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FRAPCON/FRAPTRAN Code Application NRC Office of Research

FRAPCON/FRAPTRAN Code Application NRC Office of Research. Patrick Raynaud, Ph.D. Patrick.Raynaud@nrc.gov. Outline. Background on NRC fuel behavior codes Predicting LOCA and RIA limits FRAPCON hydrogen models SCIP-2 Modeling Workshop power ramps FRAPCON/FRAPTRAN comparison

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FRAPCON/FRAPTRAN Code Application NRC Office of Research

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  1. FRAPCON/FRAPTRANCode ApplicationNRC Office of Research Patrick Raynaud, Ph.D. Patrick.Raynaud@nrc.gov

  2. Outline • Background on NRC fuel behavior codes • Predicting LOCA and RIA limits • FRAPCON hydrogen models • SCIP-2 Modeling Workshop power ramps • FRAPCON/FRAPTRAN comparison • Recommendations for power ramp modeling • Fuel dispersal effort: core-wide realistic rod burst inventory • FRAPCON/FRAPTRAN and TRACE integration • Input generator improvements for detailed coolant boundary conditions FRAPCON/FRAPTRAN User Group - Manchester, UK

  3. BackgroundNRC Fuel Behavior Codes FRAPCON-3.4 FRAPTRAN-1.4 Rapid transients Milliseconds to a few minutes Transient solution Thermal, mechanical, fission gas, rod internal pressure response, high temperature corrosion, fuel cladding interaction, cladding failure (PCMI, ballooning) Failure models (ballooning and burst, PCMI) RIA, LOCA, BWR oscillations • Steady-state and slow transients • Minutes to many days • Equilibrium solution • Thermal, mechanical, fission gas, rod internal pressure response, corrosion, hydriding, cladding creep • No failure models • Warnings and/or stops when certain limits are reached (1% hoop strain, fuel melt…) FRAPCON/FRAPTRAN User Group - Manchester, UK

  4. Predicting RIA and LOCA Limits Objectives of the Study • Obtain hydrogen content [H] as a function of rod burnup BU to investigate burnup dependence of LOCA and RIA criteria • Use FRAPCON-3.4 to predict [H] vs. BU for U.S. cladding alloys • Zircaloy-2: BU dependent hydrogen pickup model: direct [H] vs. BU relationship • Zircaloy-4, ZIRLO™, M5™: heat flux, neutron flux, temperature, and time dependent model: complex indirect [H] vs. BU relationship • Constant alloy-dependent hydrogen pickup fractions • Generate BU dependent allowable ECR and ∆h for U.S. cladding alloys for different core axial elevations and power histories • Compare BU dependent LOCA and RIA limits as a function of cladding alloy FRAPCON/FRAPTRAN User Group - Manchester, UK

  5. FRAPCON Fuel Performance Modeling • Typical GE BWR/4 • 10x10 fuel assembly, Zircaloy-2 cladding • Typical Westinghouse 4-loop PWR • 17x17 fuel assembly, ZIRLO™ cladding • Core-load patterns and rod average power from plant safety analysis reports • Best guess at representative power histories for 2-cycle and 3-cycle lifetimes • 7 PWR and 18 BWR power histories • PWR histories based on ZIRLO™ cladding used for Zircaloy-4 and M5™ BWR BWR PWR PWR FRAPCON/FRAPTRAN User Group - Manchester, UK

  6. FRAPCON Hydrogen Models • Zircaloy-2 [H] only dependent on BU: no axial-node dependence • PWR alloys: strong temperature and heat flux dependence results in large axial variations • Zircaloy-4: high oxidation and H pickup fraction • Early transition in oxidation kinetics (δoxide>2μm) • Oxidation beyond allowable limits • ZIRLO™: intermediate behavior • M5™: low oxidation and H pickup fraction FRAPCON/FRAPTRAN User Group - Manchester, UK

  7. LOCA and RIA CriteriaAlloy Model Comparison • Zircaloy-2 in BWR • Better predicted performance than Zircaloy-4 and ZIRLO™ in PWR • Rapid degradation at high burnup • PWR alloys: • Highest H pickup for Zircaloy-4: lowest margin • M5™ plants less challenged by new criteria FRAPCON/FRAPTRAN User Group - Manchester, UK

