity to modify the selections of welds for the NDE program to offset significant risk impacts that are associated with debrisinduced failure of recirculation cooling. Because Step 3.4 in the LOCA frequency procedure is deferred, the results to be used in the 2011 GSI191 evaluation will not reflect weld to weld variations due to the welds included and excluded from the NDE program.

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LOCA Frequencies for STP GSI191 Figure 54 Comparison of Weld Failure Rates Determined by Markov Model for Different Reliability Integrity Management Approaches 5.4 Total LOCA Frequencies for RISKMAN (Step 3.6)

The total LOCA frequencies were calculated using Equation (2.1) by multiplying the number of components in each category by the LOCA frequencies per category from Step 3.3. This was done using two methods. Method 1 is a mean point estimate in which the means of the failure rate, means of the CRP model distributions, and weld counts were multiplied on an Excel spreadsheet. Method 2 was an integrated Monte Carlo simulation that included the steps in the failure rate development, application of the CRP lognormal distributions, and weld counts. As noted earlier, the CRP distributions within a CRP component category were treated as fully correlated in the Monte Carlo calculations. The results are summarized in Table 55.

In Figure 55, the STP mean pipeinduced LOCA frequencies are compared against the results from NUREG1829 for pipeinduced LOCAs. As seen in this comparison, the results are in excellent agreement for Categories 1 and 4 and within a factor of 3 of each other over the whole range of LOCA categories.

For Categories 2 and 3, the STP results track somewhat lower, whereas for Categories 5 and 6, the STP results track somewhat higher. To conduct a sensitivity study, a case is plotted with the contributions from the SG inlet removed to investigate the impact of this outlier component that was observed in the failure data. There was an unusually high incidence of failures at this weld location (19 failures vs. 6 71 KNF Consulting Services LLC

LOCA Frequencies for STP GSI191 failures for the entire remaining hot leg welds in the database), all occurring in Japanese PWRs following steam generator replacement. When these outlier contributions are removed, the Category 5 and 6 results from STP and NUREG1829 are in excellent agreement.

Figure 55 is based on mean values, whereas Figure 56 compares the uncertainty distribution results, with the caveat that the STP results are for pipeinduced LOCAs and that the NUREG1829 data in this figure include both pipe and nonpipeinduced LOCAs. While there is information in NUREG1829 that breaks down pipe and nonpipe contributions, which is used in Figure 55, there is no information on uncertainty distributions for the pipeonly contributions. However, it is reasonable that the uncertainties calculated for STP are somewhat smaller than those estimated in NUREG1829, given that the STP results are for a specific plant and NUREG1829 reflects the uncertainty and variability for entire fleet of US PWR plants.

Figures 57 and 58 present the major contributions to LOCA frequency by system, using a logarithmic scale on the Yaxis. A linear perspective (i.e. not with the distortion of logarithmic scales) on the contributions to Category 6 LOCA frequencies is provided in Figure 59, which shows that the SG Inlet B F welds contribute about 74% to the total Category 6 LOCA frequency.

When making comparisons with NUREG1829, the following differences between that and the present study should be taken into account:

NUREG1829 results are from a generic study for the population of PWRs in the U.S. This includes 2loop, 3loop, and 4loop PWR plants, almost all of which have only two trains of ECCSs connected to the loop piping. The base case analyses that were performed in NUREG 1829 that were available for use as anchors for the expert elicitation were for a 3loop PWR plant. This document's results are specific to STP, a 4loop PWR plant with interfacing piping for three trains of ECCSs.

NUREG1829 results are produced from expert elicitation. The STP results have utilized NUREG 1829 information to develop the CRP distributions, but have been calculated using a different methodology and based on generic pipe failure information from the PIPExp database and from STPspecific weld counts, pipe sizes, and damage mechanisms.

Given the differences, it is interesting that the results are so comparable in magnitude. That the total LOCA frequencies calculated for STP are comparable to the NUREG1829 results provides a sanity check on the methodology used in this study and its application to STP. More specifically this comparison shows that assumptions made in using NUREG1829 data to develop the CRP distributions, in combination of the failure rate treatment and LOCA frequency methodology, have produced a set of results that do not differ appreciably from the pipe induced LOCA frequencies in NUREG1829.

