FRAPCON/FRAPTRAN Code Application NRC Office of Research Patrick Raynaud, Ph.D. Patrick.Raynaud@nrc.gov

FRAPCON/FRAPTRAN User Group - Manchester, UK 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 09/07/2012 FRAPCON/FRAPTRAN User Group - Manchester, UK

Background NRC Fuel Behavior Codes FRAPCON-3.4 FRAPTRAN-1.4 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…) 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 09/07/2012 FRAPCON/FRAPTRAN User Group - Manchester, UK

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 09/07/2012 FRAPCON/FRAPTRAN User Group - Manchester, UK

FRAPCON Fuel Performance Modeling BWR 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 PWR PWR 09/07/2012 FRAPCON/FRAPTRAN User Group - Manchester, UK

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 09/07/2012 FRAPCON/FRAPTRAN User Group - Manchester, UK

LOCA and RIA Criteria Alloy 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 09/07/2012 FRAPCON/FRAPTRAN User Group - Manchester, UK

SCIP-2 Modeling Workshop Ramps 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 09/07/2012 FRAPCON/FRAPTRAN User Group - Manchester, UK

FRAPCON Modeling Methodology Slow 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 09/07/2012 FRAPCON/FRAPTRAN User Group - Manchester, UK

FRAPTRAN for Slow Power Ramps Evaluation 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) 09/07/2012 FRAPCON/FRAPTRAN User Group - Manchester, UK

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 09/07/2012 FRAPCON/FRAPTRAN User Group - Manchester, UK

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 09/07/2012 FRAPCON/FRAPTRAN User Group - Manchester, UK

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 09/07/2012 FRAPCON/FRAPTRAN User Group - Manchester, UK

FRAPTRAN for Slow Power Ramps Summary 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 Relative Agreement Between FRAPCON and FRAPTRAN Gap Size Gap HTC RIP FGR Free Volume Cladding Hoop Strain Fuel Temperature FRAPTRAN Model Transient FGR Better No Change Imposed RIP Lesser Best Worst Imposed FGR 09/07/2012 FRAPCON/FRAPTRAN User Group - Manchester, UK

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 09/07/2012 FRAPCON/FRAPTRAN User Group - Manchester, UK

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 09/07/2012 FRAPCON/FRAPTRAN User Group - Manchester, UK

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 09/07/2012 FRAPCON/FRAPTRAN User Group - Manchester, UK

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 09/07/2012 FRAPCON/FRAPTRAN User Group - Manchester, UK

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 09/07/2012 FRAPCON/FRAPTRAN User Group - Manchester, UK

FRAPCON/FRAPTRAN User Group - Manchester, UK 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 09/07/2012 FRAPCON/FRAPTRAN User Group - Manchester, UK

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 09/07/2012 FRAPCON/FRAPTRAN User Group - Manchester, UK

FRAPCON/FRAPTRAN User Group - Manchester, UK 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… 09/07/2012 FRAPCON/FRAPTRAN User Group - Manchester, UK