ML21230A361: Difference between revisions

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{{#Wiki_filter:xLPR Loss of Coolant Accident Frequency Estimates xLPR User Group Meeting August 18, 2021 1
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Introduction The approach would be supported by the application of the Extremely Low Probability of Rupture (xLPR) leak-before-break (LBB) analysis tool, developed jointly by EPRI and the NRC Office of Nuclear Regulatory Research for Large Break- (LB) LOCAs. This tool could be used within this generic methodology to inform the probability of LB-LOCAs and that LB-LOCAs may be detected in sufficient time to allow for reactor shutdown before a reactor coolant system (RCS) piping rupture occurs. This approach facilitates the demonstration of no fuel rod burst (that is, no FFRD) and estimates the changes in CDF.
  -  From: Alternative Licensing Approaches for Higher Burnup Fuel: A Scoping Study on Deterministic and Risk-Informed Alternatives Supporting Fuel Discharge Burnup Extension. EPRI, Palo Alto, CA: 2020. 3002018457.
2
 
xLPR Work Scope
* Objective: Perform Probabilistic Fracture Mechanics (PFM) evaluation using xLPR to calculate the probabilities of Loss-of-Coolant-Accidents (LOCAs) as a function of line size
    -  Use xLPR to benchmark NUREG-1829, Vol. 1, Estimating Loss-of-Coolant Accident (LOCA) Frequencies Through the Elicitation Process
* Validate (or replace) NUREG-1829 LOCA frequency estimates for use in high burnup fuel licensing
    -  Evaluate the time between detectable leakage and rupture to further inform the fuels licensing effort
* Approach: Project to be performed in phases
    -  Phase 1 consists of a PFM evaluation of two line sizes as a proof-of-concept and to develop methodology for possible expansion to other line sizes (detailed in this presentation)
    -  Phase 2 will expand the study to a variety of line sizes 3
 
Phase 1 Proof-of-Concept Study
* Focus on the following:
: 1. Methodology for using xLPR to evaluate the probability of LOCAs as a function of line size
: 2. Estimation of the frequency of LOCA events
: 3. Statistics on time between detectable leakage and unstable pipe rupture
* Two lines for initial evaluations were selected considering:
    - xLPR computational abilities
    - NUREG-1829 LOCA frequency insights (smaller lines have higher freq. of rupture)
    - Line size (mid-range of greatest interest to fuels licensing project)
    - Input availability
    - Degradation mechanisms (i.e., fatigue and primary water stress-corrosion cracking (PWSCC))
* Scope is limited to Pressurized-Water Reactors (PWRs)
* Base cases for each line include initial postulated flaws, fatigue and PWSCC crack growth (where applicable), and seismic occurrences
* Sensitivity cases were defined considering both inputs known to have influence on xLPR results and assumptions made during input development 4
 
Lines Selected for Evaluation
* Emphasis was placed on lines in the 6-10 in. (DN 150-250) range, as this range is of particular interest for fuel fragmentation, release, and dispersal (FFRD)
* Available input sources were reviewed to identify lines for which inputs to xLPR were readily available
      - Licensee submittals in the NRCs Agencywide Documents Access and Management System
      - EPRI reports
      - xLPR documentation
* It was determined that of the two lines evaluated, one should be susceptible to PWSCC and the other only susceptible to fatigue since the population of dissimilar-metal welds is small Stainless Steel Weld                              Alloy 82/182 Weld
* Nominal pipe size (NPS) 6 (DN 150)
* NPS 12 (DN 300) SCH 140 or 160, resulting in ~10
* NUREG-1829 category:                                in. (250 mm) inside diameter
* Direct Volume Injection
* NUREG-1829 Category:
* Represented by Westinghouse Safety
* High Pressure Safety Injection Injection
* Represented by Combustion Engineering Safety
* Population includes both hot leg and cold          Injection/Accumulator leg temperature locations
* Cold leg temperature, unmitigated at most plants 5
 
Initial Results
* Runs are completed for the two lines selected for Phase 1 - EPRI Technical Update to be published in October 2021
    - Phase 2 work extending the approach to additional lines will begin shortly -
EPRI report with results to be published in 2022
* As expected, LOCA frequencies are higher for lines susceptible to PWSCC
    - When fatigue is the only active degradation mechanism, the number of cases with leakage/rupture is extremely low
* Results show rupture frequencies on similar order of magnitude or lower than NUREG-1829 LOCA frequencies
    - Results are conservative as inservice inspection and leak rate detection were not credited
    - It is noted that NUREG-1829 expert elicitation considered other degradation mechanisms in addition to those modeled in these xLPR analyses
* Time between leakage and rupture data analysis indicated notable margin for leak-before-break 6
 
Lessons Learned
* Performing sensitivity studies on key inputs as well as on inputs for which modeling decisions were made during input development adds confidence to the overall xLPR analysis results
* The xLPR analyses decoupled crack initiation and growth to work within xLPR memory limitations, significantly reducing the number of necessary realizations
* A methodology has been established that is able (with some refinements) to evaluate LOCA probabilities and time between detectable leakage and rupture for other PWR lines 7}}

Revision as of 01:04, 17 January 2022

Xlpr LOCA Frequencies Presentation for Xlpr User Group
ML21230A361
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Issue date: 08/18/2021
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