ML22166A349
| ML22166A349 | |
| Person / Time | |
|---|---|
| Issue date: | 06/14/2022 |
| From: | Robert Tregoning Office of Nuclear Regulatory Research |
| To: | |
| References | |
| Download: ML22166A349 (22) | |
Text
Passive System LOCA Frequency Estimation and Break Size Selection Robert L. Tregoning NRC\\RES Public Meeting to Discuss Use of xLPR for LOCA Estimation June 14, 2022
June 14, 2022 Public Meeting to Discuss Use of xLPR for LOCA Estimation Page 2 Presentation Objectives
- 1. Outline LOCA elicitation chronicled in draft NUREG-1829 and used as part of the technical basis supporting the proposed 50.46 rule revision and for general use in PRA plant modeling
- 2. Summarize other activities associated with selecting the transition break size (TBS) for proposed 50.46 rule revision Considerations associated with selecting TBS NUREG-1903: Seismic considerations Development of draft guidance
June 14, 2022 Public Meeting to Discuss Use of xLPR for LOCA Estimation Page 3
LOCA frequencies previously developed from operating history.
Notable Previous Evaluations:
WASH-1400 (1975): Estimates largely based on experience in other industries
NUREG-1150 (1987): Updated the WASH-1400 distributions to account for the additional service since WASH-1400
NUREG/CR-5750, Appendix J (1998): Updated original WASH-1400 study for SB LOCAs while MB and LB LOCA frequencies were calculated from precursor leaks in class 1 systems
Barsebck-1 Study (1998): Determined estimates using piping reliability attribute and influence characteristics for each degradation mechanism
Operating history, by itself, may not accurately reflect future performance and requires significant extrapolation for MB and LB LOCA frequencies.
Historical LOCA Frequency Evaluation
June 14, 2022 Public Meeting to Discuss Use of xLPR for LOCA Estimation Page 4
Develop piping and non-piping passive system LOCA frequencies as a function of leak rate and operating time up to the end of the license extension period using expert elicitation
LOCAs which initiate in unisolable portion of reactor coolant system
LOCAs related to passive component aging, tempered by mitigation measures
Determine LOCA frequency distributions for typical plant operational cycle and history
Assume that no significant changes will occur in future plant operating profiles NUREG-1829: Scope and Objectives
June 14, 2022 Public Meeting to Discuss Use of xLPR for LOCA Estimation Page 5 Expert Elicitation Process
Classical approaches
Operating experience: LOCA events are rare
Plant modeling: Number and diversity of possible failure modes is too complex to accurately model
Expert elicitation is a formal process for providing quantitative estimates for the frequency of physical phenomena when the required data is sparse and when the subject is too complex to accurately model.
Elicitation has often been used at NRC
Development of seismic hazard curves
Performance assessments for high-level radioactive waste repository
Determination of reactor pressure vessel flaw distributions
June 14, 2022 Public Meeting to Discuss Use of xLPR for LOCA Estimation Page 6 NUREG-1829: LOCA Size Classification
LOCA sizes based on flow rate to group plant system response characteristics.
First three categories similar to NUREG-1150 and NUREG/CR-5750.
Three additional LBLOCA categories used to determine larger break frequencies.
Correlations developed to relate flow rate to effective break area.
Three time periods evaluated
Current day (average 25 years of operation)
End of design life (next 15 years of operation)
End of life extension (following 20 years of operation)
Category Flow Rate Threshold (gpm)
LOCA Size 1
> 100 SB 2
> 1500 MB 3
> 5000 LB 4
> 25,000 LB a 5
> 100,000 LB b 6
> 500,000 LB c
June 14, 2022 Public Meeting to Discuss Use of xLPR for LOCA Estimation Page 7
Elicitation focuses on passive system LOCAs.
Important piping and non-piping variables identified.
Elicitation structure supports top down and bottom up analysis.
Passive System LOCAs Plant Piping Systems Component Non-Piping Contribution Piping Contribution Aging Mechs.
Mitigation
& Maint.
Geometry Materials Loading History Valves Pressure Vessel Pumps Press.
Steam Gen.
Active System LOCAs Service History LOCA Contributions Top Down Bottom Up NUREG-1829: General Issue Classification
June 14, 2022 Public Meeting to Discuss Use of xLPR for LOCA Estimation Page 8 NUREG-1829: Piping Base Cases
The base cases were available for anchoring the elicitation responses.
Base case conditions specify the piping system, piping size, material, loading, degradation mechanism(s), and mitigation procedures.
Five base cases defined.
Recirculation System (BWR-1)
Feedwater System (BWR-2)
Hot Leg (PWR-1)
Surge Line (PWR-2)
High Pressure Injection makeup (PWR-3)
The LOCA frequency for each base case condition is calculated as a function of flow rate and operating time.
Four panel members individually estimated frequencies: two using operating experience and two using probabilistic fracture mechanics.