  8. SCIP-2 Modeling WorkshopRamps and Fission Gas Release • Studsvik slow power ramps performed in R-2 reactor • 16 rods modeled • 12 rods modeled under FRAPCON only • KKL-1, M5-H2, O-2, Z-4 • OL1-1, OL1-2, OL1-3, OL1-4 • OA1-1, OA1-2, OA1-3, OA1-4 • 4 rods with FRAPCON and then with FRAPTRAN initialized by FRAPCON • GE-1 (feasibility scoping study) • xM1, xM2, xM3 • FRAPCON and FRAPTRAN predictions compared for 4 cases • Trends based on ramp characteristics were investigated • No specific PCI models in FRAPCON and FRAPTRAN FRAPCON/FRAPTRAN User Group - Manchester, UK

  9. FRAPCON Modeling MethodologySlow Power Ramps • Base irradiation and ramp simulated in same run • Used automatic input generator • Input given power histories and shapes and then made small adjustments to power level to match discharge burnups • Initial rod internal pressure adjusted to match refabricated rodlet pressure after base irradiation and before ramp test • Initial and final step at cold zero power • Allows for free volume calibration and residual hoop strain and gap predictions • Fission gas release (FGR) • 1st run for base FGR: FGR turned ON for base irradiation and ON for ramp test • 2nd run for ramp FGR: FGR turned OFF for base irradiation and ON for ramp test • Reminder: stepwise ramp approximation in FRAPCON FRAPCON/FRAPTRAN User Group - Manchester, UK

  10. FRAPTRAN for Slow Power RampsEvaluation with the GE-1 Case • FRAPTRAN is less suited to model phenomena over a long time scale • No creep • No steady-state fission gas release models • No pellet radial relocation and relaxation models • Different hypotheses were investigated to determine if FRAPTRAN can be used to model slow power ramps: • Default FRAPTRAN models • Transient FGR model • User-input rod internal pressure (RIP) to match FRAPCON predictions (no FGR modeled) • User-input fission gas release (FGR) to match FRAPCON predictions (FRAPTRAN default pressure calculation) FRAPCON/FRAPTRAN User Group - Manchester, UK

  11. FRAPTRAN for Slow Power Ramps GE-1 Plenum Pressure • Poor agreement during ramp unless RIP or FGR is imposed • Imposing RIP does not match FGR, imposing FGR matches RIP and FGR FRAPTRAN-1.4 defaults FRAPTRAN-1.4 transient FGR FRAPCON shown in red FRAPTRAN-1.4 imposed RIP FRAPTRAN-1.4 imposed FGR FRAPTRAN shown in blue Good Agreement Good Agreement FRAPCON/FRAPTRAN User Group - Manchester, UK

  12. FRAPTRAN for Slow Power Ramps GE-1 Gap Conductance or HTC • Poor agreement during preconditioning (gap open/closed) • Good agreement once gap closes in FRAPTRAN for imposed FGR FRAPTRAN-1.4 defaults FRAPTRAN-1.4 transient FGR FRAPCON shown in red FRAPTRAN-1.4 imposed RIP FRAPTRAN-1.4 imposed FGR Good agreement during most of ramp FRAPCON/FRAPTRAN User Group - Manchester, UK

  13. FRAPTRAN for Slow Power Ramps GE-1 Cladding Perm. Hoop Strain • Similar trends predicted, but FRAPTRAN under-prediction • Improved agreement if FGR is imposed FRAPTRAN-1.4 defaults FRAPTRAN-1.4 transient FGR FRAPCON shown in red FRAPTRAN-1.4 imposed RIP FRAPTRAN-1.4 imposed FGR FRAPTRAN shown in blue Best Agreement FRAPCON/FRAPTRAN User Group - Manchester, UK

  14. FRAPTRAN for Slow Power RampsSummary Based on GE-1 Case • Large differences between the two codes • Gap closure and heat transfer coefficient, rod internal pressures, cladding stresses, etc… • Agreement between the codes can be improved • Turning FGR ‘on’ in FRAPTRAN • Small improvement on residual gap, gap conductance, and RIP • Imposing RIP • Same RIP but degraded free volume and permanent cladding hoop strain • Imposing FGR • Large improvements in gap conductance, fuel temperature, RIP, and cladding permanent hoop strain FRAPCON/FRAPTRAN User Group - Manchester, UK

