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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Homiack M
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xLPR Loss of Coolant Accident Frequency Estimates xLPR User Group Meeting August 18, 2021 1

Introduction 2

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.

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 4

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

Lines Selected for Evaluation 5

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 Nominal pipe size (NPS) 6 (DN 150)

NUREG-1829 category:

Direct Volume Injection Represented by Westinghouse Safety Injection Population includes both hot leg and cold leg temperature locations Alloy 82/182 Weld NPS 12 (DN 300) SCH 140 or 160, resulting in ~10 in. (250 mm) inside diameter NUREG-1829 Category:

High Pressure Safety Injection Represented by Combustion Engineering Safety Injection/Accumulator Cold leg temperature, unmitigated at most plants

Initial Results 6

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

Lessons Learned 7

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