ML21230A361

Xlpr LOCA Frequencies Presentation for Xlpr User Group

ML21230A361
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Issue date:	08/18/2021
From:	NRC/RES/DE
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xLPR Loss of Coolant Accident Frequency Estimates xLPR User Group Meeting August 18, 2021 1

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