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LOCA Frequencies for STP GSI191 Table 55 Results for Total Pipe BreakInduced LOCA Frequencies LOCA Break Point LOCA Frequency per ReactorCalendar Year Range Category[1] Size (in.) Estimate[2] Mean 5%tile 50%tile 95%tile Factor[3]

Small LOCA 0.5 to 2.0 3.59E04 3.54E04 1.42E04 3.11E04 7.03E04 2.2 Medium LOCA 2.0 to 6.0 2.01E05 2.00E05 1.44E06 1.14E05 6.53E05 6.7 Large LOCA > 6.0 2.29E06 2.09E06 1.80E07 9.53E07 7.18E06 6.3 Category 1 0.5 3.82E04 3.76E04 1.57E04 3.30E04 7.39E04 2.2 Category 2 1.5 3.91E05 3.90E05 7.00E06 2.37E05 1.18E04 4.1 Category 3 3 9.24E06 9.09E06 1.07E06 5.04E06 2.94E05 5.2 Category 4 6.75 1.84E06 1.82E06 2.00E07 9.69E07 5.83E06 5.4 Category 5 14 4.40E07 4.31E07 4.45E08 2.25E07 1.39E06 5.6 Category 6 0.5 4.48E08 4.50E08 1.61E09 1.44E08 1.65E07 10.1 Notes:

[1] Small, Medium, and Large LOCA categories consistent with STP PRA model; Categories 16 defined in NUREG1829 (see Table 41).

[2] Point estimate obtained with mean failure rate and CRP lognormal distributions and weld counts.

[3] Range Factor = SQRT(95%tile/5%tile).

Figure 55 Comparison of LOCA Frequencies for Pipes: STP vs. NUREG1829 73 KNF Consulting Services LLC

LOCA Frequencies for STP GSI191 Figure 56 Comparison of Uncertainty Distributions for STP PipeInduced LOCA and NUREG1829 Total LOCA Frequencies Figure 57 Contributions to Mean LOCA Category Frequencies by System 74 KNF Consulting Services LLC

LOCA Frequencies for STP GSI191 Figure 58 System Contributions to Mean LOCA Initiating Event Frequencies Figure 59 System Contribution to LOCA Category 6 Frequencies 75 KNF Consulting Services LLC

LOCA Frequencies for STP GSI191 5.5 LOCA Frequency Summary The technical approach to estimation of LOCA frequencies for the STP GSI191 project has been described in section 5, with results for each step. The specific capabilities that have been demonstrated include:

The capability to estimate LOCA frequencies as a function of break size at each location.

The capability to utilize information from NUREG1829 to characterize epistemic uncertainty associated with LOCA frequencies.

A method that incorporates via Bayes uncertainty analysis the service data on pipe failures and component exposures.

A quantification of epistemic uncertainties associated with estimating the input parameters in the model equations, including both parametric and modeling sources of uncertainty.

The capability to quantify the impacts of information on degradation mechanism susceptibility at each location, based on insights from service data and results of RIISI evaluation.

The results that have been generated for LOCAspecific as well as total LOCA frequencies are reasonable and consistent with those developed in previous studies.

Prior to completion of the LOCA frequency task for GSI191, the following issues need to be and will be addressed in a future update of this report.

Nonisolatable LOCAs caused by failures of nonpipe components need to be addressed. These include control rod drive standpipes, instrument lines, and other components welded to the reactor pressure vessel, pump and valve bodies, pressurizer safety and relief valve leaks, and reactor coolant pump seals.

Isolatable LOCAs need to be addressed. These involve failures in Class 2 piping systems that can be isolated, including CVCS charging and letdown lines, RCP seal return lines, etc.

Pipe breaks in steam and feedwater lines inside the containment that could generate debris and lead to a need for recirculation cooling and/or containment spray actuation need to be addressed.

Execution of Step 3.4 to apply the Markov model to evaluate the impact of inspected and non inspected NDE locations on the LOCA frequencies needs to be completed.

The current study is based on rough estimates of weld counts and pipe sizes for small bore pipes. If small bore pipes are found to contribute significantly to the risk of debrisinduced ECCS failures, more detailed review of the small bore piping configurations needs to be completed.

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LOCA Frequencies for STP GSI191

6. References

[1] Tregoning, R., L. Abramson, and P. Scott, Estimating LossofCoolant Accident (LOCA)

Frequencies through the Elicitation Process, NUREG1829, U.S. Nuclear Regulatory Commission, Washington, DC, April 2008.

[2] Lydell, B. O. Y., PIPExp/PIPE2010: Monthly Summary of Database Content (Status as of 31 July2011), SigmaPhase Inc., Vail AZ. Monthly summary reports have been issued since January 1999.

