FuelSim
Steady-State Nuclear Fuel Rod Performance Analysis
What is FuelSim?
FuelSim is a steady-state nuclear fuel rod performance analysis code that predicts the thermal and mechanical behavior of light water reactor (LWR) fuel rods during normal operating conditions. It calculates temperature distributions, fission gas release, rod internal pressure, cladding deformation, and other key parameters as a function of burnup over the operating lifetime of the fuel.
FuelSim is derived from FRAPCON-3, the fuel rod analysis code developed by Pacific Northwest National Laboratory (PNNL) for the U.S. Nuclear Regulatory Commission (NRC). FuelSim retains the extensively validated computational core while adding improved variable naming for clarity, a modernized input format, additional output capabilities, and a desktop graphical user interface.
NRC Heritage
Built on FRAPCON-3, the NRC's standard analytical tool for predicting fuel rod behavior under steady-state conditions since the late 1990s.
Extensively Validated
Validated against international irradiation data from Halden, Riso, BR-3, and commercial BWR/PWR fuel programs.
Desktop GUI & 3D Visualization
Graphical interface with one-click execution, real-time output viewing, interactive charting, 3D fuel rod visualization, safety limits checking, parametric sweeps, and PDF report generation.
What Physical Phenomena Does FuelSim Model?
Coupled thermal, mechanical, and chemical phenomena in an operating fuel rod
Thermal Analysis
Radial temperature distribution through fuel pellet, gap, cladding, and coolant with burnup-dependent thermal conductivity.
- Gap conductance (gas, radiation, contact)
- Crud and oxide layer thermal resistance
- PWR and BWR coolant heat transfer
Fuel Behavior
Comprehensive fuel pellet modeling including densification, swelling, relocation, cracking, and fission gas release.
- Fission gas diffusion and release (Xe, Kr)
- Fuel densification and swelling
- Fuel relocation, cracking, and grain growth
Cladding Behavior
Time-dependent cladding deformation under stress including creep, corrosion, irradiation growth, and mechanical response.
- Thermal and irradiation-enhanced creep
- Oxide layer buildup and corrosion
- Elastic and plastic deformation
Pellet-Cladding Interaction
Gap closure mechanics, contact pressure calculation, and cladding stress/strain from pellet-cladding mechanical interaction (PCMI).
- Gap closure tracking
- Interfacial contact pressure
- Rod internal pressure tracking
What Materials Does FuelSim Support?
FuelSim incorporates the MATPRO-11 material properties package providing temperature- and irradiation-dependent properties.
UO2 Fuel
Thermal conductivity, specific heat, thermal expansion, elastic modulus, emissivity, densification, swelling, creep, fission gas diffusion
Zircaloy-2 & Zircaloy-4
Thermal conductivity, specific heat, thermal expansion, elastic modulus, yield strength, creep, oxidation, irradiation growth
M5 & ZIRLO Alloys
Modern cladding properties for Framatome (M5) and Westinghouse (ZIRLO) fuel designs
Fill Gas Mixtures
Thermal conductivity and viscosity for He, Xe, Kr, Ar, N2, H2, H2O, air, and mixtures
Key Capabilities
- Up to 400 timesteps tracking full irradiation history
- Up to 50 axial nodes for axial power/temperature variations
- Configurable radial nodes through the fuel pellet
- Variable power history with multiple axial power shapes
- High burnup validated (>60 GWd/MTU)
- Solid and annular fuel pellet geometries
- Safety limits checking against fuel melt, cladding, oxide, and stress limits
- Parametric sweeps for sensitivity studies on any input parameter
- Batch execution with automated results extraction and error reporting
Reactor Types Supported
PWR
2000–2250 psia single-phase forced convection
BWR
~1000 psia two-phase coolant conditions
How Does FuelSim Visualize Results in 3D?
Interactive 3D surface plots and cross-section views built into the desktop GUI
Rod Temperature View
3D surface plot of radial temperature distribution across the entire fuel rod at a selected timestep. Reveals hot spots and thermal gradients from fuel centerline to cladding outer surface.
- Radial position vs. axial node vs. temperature
- Navigate timesteps with slider control
Time-Axial Surface
3D surface showing how any nodal variable evolves over time and across axial positions. Captures the full irradiation history in a single plot to identify when and where peak values occur.
- Time vs. axial node vs. any variable
- Select from all nodal output variables
Cross-Section View
Polar contour heatmap of the radial temperature distribution at a specific axial node and timestep, as if looking down the axis of the fuel rod. Intuitive view of radial thermal gradients.
