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
– 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
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Background
NRC Fuel Behavior Codes
FRAPCON-3.4
FRAPTRAN-1.4
• Steady-state and slow transients
• Rapid 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…)
09/07/2012
– 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
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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
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FRAPCON Fuel Performance
Modeling
• Typical GE BWR/4
BWR
BWR
– 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
PWR
PWR
– 7 PWR and 18 BWR power histories
• PWR histories based on ZIRLO™
cladding used for Zircaloy-4 and M5™
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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
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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
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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
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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
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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:
1. Default FRAPTRAN models
2. Transient FGR model
3. User-input rod internal pressure (RIP) to match FRAPCON predictions (no
FGR modeled)
4. User-input fission gas release (FGR) to match FRAPCON predictions
(FRAPTRAN default pressure calculation)
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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
Good Agreement
09/07/2012
FRAPTRAN
shown in blue
FRAPTRAN-1.4 imposed FGR
Good Agreement
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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
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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
shown in blue
FRAPTRAN-1.4 imposed FGR
Best Agreement
09/07/2012
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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
FRAPTRAN
Model
Relative Agreement Between FRAPCON and FRAPTRAN
Cladding Hoop
Fuel
Strain
Temperature
Gap Size
Gap HTC
RIP
FGR
Free Volume
Transient FGR
Better
Better
Better
No Change
Better
No Change
No Change
Imposed RIP
Lesser
No Change
Best
No Change
Worst
Lesser
No Change
Imposed FGR
Best
Best
Best
Best
Lesser
Best
Best
09/07/2012
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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
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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
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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
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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
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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
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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
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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
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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…
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FRAPCON/FRAPTRAN User Group - Manchester, UK
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