ML112350883

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LOCA Initiating Event Frequencies and Uncertainties Status Report
ML112350883
Person / Time
Site: South Texas  STP Nuclear Operating Company icon.png
Issue date: 08/22/2011
From: Kreslyon Fleming, Lydell B
KNF Consulting Services, Scandpower
To:
Office of Nuclear Reactor Regulation
Singal, B K, NRR/DORL, 301-415-301
Shared Package
ML112350857 List:
References
TAC ME5358, GSI-191, TAC ME5359
Download: ML112350883 (19)
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LOCA Initiating Event Frequencies and Uncertainties Status Report Risk Informed GSI-191 Resolution Monday, August 22, 2011 8:00 am - 5:00 p.m EDT Public Meeting with STP Nuclear Operating Company Karl N. Fleming KNF Consulting Services LLC Bengt O. Y. Lydell 8/22/11 Pre-Licensing Meeting 1

Risk Informed GSI-191 Discussion Topics

  • Current Status
  • Refinements of Approach for Conditional Rupture Probabilities
  • Preliminary Results
  • Independent Review by Ali Mosleh
  • Issues to Complete 8/22/11 Pre-Licensing Meeting 2

Risk Informed GSI-191 LOCA Frequencies Objectives

  • Incorporate insights from previous work on LOCA frequencies

- Specific components, materials, dimensions

- Specific locations

- Range of break sizes

- Damage / Degradation mechanisms and mitigation effectiveness

- Other break characteristics, e.g. speed

  • Quantify both aleatory and epistemic uncertainties; augment with sensitivity studies
  • Support interfaces with other parts of the GSI-191 evaluation

- LOCA initiating event frequencies for PRA modeling

- Break characterization for evaluation of debris formation

  • Participate in NRC workshops 3 8/22/11 Pre-Licensing Meeting

Current Status

  • Defined homogenous pipe failure rate categories
  • Refined method for deriving conditional rupture probabilities vs. break size
  • Obtained preliminary results for each pipe category
  • Obtained preliminary results for total LOCA frequencies from pipe failures
  • Independent reviews by MIT and Ali Mosleh in progress 8/22/11 Pre-Licensing Meeting 4

Homogeneous Pipe Failure Rate Cases 8/22/11 Pre-Licensing Meeting 5

Step by Step Procedure

1. Determination of weld types (i)
2. Perform data query for failure counts (n)
3. Estimate component exposure (T) and uncertainty
4. Develop component failure rate prior distributions for each DM
5. Perform Bayes update for each exposure case (combination of weld count and DM susceptibility)
6. Apply posterior weighting to combine results for different hypothesis yield conditional failure rate distributions; compute unconditional failure rates for locations with uncertain DM status
7. Develop conditional probability of rupture size given failure probabilities for each weld type and associated epistemic uncertainties
8. Combine the results of Step 6 and Step 7 by Monte Carlo in Eq. (1) for component LOCA frequencies and total LOCA frequencies for each component
9. Apply Markov Model to specialize rupture frequencies for differences in integrity management
10. For intermediate LOCA categories and break sizes, interpolate the results of Step 10 via log-log linear interpolation
11. Calculate total LOCA frequencies from all components and reconcile differences with earlier LOCA frequency estimates 8/22/11 Pre-Licensing Meeting 6

Step 7 Conditional Probability of Pipe Rupture

  • Step 7.1 Benchmark of Lydells Base Case LOCA frequencies for PWR hot leg, surge line, and HPI line
  • Step 7.2 Compare results of individual expert elicitation LOCA Frequencies from NUREG-1829 to base case
  • Step 7.3 Set Target LOCA frequencies that encompass elicitation results (method revised since July meeting)
  • Step 7.4 Derive conditional rupture probability distributions that when combined with Lydell failure rate estimates match the target LOCA frequencies
  • Step 7.5 Perform Bayes updates that incorporate evidence on pipe failures without LOCAs 8/22/11 Pre-Licensing Meeting 7

Selection of Target LOCA Frequencies

- Input from 9 experts for component level LOCA frequencies at different plant ages; using data for 40 years

- Bengt Lydell base case results in Appendix D

  • Evaluated alternative approaches to aggregating into composite distributions for different components

- Mixture distribution of NUREG-1829 data - rejected

- Geometric mean distribution of NUREG-1829 data

  • Geometric means of 50th percentiles and range factors - accepted
  • Geometric means of input 5%tiles and 95%tiles - rejected

- Hybrid of geometric means and Lydell base

  • Use of worst case 5% tiles and 95%tiles (Method described in July meeting)
  • Mixture distribution of geometric mean and Lydell (Method recommended by Dr.

