ML25253A427
| ML25253A427 | |
| Person / Time | |
|---|---|
| Site: | Westinghouse |
| Issue date: | 09/11/2025 |
| From: | Jeremy Dean, John Lehning, Ekaterina Lenning, Polder J, Benjamin Wise Licensing Processes Branch |
| To: | |
| References | |
| WCAP-18850-P/NP | |
| Download: ML25253A427 (1) | |
Text
NRC Staffs Review of Westinghouse Topical Report WCAP-18850-P, Adaptation of the FULL SPECTRUM LOCA (FSLOCA )
Evaluation Methodology to Perform Analysis of Cladding Rupture for High Burnup Fuel Open Presentation to Advisory Committee on Reactor Safeguards, Accident Analysis Subcommittee September 16, 2025 J. Dean, U.S. NRC B. Wise, U.S. NRC J. Vande Polder, U.S. NRC J. Lehning, U.S. NRC
Presentation Outline Topic
- of Slides Introduction / Background 4
Review History 1
Requirements and Guidance 1
Technical Evaluation (open portion) 7
- Modeling Basics (1)
- Phenomenon Identification and Ranking (1)
- Fuel Rod Modeling Updates (1)
- Kinetics and Decay Heat Model Updates (1)
- Cladding Rupture Methodology (3)
Applicability, Limitations and Conditions 3
Conclusions 1
Acronyms 1
Presentation Total 18 2
Introduction
- Westinghouse proposed WCAP-18850-P to extend its approved FULL SPECTRUM LOCA methodology to higher fuel enrichment and burnup levels 3
- One means of addressing fuel dispersal is preventing rupture of fuel rods susceptible to fragmentation
- WCAP-18850-P provides a method for applying FSLOCA framework to determine with high probability that cladding rupture will not occur during a LOCA
FSLOCA Background WCAP-16996-A
- FSLOCA is a realistic LOCA evaluation model that accounts for uncertainty
- Methodology addresses scenarios across entire postulated spectrum of break sizes
- Current methodology applicable to Westinghouse 3-and 4-loop pressurized-water reactors was approved in 2017
- Extensions currently being pursued for additional pressurized-water reactor designs
- Licensed FSLOCA methodology does not address fuel dispersal
- Models are included for fragmentation and relocation 4
Review Timeline 5
WCAP-18850-P Submitted Feb May Acceptance Review Complete Oct Audit Nov RAI RAI Responses
- 1 #2 Jan Mar July Draft SE Sep ACRS SC Meeting NRC Westinghouse 2024 2025
Key Regulatory Requirements and Guidance for LOCA
- NUREG-0800, Standard Review Plan
- Chapter 15.6.5, Loss-of-Coolant Accident
- Chapter 15.0.2, Review of Transient and Accident Analysis Methods 6
- Regulatory Guide 1.157, Best-Estimate Calculations of Emergency Core Cooling System Performance
- Regulatory Guide 1.203, Transient and Accident Analysis Methods
Loss-of-Coolant Accident Analysis Methods
- WCAP-18850-P methodology must be capable of predicting occurrence of cladding rupture during a LOCA
- Modeling based on FULL SPECTRUM LOCA methodology (WCAP-16996-P-A)
- WCOBRA/TRAC-TF2 code 7
- Westinghouse evaluated PIRT phenomena for impacts of increased burnup
- Key model updates in two main areas
- Fuel rod modeling
- Kinetics and decay heat modeling
Phenomena Identification and Ranking
- WCAP-18850-P evaluated the FSLOCA PIRT and an industry high burnup PIRT.
- No new phenomena identified
- Updates to WCOBRA/TRAC-TF2 and other models where appropriate
- Fuel Rod Models, Kinetics and Decay Heat, Burst Criteria
- Burst size, location, etc. determined to be unimportant.
- The NRC staff found that the PIRT evaluation is comprehensive and adequately identifies where model updates are needed to account for FFRD and high burnup and enrichment.
8
Fuel Rod Model Updates
- WCAP-18850-P addresses fuel rod models that could be impacted by increased burnup:
- Pellet-cladding gap conductance
- Cladding deformation
- Cladding rupture
- Fuel rod initialization
- Susceptibility to fine fragmentation
- Transient fission gas release
- Pre-burst fuel relocation
- Fuel rod material properties
- Proprietary details discussed further in closed session 9
WCOBRA/TRAC-TF2 Kinetics and Decay Heat Model Updates Westinghouse updated the models to incorporate increased enrichment and higher burnup
- Nuclear physics data provided by PARAGON2 (Approved June 2021)
Similar model updates were previously approved in:
- WCAP-18446-P-A (Approved June 2024)
- WCAP-18773-P (Final SE Issued July 2025)
The NRC Staff found the model updates to be acceptable because they were consistent with previous approvals. Unique characteristics associated with the expanded range of applicability and cladding rupture calculations were examined.