  15. FRAPTRAN for Slow Power Ramps Comparison with FRAPCON • Large differences between the two codes related to mechanical models • Fuel relocation recovery in FRAPCON but not FRAPTRAN • Absence of creep and differences in rod internal pressure in FRAPTRAN • Differences in predicted permanent hoop strain (creep + plastic in FRAPCON versus just plastic in FRAPTRAN) • Impact on gap closure and stresses • Fission Gas Release absent in FRAPTRAN • Must be imposed manually based on FRAPCON calculation • Impact of FGR and mechanical predictions on thermal predictions are significant • Very different gap heat transfer coefficients due to very different gap sizes and different RIP and gas composition • Different fuel temperature predictions >100 K FRAPCON/FRAPTRAN User Group - Manchester, UK

  16. FRAPTRAN for Slow Power Ramps Conclusions & Recommendations • FRAPCON overall better captures the phenomena at play during a relatively slow transient such as a power ramp • Creep, fission gas release, fuel relocation recovery, gap heat transfer • FRAPTRAN can be used but with caution • Fission gas release should be manually added to each time step • Gap size and heat transfer coefficient should be looked at closely and matched with FRAPCON when possible to improve predictions • For events longer than 3-8 seconds, such as the SCIP power ramps, thermal equilibrium will be reached in the fuel rod • FRAPCON is preferred over FRAPTRAN for the SCIP power ramps FRAPCON/FRAPTRAN User Group - Manchester, UK

  17. FRAPCON/FRAPTRAN and TRACE for LOCA Rod Burst Inventory • Objective: calculate a best-estimate number of fuel rods that rupture • Supports efforts to assess impact of fuel dispersal during a LOCA • 1st case chosen: large-break LOCA in a 4-loop PWR with large dry containment FRAPCON/FRAPTRAN User Group - Manchester, UK

  18. FRAPCON Power History Modeling • Generate power bins and histories to maximize core average discharge burnup (final core average BU ~ 51 GWd/MTU) • Keep track of every assembly throughout life in the core Peak assembly (center of core) Cycle 1 power: 1G Cycle 2 power: 2L History: 1G2L3A Cycle 3 power: 3A Discharge BU: 62.458 GWd/MTU FRAPCON/FRAPTRAN User Group - Manchester, UK

  19. TRACE Output Processing for FRAPTRAN • 8 coolant zones, with corresponding fuel rod heat structures • 1-to-1 axial zone correspondence (14 axial nodes) • Extracted variables for coolant boundary conditions: • ‘Coolant’ option • Attempt to accurately model coolant conditions • Inlet pressure, enthalpy, and mass flux • ‘Heat’ option • Trick to impose cladding OD temperature • Coolant pressure, temperature, and HTC • Impose coolant temperature = cladding OD temperature • Impose very high HTC (to force cladding OD temperature equal to coolant temperature) • Reflood option • Determine reflood rate based on core level vs. time • ‘coolant’ or ‘heat’ options ignored once reflood begins FRAPCON/FRAPTRAN User Group - Manchester, UK

  20. FRAPTRAN Modeling • Initialize FRAPTRAN with FRAPCON base irradiation runs at MOC • 22 different possible power histories • For a given power history, choice restart time determines the assembly burnup (1st cycle, 2nd cycle, or 3rd cycle) • Use coolant boundary conditions from TRACE • Run FRAPTRAN until after quench and determine whether rod has ruptured • 43 groups of rods * 8 azimuthal coolant sectors = 344 FRAPTRAN runs • 10 first cycle power bins • 22 second cycle power bins • 11 third cycle bins FRAPCON/FRAPTRAN User Group - Manchester, UK

  21. Input Generator Enhancements for Advanced Coolant Modeling • ‘coolant’ option: up to 50 time/parameter pairs • ‘heat’ option: up to 100 time/parameter pairs and 20 axial zones • ‘reflood’ option: up to 20 time/parameter pairs for inlet temperature and pressure, and up to 100 time/parameter pairs for reflood rate • Except for increased number of coolant zones in ‘heat’ option, capabilities already existed in the code, but were added to input generator • Coolant zones were increased from 10 to 20 for ‘heat’ option FRAPCON/FRAPTRAN User Group - Manchester, UK

  22. Summary • NRC is continuing in-house use of the code • PNNL continues to be very supportive of knowledge transfer activities to NRC, and as a result, NRC is actively participating in the code development effort that it sponsors at PNNL • FRAPCON/FRAPTRAN analyses support regulatory decision-making as well as safety scoping studies, and benchmarking exercises • NRC and PNNL are seeking additional opportunities to collaborate and exchange with other code users • Debugging, novel code applications, code interfacing, etc… FRAPCON/FRAPTRAN User Group - Manchester, UK

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