[3] Fleming, K. N. and B. O. Y. Lydell, Database Development and Uncertainty Treatment for Estimating Pipe Failure Rates and Rupture Frequencies, Reliability Engineering and System Safety, 86: 227-246, 2004.

[4] Fleming, K. N. and B. O. Y. Lydell, Pipe Rupture Frequencies for Internal Flooding PRAs, Revision 1. EPRI, Palo Alto, CA: 2006. 1013141.

[5] Fleming, K. N. and B. O. Y. Lydell, Pipe Rupture Frequencies for Internal Flooding PRAs, Revision 2. EPRI, Palo Alto, CA: 2010. 1021086.

[6] Fleming, K. N. et al., Piping System Reliability and Failure Rate Estimation Models for Use in RiskInformed InService Inspection Applications, EPRI, Palo Alto, CA: 1998. TR110161.

[7] Fleming, K. N. and T.J. Mikschl, Piping System Failure Rates and Rupture Frequencies for Use in RiskInformed InService Inspection Applications, EPRI, Palo Alto, CA: 1999. TR111880.

[8] Mosleh, A. and F. Groen, Technical Review of the Methodology of EPRI TR110161, University of Maryland report for EPRI, published as an Appendix to EPRI TR110161 (Reference [6]).

[9] Electric Power Research Institute, Revised RiskInformed InService Inspection Procedure, EPRI, Palo Alto, CA: 1999. TR112657, Rev. BA.

[10] U.S. Nuclear Regulatory Commission, Safety Evaluation Report Related to Revised Risk Informed InService Inspection Evaluation Procedure: EPRI TR112657, Rev. B, July 1999, Washington, DC, 1999. Published as a forward to TR112657 (Reference [9]).

[11] Martz, H., Final (Revised) Review of the EPRIProposed Markov Modeling/Bayesian Updating Methodology for Use in RiskInformed InService Inspection of Piping in Commercial Nuclear Power Plants, Los Alamos National Laboratory, June 1999. TSA1/99164.

[12] Fleming, K. N., Markov Models for Evaluating RiskInformed InService Inspection Strategies for Nuclear Power Plant Piping Systems, Reliability Engineering and System Safety, 83(1): 27-45, 2004.

[13] Fleming, K. N. et al., Treatment of Passive Component Reliability in RiskInformed Safety Margin Characterization - Fiscal Year 2010 Status Report, INL/EXT1020013, report prepared by Pacific Northwest National Laboratory for the U.S. Department of Energy, September 2010.

[14] U.S. Nuclear Regulatory Commission, Supporting information for NUREG1829 (Reference [1])

on Individual Experts Estimates of LOCA Frequencies for Specific Components and LOCA Categories, available on ADAMS Accession Numbers ML080560008, ML080560010, ML080560011, ML080560013.

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LOCA Frequencies for STP GSI191

[15] Scandpower Risk Management, Inc., GSI191 Component Database, Microsoft Access Database of South Texas Project Components considered in LOCA Frequency Analysis, September 16, 2011.

[16] Andresen, P.L. et al, 2007. Expert Panel Report on Proactive Materials Degradation Assessment, NUREG/CR6923, U.S. Nuclear Regulatory Commission, Washington (DC).

[17] OECD Nuclear Energy Agency, 2011. Technical Basis for Commendable Practices on Ageing Management - SCC and Cable Ageing Project (SCAP), NEA/CSNI/R(2010)5, IssylesMoulineaux (France).

[18] Scott, P.M., 2010. Primary Water Stress Corrosion Cracking of Nickelbase Alloys, Technical Report Prepared for the SCAPSCC Working Group, Noisy le Roi (France).

[19] OECD Nuclear Energy Agency, 1998. Proc. Specialists Meeting on Experience with Thermal Fatigue in LWR Piping Caused by Mixing and Stratification, NEA/CSNI/R(1998)8, Issyles Moulineaux (France).

[20] Lydell, B. and Olsson, A., 2008. Reliability Data for Piping Components in Nordic Nuclear Power Plants. "RBook" Project Phase I, SKI Report 2008:1, Swedish nuclear Power Inspectorate, Stockholm (Sweden).

[21] Letter, dated December 30, 1999, as supplemented April 17, 2000, T. J. Jordan (South Texas Project, Units 1 and 2, Manager, Systems Engineering), to U.S. Nuclear Regulatory Commission, containing RiskInformed Inservice Inspection Program Plan South Texas Project Units 1 and 2.

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