10
Cladding Rupture Methodology
- Uses the FSLOCA EM as a base methodology and adapts it as appropriate
- The purpose of the methodology is to preclude cladding rupture of rods susceptible to fine fragmentation.
- The WCAP-18850-P cladding rupture analysis is performed parallel to the WCAP-16996-A FSLOCA base analysis that demonstrates compliance with 10 CFR 50.46(b)(1)-(b)(3).
- Defines a new break spectrum region, Region IB
- Proposes a methodology for analyzing cladding rupture in Region IB
- A mix of Regions I and II, with some unique characteristics
- Proposes changes to Region I and II uncertainty analyses 11
Cladding Rupture Methodology
- The NRC Staff
- Found the adaptation of the FSLOCA EM base methodology appropriate to perform cladding rupture calculations. Several models were updated to incorporate the increase range of applicability for burnup and enrichment
- Found the methodology to be supported by significant data and is acceptable for predicting the occurrence of cladding rupture of rods susceptible to fine fragmentation.
12
Cladding Rupture Methodology
- The NRC Staff
- Found the definition of Region IB and its treatment in the FSLOCA EM to be comprehensively described and acceptable.
- Found the proposed changes to Region I and II uncertainty analyses acceptable.
- The proposed treatments are supported by sensitivity studies, comparison calculations, and engineering judgment.
13
Review of Limitations and Conditions from FSLOCA (WCAP-16996, R1)
- Westinghouse assessed the 15 limitations and conditions from WCAP-16996, R1, for applicability to the derivative WCAP-18850-P methodology
- Applicable limitations and conditions from WCAP-16996, R1, were propagated into WCAP-18850-P topical report
- Direct propagation of FSLOCA L&Cs 3, 12, and 13
- Others adopted in modified form (FSLOCA L&Cs 2, 4, 5, 6, 8, 11, 15) 14
Limitations and Conditions for WCAP-18850-P
- The NRC staffs draft safety evaluation contains 11 limitations and conditions that licensees adopting the methodology must address
- These limitations were originally proposed by Westinghouse in WCAP-18850-P
- NRC staff modified several of these limitations
- Three limitations and conditions will be covered in the open presentation
- Remaining eight will be discussed during the closed presentation 15
Limitations and Conditions:
Methodology Applicability Per Limitation and Condition 1, the methodology can only be applied to Westinghouse 2-Loop and Combustion Engineering plants after
- The base model is approved for these applications
- Applicable differences and deviations have been addressed Per Limitation and Condition 9, WCAP-18850-P is only applicable to fuel products with
- Uranium dioxide or ADOPT fuel pellets
- AXIOM cladding Per Limitation and Condition 10, WCAP-18850-P is only applicable to
- Unpoisoned fuel
- Fuel with integral fuel burnable absorbers
- Fuel with gadolinia (this limitation does not preclude use of discrete burnable absorbers) 16
Conclusions
- The NRC staff found the WCAP-18850-P methodology provides an acceptable approach for determining, with high probability, that cladding rupture will not occur for fuel rods susceptible to fine fragmentation
- The NRC staffs conclusions are predicated upon
- The methodology being used within its approved range of applicability
- Licensees acceptably addressing limitations and conditions in Section 4.0 of the staffs safety evaluation 17
Table of Abbreviations 10 CFR Title 10 of the Code of Federal Regulations ACRS Advisory Committee on Reactor Safeguards FULL SPECTRUM LOCA, FSLOCA WCAP-16996-P-A, Revision 1, 'Realistic LOCA Evaluation Methodology Applied to the Full Spectrum of Break Sizes (Full Spectrum LOCA Methodology)
L&C Limitation and Condition LOCA Loss-of-Coolant Accident NRC U. S. Nuclear Regulatory Commission PIRT Phenomenon Identification and Ranking Table RAI Request for Additional Information SC Advisory Committee on Reactor Safeguards Subcommittee SE Safety Evaluation WCAP-18850 WCAP-18850-P/NP, Revision 0, Adaptation of the FULL SPECTRUM LOCA (FSLOCA) Evaluation Methodology to Perform Analysis of Cladding Rupture for High Burnup Fuel 18