Mosleh)

  • Performing sensitivity studies on alternative models 8/22/11 Pre-Licensing Meeting 8

Comparison of Geometric Mean and Mixture Distributions from NUREG-1829 Data 8/22/11 Pre-Licensing Meeting 9

Use of Worst-Case Percentiles from NUREG-1829 GM and Lydell Base Case 8/22/11 Pre-Licensing Meeting 10

Selected Approach for Target LOCA Frequencies

  • Probabilistic mixture of two models

- Model 1 Geometric mean of 9 expert distributions

  • Develop 40 year composite distribution of 9 experts using geometric mean method
  • Combined lognormal distribution for Current day and 40yr multipliers for each expert preserving median and range factors
  • Developed composite distribution based on geometric means of each experts medians and range factors

- Model 2 Bengt Lydell Base Case analysis

- Results of Models 1 and 2 combined giving equal weight to each yielding a mixture distribution of the two models

- This method produces somewhat greater uncertainties than using Model 1 by itself 8/22/11 Pre-Licensing Meeting 11

Comparison of Hybrid Methods STP Hot Leg Target LOCA Model Worst Case 5%tile and 95%tile LOCA Cat. Break Size Mean 5%tile 50%tile 95%tile RF 1 0.5 5.79E07 3.55E09 8.72E08 2.14E06 24.6 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 4 6.76 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 STP Hot Leg Target LOCA Model Probabilistic Mixture 1 0.5 5.08E07 5.30E09 1.05E07 1.91E06 19.0 2 1.5 9.32E08 3.91E10 1.46E08 3.68E07 30.7 3 3 4.54E08 1.60E10 6.39E09 1.76E07 33.1 4 6.76 1.64E08 5.73E11 2.05E09 6.32E08 33.2 5 14 8.37E09 2.03E11 7.64E10 2.92E08 37.9 6 31.5 1.80E09 5.85E12 1.80E10 5.83E09 31.6 8/22/11 Pre-Licensing Meeting 12

Preliminary Results

  • Results shown here use worst case percentile method for combining the NUREG-1829 GM and Lydell base case distributions
  • Some modest reductions in means and range factors expected from incorporation of mixture method
  • Current results only address LOCAs caused by pipe failures
  • Non-pipe contributions to be considered in 2012 8/22/11 Pre-Licensing Meeting 13

Example Results - Hot Leg B-F Weld at RPV Nozzle BF Weld Mean 5%tile 50%tile 95%tile RF Failure Rate 2.72E-04 1.04E-04 2.32E-04 5.77E-04 2.4 0.5 4.42E-07 1.89E-08 1.74E-07 1.63E-06 9.3 1.5 1.46E-07 8.90E-10 2.22E-08 5.45E-07 24.7 1.99 1.13E-07 5.98E-10 1.57E-08 4.14E-07 26.3 2.0 1.15E-07 5.96E-10 1.58E-08 4.19E-07 26.5 3.0 8.21E-08 3.44E-10 9.92E-09 2.89E-07 29.0 4.0 5.81E-08 2.42E-10 7.00E-09 2.02E-07 28.9 Cumulative LOCA Frequencies 5.99 3.49E-08 1.45E-10 4.22E-09 1.25E-07 29.3 Vs. Break Size (in.)

6.0 3.41E-08 1.43E-10 4.24E-09 1.20E-07 29.0 6.8 2.89E-08 1.25E-10 3.60E-09 1.05E-07 29.0 14.0 1.57E-08 4.67E-11 1.57E-09 5.41E-08 34.0 20.0 8.93E-09 2.65E-11 8.93E-10 3.07E-08 34.0 31.5 4.15E-09 1.26E-11 4.21E-10 1.42E-08 33.7 44.5 3.02E-09 8.95E-12 3.12E-10 1.04E-08 34.2 8/22/11 Pre-Licensing Meeting 14

Example Results - Hot Leg B-F Weld at RPV Nozzle 8/22/11 Pre-Licensing Meeting 15

Preliminary Results for Initiating Event Frequencies from Pipe Breaks Initiating Event Break Size Mean 5%tile 50%tile 95%tile RF SLOCA .5 to 2" 2.68E04 4.44E05 1.75E04 7.85E04 4.2 MLOCA 2" to 6" 6.69E05 2.94E06 2.53E05 2.44E04 9.1 LLOCA > 6" 3.34E06 3.49E08 6.37E07 1.23E05 18.8 8/22/11 Pre-Licensing Meeting 16

Preliminary Results for Total LOCA Frequency from Pipes 8/22/11 Pre-Licensing Meeting 17

Preliminary Results - System Contributions to LOCA Frequency 8/22/11 Pre-Licensing Meeting 18

Major Tasks to Complete

  • Provide input to CASAGRANDE with appropriate conditional probabilities vs. break size given the initiating event

- Conditional probability that the break occurs in each location

- Conditional probability that the break is in different size at each location

  • Provide input to RISKMAN on the initiating event frequencies and uncertainties
  • Finalize the draft report and submit for NRC review meeting in mid-September 8/22/11 Pre-Licensing Meeting 19
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