- Polar projection of radial temperature
- Select axial node and timestep
3D Visualization Controls
Interactive Navigation
Timestep slider, axial node selector, and variable dropdown. Rotate, zoom, and pan all 3D plots.
Colormap Options
Choose from viridis, hot, coolwarm, jet, plasma, or inferno colormaps for optimal data visualization.
Dual Unit Support
Toggle between English (in, °F) and SI (mm, K) units for all axes and data displays.
What Desktop Application Features Does FuelSim Include?
A complete analysis environment with productivity tools beyond basic simulation execution
Safety Limits Checking
Automatic flagging when results exceed or approach regulatory safety limits with color-coded indicators in the Summary tab.
- Fuel centerline melt temperature
- Cladding temperature limits
- Oxide thickness and hoop stress limits
Parametric Sweep Studies
Built-in sensitivity analysis by sweeping any input parameter across a user-defined range. Automatically runs multiple cases and overlays results on a single chart.
- Vary any input parameter across a range
- Overlay results for comparison
PDF Report Generation
Export multi-page PDF summaries with case title, key input parameters table, peak values with safety limit flags, and user-selected variable plots including comparison and sweep overlays.
Batch Execution
Queue multiple input files for sequential execution with per-case error reporting and automated results summary extraction.
Baseline Comparison
Load a reference strip file and overlay its results on charts for side-by-side comparison against current simulation output.
Input Editor & Regression Testing
Built-in form editor for modifying input file parameters with validation. Regression testing compares current results against reference datasets with pass/fail tolerance checking.
What is FuelSim UA?
FuelSim UA is a statistical uncertainty analysis tool that automates Monte Carlo sampling, runs hundreds of FuelSim simulations, and quantifies how manufacturing tolerances, measurement uncertainties, and operating condition variability affect fuel rod performance predictions.
Why Uncertainty Analysis?
Nuclear fuel rod performance codes use dozens of input parameters — fuel density, cladding dimensions, gap thickness, coolant conditions, power levels, and more. In practice, none of these values are known exactly:
Manufacturing Tolerances
Variability in pellet density, enrichment, cladding dimensions, and gap size
Operating Conditions
Coolant temperature, pressure, and flow rate differ from rod to rod across the core
Measurement Uncertainty
Even “known” values carry some error from measurement processes
A single best-estimate calculation tells you what happens with one set of nominal inputs. Uncertainty analysis tells you how much the answer could change given realistic input variability — and whether the results remain within safety limits with high confidence.
Input Parameters
Fuel enrichment, density, cladding dimensions, gap thickness, fill gas pressure, coolant conditions, power, and more
Distribution Types
Normal, Uniform, Triangular, Lognormal, and Fixed distributions for flexible uncertainty specification
Output Metrics
Burnup, FGR, fuel/cladding temperatures, hoop stress, oxide thickness, plenum pressure, stored energy
Interactive Plot Types
Histograms, CDFs, Time-Series Overlay, Scatter/Correlation, Tornado Diagrams, and Results Table
Max Samples
Scalable from quick screening studies (20–50 samples) to high-fidelity analyses (1,000+)
CSV & PDF Export
Full results matrix to CSV; multi-page PDF reports with statistics and histograms
How Does FuelSim UA Work?
Load Template
Select any FuelSim input file as baseline
Configure
Choose inputs to vary and set probability distributions
Sample
Monte Carlo random sampling from each distribution
Run Cases
FuelSim executed automatically for each sample
Analyze
Statistical analysis of output metrics performed automatically
Export
Interactive charts, CSV data, and PDF reports
What Visualizations Does FuelSim UA Provide?
Six interactive chart types for exploring uncertainty analysis results
Histograms
Frequency distributions of each output metric with mean, 5th/95th percentile lines, and safety limit markers. Statistics text box shows N, Mean, Std, Min, Max, and exceedance probability.
Cumulative Distribution Functions
Empirical CDF plots showing the probability that each output metric is below any given value. Annotated with P5, P50, P95 percentiles and safety limit exceedance probability.
Time-Series Overlay
All sample time-series plotted on a single chart showing how uncertainty fans out over the irradiation history. Includes mean curve and shaded 5th–95th percentile envelope.
Scatter & Correlation
Input parameter vs. output metric scatter plots with linear trend lines and Pearson correlation coefficients. Side panel ranks all inputs by correlation strength.
Tornado Diagrams
Horizontal bar charts ranking input parameters by their influence (Pearson r) on each output metric. Positive correlations in red, negative in blue — a single-glance view of which inputs matter most.
Results Table
Sortable table of all samples showing input values, output metrics, and pass/fail status with color coding for safety limit violations.