June 14, 2022 Public Meeting to Discuss Use of xLPR for LOCA Estimation Page 9 Piping Base Case Summary Results:
25 Year Operating Period
Large variability due to inconsistencies in both the conditions evaluated and differences in approaches.
Each base case participant presented their approach and results to entire panel.
Each panel member was asked to critique approaches & results during their elicitation session.
LOCA Category 1
2 3
4 5
6 Failure Frequency (R-yr-1) 100 10-2 10-4 10-6 10-8 10-10 10-12 10-14 10-16 10-18 BWR-1 BWR-2 LOCA Category 1
2 3
4 5
6 Failure Frequency (R-yr-1) 100 10-2 10-4 10-6 10-8 10-10 10-12 10-14 10-16 10-18 PWR-1 PWR-2 PWR-3 BWR Base Cases PWR Base Cases
June 14, 2022 Public Meeting to Discuss Use of xLPR for LOCA Estimation Page 10 NUREG-1829: Non-Piping Base Cases
The variety and complexity of the non-piping failure mechanisms makes the piping base case approach intractable.
Approach
Develop general non-piping precursor database
Use PFM modeling to develop LOCA frequencies for targeted degradation mechanisms CRDM ejection BWR vessel rupture: normal operating and LTOP PWR vessel rupture: PTS
Analysis requirements
Choose appropriate base case: non-piping precursor, piping precursor, piping base case, or non-piping base case
Determine relative likelihood of each non-piping failure scenario compared to chosen base case
June 14, 2022 Public Meeting to Discuss Use of xLPR for LOCA Estimation Page 11 Analysis of Elicitation Responses:
Framework
Calculate individual estimates for each panelist.
Total BWR and PWR LOCA estimates
Approach is most self-consistent
Aggregate individual estimates: Philosophy
Group results more accurate than any single estimate.
Outliers should not dominate quantitative estimates.
Aggregate individual estimates: Approach
Combine parameters (mean, median, 5th & 95th percentiles) of individual distributions
Calculate confidence bounds associated with each parameter estimate
Final LOCA distributions reflect uncertainty and variability.
Uncertainty: Individual panel member responses
Variability: Range of individual responses
June 14, 2022 Public Meeting to Discuss Use of xLPR for LOCA Estimation Page 12 Threshold Break Diameter (in) 0.1 1
10 100 LOCA Frequency ({cal-yr}-1) 10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 Median Mean 95th Percentile BWR: Baseline Results median and 95th percentile results offset slightly for clarity Threshold Break Diameter (in) 0.1 1
10 100 LOCA Frequency ({cal-yr}-1) 10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 Median Mean 95th Percentile PWR: Baseline Results median and 95th percentile results offset for clarity NUREG-1829: Total LOCA Frequencies
95% confidence bounds (i.e., error bars) account for diversity among panelists
Differences between median and 95th percentiles reflect individual panelist uncertainty
June 14, 2022 Public Meeting to Discuss Use of xLPR for LOCA Estimation Page 13 NUREG-1829: Summary
Formal elicitation process used to estimate generic BWR and PWR passive-system LOCA frequencies associated with material degradation during normal operations
Piping and non-piping base cases were developed and evaluated for anchoring elicitation responses.
Panelists provided quantitative estimates supported by qualitative rationale in individual elicitations for underlying technical issues.
Generally good agreement on qualitative LOCA contributing factors.
Large individual uncertainty and panel variability in quantitative estimates.
Results are generally comparable to NUREG/CR-5750 estimates.
Group results determined by aggregating individual panelists estimates.
Geometric mean aggregated results are consistent with elicitation objectives and results are generally comparable with NUREG/CR-5750 estimates.
Alternative aggregation schemes can result in higher LOCA frequencies.
June 14, 2022 Public Meeting to Discuss Use of xLPR for LOCA Estimation Page 14 Selection of Transition Break Size (TBS)
Use NUREG-1829 results as a starting point.
There is a range of pipe sizes which correlate to pipe break frequency of < 1E-5/cal-yr
Selection should accommodate various uncertainties
Other types of LOCAs should be considered in determining TBS
Active LOCAs
Load-generated LOCAs (i.e., dropped heavy loads, water hammer)
Seismically induced LOCAs
Actual plant piping design and operating experience should be considered in final selection
June 14, 2022 Public Meeting to Discuss Use of xLPR for LOCA Estimation Page 15 TBS Selection
TBS is selected as the size of the largest pipe attached to the main coolant loop
For PWRs, based on the size of the largest pipe attached to the cold or hot leg main loop piping
For BWRs, based on the size of the largest pipe in either of the RHR or Feedwater systems inside primary containment
Next larger pipes are significantly less likely to break
Accommodates uncertainties and provides regulatory stability
TBS is defined in the proposed 50.46a rule as twice the cross-sectional flow areas of these size pipes
Page 16 NUREG-1903: Objective and Approach
Objective
Determine if seismic risk is acceptable for breaks greater than transition break size (TBS)
Approach
Use of hybrid deterministic and probabilistic approaches
Six supporting activities Unflawed piping Flawed piping Indirect failures Review of past earthquake experience Review of past PRAs Review of a LLNL study conducted in connection with revision to GDC4 June 14, 2022 Public Meeting to Discuss Use of xLPR for LOCA Estimation
June 14, 2022 Public Meeting to Discuss Use of xLPR for LOCA Estimation Page 17 NUREG-1903: Summary
Reviewed prior PRA, seismic studies and earthquake experience
Analyzed direct piping failure associated with rare seismic events
Evaluated unflawed and flawed piping systems with diameter larger than the TBS (e.g., hot leg, cold leg, and cross-over leg)
Used updated seismic-hazard curves for plants east of Rocky Mountains
Used hybrid deterministic and probabilistic method to determine component stresses
Considered 10-5/yr and 10-6/yr seismic events
Analyzed indirect piping failure associated with rare seismic events
Analyzed large component support failures that may lead to piping failure
Assumed that support failure leads directly to piping failure
Updated results from prior LLNL study to reflect new hazard and ground motion information
Determined mean failure probability of component supports
June 14, 2022 Public Meeting to Discuss Use of xLPR for LOCA Estimation Page 18 NUREG-1903: Summary, cont.