Input Parameters Available
Output Metrics Monitored
How is FuelSim UA Used?
Statistical uncertainty analysis for licensing, design, and manufacturing decisions
Licensing Support
Demonstrate with statistical confidence that safety limits are met across the range of expected input variability (e.g., “95% of rods remain below fuel melt temperature with 95% confidence”).
Design Margin Assessment
Quantify how much margin exists between predicted performance and regulatory limits to optimize fuel rod designs.
Manufacturing Specification
Determine which manufacturing tolerances have the greatest impact on fuel performance and whether tighter specifications are needed.
Sensitivity Screening
Rapidly identify the 2–3 input parameters that drive 80%+ of output variability, focusing future analysis and testing efforts.
Additional FuelSim UA Features
Safety Limit Checking
Computes the probability of exceeding regulatory limits on fuel melt, cladding temperature, oxide thickness, hoop stress, and strain.
Sensitivity Ranking
Pearson correlation coefficients identify which input parameters drive the most output uncertainty via tornado diagrams.
Reproducibility
Configurable random seed and saved configuration file for exact reproduction of any analysis.
How Does FuelSim Improve on FRAPCON-3?
Same validated physics with a modernized user experience
Descriptive Variables
Self-descriptive input names like FuelDensity instead of cryptic codes like den.
Organized Input
Input grouped by physical category with clear section headers for easier model setup.
Expanded Output
Additional variables in strip files (60+) for comprehensive post-processing and plotting.
Desktop GUI
Graphical front-end with file browser, drag-and-drop, one-click execution, and interactive charting.
3D Visualization
Interactive 3D surface plots and polar cross-section views with multiple colormaps and dual unit support.
Safety Limits Checking
Automatic flagging of results exceeding regulatory limits with color-coded visual indicators.
Parametric Sweeps
Built-in sensitivity analysis by varying input parameters and overlaying results.
PDF Reports
Export multi-page reports with input parameters, peak values, safety flags, and variable plots.
Batch & Regression
Batch execution of multiple cases and regression testing against reference datasets.
Input Validation
Pre-run checking of input parameters against physical bounds with error and warning messages.
Baseline Comparison
Load reference strip files and overlay results for side-by-side validation comparison.
Simplified Execution
Single executable with integrated input/output file management.
How is FuelSim Used in Industry?
Fuel Design Evaluation
Assess new fuel rod designs against thermal and mechanical performance criteria.
Licensing Analysis
Demonstrate compliance with regulatory limits on fuel temperature, pressure, strain, and oxide thickness.
Operating Limits
Establish power and burnup limits for safe fuel operation.
Experimental Validation
Validate fuel performance models against irradiation test data from international programs.
Parametric Studies
Investigate sensitivity of fuel behavior to design parameters and operating conditions.
Fuel Failure Analysis
Understand conditions leading to fuel rod failures for root cause investigation.
Technical Specifications
Proven fuel performance technology with modernized usability
Platform Support
macOS, Linux, Windows
GUI Requirements
Python 3 with Tkinter; matplotlib (with mplot3d) and NumPy for charting, 3D visualization, and PDF report generation
Input Format
Fortran NAMELIST with self-descriptive variable names
Output Formats
Printed output (.out), strip file (.strip) with 60+ variables, interactive 3D visualization, and PDF report export
Disk Space
Minimal (~10 MB for code and test cases)
Fission Gas Release Models
- ANS 5.4 model — Standard model from the American Nuclear Society
- Modified Forsberg-Massih — Diffusion-based model tracking intragranular and grain boundary gas (default)
- Additional models — Empirical and mechanistic options selectable via input
Validated Test Cases
| Test Case | Description |
|---|---|
| TypicalPWRRod | Generic PWR fuel rod |
| TypicalBWRRod | Generic BWR fuel rod |
| PWRHighBurnup | High-burnup PWR rod (56 steps) |
| BWRHighBurnup | High-burnup BWR rod (43 steps) |
| IFA-432 | Halden Reactor (Norway) |
| Riso F7-3 | Riso National Lab (Denmark) |
| BR-3 24I6 | BR-3 Reactor (Belgium) |
| GE-A1 | GE BWR test rod |
Related Software Solutions
FuelSim works alongside ISS system codes for complete nuclear safety analysis
RELAP5/SCDAPSIM
Use FuelSim steady-state results as initial conditions for RELAP5/SCDAPSIM transient and severe accident analysis.
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Visual drag-and-drop interface for building and analyzing RELAP5/SCDAPSIM and ASYST models.
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Next-generation BEPU code for advanced reactor types with integrated uncertainty analysis.
Discover ASYSTInterested in FuelSim?
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