Results
Unflawed piping: Failure frequency is much lower than 10-5/yr
Flawed piping Critical flaws for long, circumferential flaws (/ = 0.8) are generally large 40% of wall thickness for 10-5/yr seismic event 30% of wall thickness for 10-6/yr seismic event Conditional probability of breaks larger than the TBS should be less than 10-5/yr
Indirect failures Only two cases analyzed (W and CE plants)
Piping failure induced by major component support failure has a mean probability of approximately 10-6/yr.
June 14, 2022 NUREG-1903: Use of Results
Use in §50.46a
Risks of seismically induced LOCAs are expected to be acceptable
TBS selection is appropriate
Limitations
Analyses may not be generically applicable
Indirect failure risks not generically evaluated Public Meeting to Discuss Use of xLPR for LOCA Estimation Page 19
Consider issues and implications associated with generic aspects of NUREGs
Assumptions
Approach
Analysis
Guidance proposed in several areas that may be affected by plant-specific factors
NUREG-1829 Applicability Safety culture Current plant operation Changes in plant operation that may affect LOCA frequencies
NUREG-1903 Applicability Frequencies associated with direct piping failures caused by seismic loading Frequencies associated with indirect piping failures caused by seismic loading DG-1216 - Plant-Specific Applicability of TBS: Scope Public Meeting to Discuss Use of xLPR for LOCA Estimation June 14, 2022 Page 20
DG-1216: Summary
Guidance provided in areas that may be affected by plant-specific factors
NUREG-1829 Applicability Current plant operation Changes in plant operation that may affect LOCA frequencies
NUREG-1903 Applicability Frequencies of seismically induced direct piping failures
Evaluation required only for breaks larger than the proposed TBS
Uses information submitted under other programs wherever possible
No current guidance to ensure that frequencies of seismically induced indirect piping failures remain acceptable
Modified draft final rule based on ACRS feedback
Sample evaluation method has been proposed
Plan to add method to DG-1216
Plan to publish interim RG in 2011 coincident with final rule
Soliciting pilot applications for exercising RG approach Public Meeting to Discuss Use of xLPR for LOCA Estimation June 14, 2022 Page 21
June 14, 2022 TBS Development for Risk-Informed Revision of 10 CFR 50.46
Commission Communication
SECY-01-0133 (Feasibility): ML011800524
SECY-02-0057 (Recommended Changes): ML020660607 / SRM: ML030910476
SECY-04-0037 (Request for Policy Direction): ML040490133 / SRM: ML041830412
SECY-05-0052 (Initial Proposed Rule): ML050480172 / SRM: ML052100416
SECY-07-0082 (Supplemental Proposed Rule): ML070180692 / SRM: ML072220595
SECY-10-0161 (Final Rule): ML102300252 / SRM: ML12117A121
Reports
NUREG-1829 - Vol. 1/Vol. 2: ML080630015
NUREG-1903: ML080880140
DG-1216: ML100430356
ACRS Meetings
Minutes of the 518th ACRS Meeting, Dec. 2-4, 2005: ML081830134 (Selecting TBS)
Minutes of ACRS Subcommittee on Reliability and PRA, Nov. 27, 2007: ML082530561 (NUREG-1829/1903)
Minutes of the 548th ACRS Meeting, Dec. 6-8, 2007: ML080640924 (NUREG-1829/1903)
Transcript of ACRS Regulatory Policies and Practices Subcommittee, Sep. 22, 2010: ML102910759 (Rule)
Meeting Minutes of the 576th ACRS Meeting, Oct. 7-9, 2010: ML103120527 (DG-1216)
Public Meetings
Information required to apply for license changes under 10 CFR 50.46a, Feb. 27, 2009: ML090550073
DG-1216, Sep. 30, 2010: ML102910247 Public Meeting to Discuss Use of xLPR for LOCA Estimation Page 22