FuelSim UA is a statistical uncertainty analysis tool for FuelSim that automates Monte Carlo sampling of 18 input parameters from probability distributions, runs up to 10,000 FuelSim simulations, and analyzes output distributions across 11 interactive analysis tabs including histograms, CDFs, tornado diagrams, Sobol sensitivity indices, response surface modeling, surrogate optimization, and experimental calibration.

What input parameters can FuelSim UA vary?

FuelSim UA can vary 18 input parameters across 7 categories: fuel pellet (enrichment, density, pellet height), cladding (outside diameter, wall thickness), gap (gap thickness, fill gas pressure), coolant (system pressure, inlet temperature, mass flux), power (rod power, power ratio), geometry (bundle pitch, crud thickness), and model bias multipliers (fuel conductivity, fuel swelling, cladding oxidation, cladding creep).

What analysis visualizations does FuelSim UA provide?

FuelSim UA provides 11 interactive analysis tabs: Histograms with safety limit markers, Cumulative Distribution Functions (CDFs), Time-Series Overlay with percentile envelopes, Scatter and Correlation plots, Tornado Diagrams, Sensitivity Analysis (Sobol, Spearman, Pearson, partial correlations), Response Surface modeling, Surrogate-based Optimization, Box-Whisker Comparison, Calibration against experimental data, and a sortable Results Table.

COMPANION TO FUELSIM

FuelSim UA

Q: How does FuelSim UA support BEPU licensing?

FuelSim UA supports Best-Estimate Plus Uncertainty (BEPU) licensing methodologies by demonstrating with statistical confidence that safety limits are met across the range of expected input variability, computing exceedance probabilities for regulatory limits on fuel melt temperature, cladding temperature, oxide thickness, and hoop stress.

Q: How does FuelSim UA integrate with FuelSim?

FuelSim UA uses the same input format, executable, strip file format, and safety limits as FuelSim. Any FuelSim .in file can be used as a UA template. FuelSim UA can be launched independently or from the FuelSim desktop GUI, requiring no separate solver or file conversion.

Statistical Uncertainty Analysis for Nuclear Fuel Rod Performance

Back to FuelSim

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 model bias multipliers

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 Analysis Tabs

Histograms, CDFs, Time-Series, Scatter, Tornado, Sensitivity, Response Surface, Optimization, Box-Whisker, Calibration, Results Table

10K

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?

Eleven interactive analysis tabs 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.

Sensitivity Analysis

Advanced sensitivity methods with selectable algorithm: Sobol indices for nonlinear effects, Spearman rank for monotonic relationships, partial correlations to isolate individual parameter effects, and Pearson coefficients.

Response Surface

Polynomial or RBF surrogate model fitted to sample data, visualized as 2D filled contour or interactive 3D surface plot. Toggle between views with R-squared goodness of fit displayed.

Optimization

Surrogate-based design optimization to find input parameter values that minimize or maximize any output metric, with optional constraints on other metrics using L-BFGS-B on the fitted response surface.

Box-Whisker Comparison

Side-by-side box plots of all output metrics with P5/P95 whiskers. Subplots or normalized z-score mode for cross-comparison, with optional data point overlay and safety limit lines.

Calibration

Load experimental measurements from CSV, map to output metrics, select parameters to calibrate, and find optimal values using weighted least-squares minimization on response surfaces.

Input Parameters Available

Category	Parameters
Fuel Pellet	Enrichment, Density, Pellet Height
Cladding	Outside Diameter, Wall Thickness
Gap	Gap Thickness, Fill Gas Pressure
Coolant	System Pressure, Inlet Temperature, Mass Flux
Power	Rod Power, Power Ratio
Geometry	Bundle Pitch, Crud Thickness
Model Bias	Fuel Conductivity, Fuel Swelling, Cladding Oxidation, Cladding Creep Multipliers

Output Metrics Monitored

Metric	Description
Burnup	End-of-life burnup (MWd/MTU)
Fission Gas Release	Cumulative FGR fraction (%)
Peak Fuel CL Temp	Max fuel centerline temperature
Clad ID/OD Temp	Max cladding surface temperatures
Peak Hoop Stress	Peak cladding hoop stress
Oxide Thickness	Max cladding oxide layer
Plenum Pressure	Rod internal gas pressure
Stored Energy	Total stored energy in fuel rod

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.

Best-Estimate Plus Uncertainty (BEPU)

Support modern BEPU licensing methodologies as an alternative to conservative bounding analyses, as recommended by the IAEA and U.S. NRC.

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.

Advanced Sensitivity Ranking

Sobol indices, Spearman rank correlations, partial correlations, and Pearson coefficients identify which parameters drive uncertainty.

Reproducibility

Configurable random seed and saved configuration file for exact reproduction of any analysis.

How Does FuelSim UA Integrate with FuelSim?

FuelSim UA is designed to work seamlessly with the main FuelSim application — no separate solver or file conversion required.

Same input format — any FuelSim .in file can be used as a template
Same executable — shares the FuelSim solver, no separate installation
Same strip file format — reads standard FuelSim output for results extraction
Same safety limits — uses FuelSim’s built-in regulatory limits for pass/fail checking
Standalone or alongside — can be launched independently or from the FuelSim desktop UI

FuelSim UA System Requirements

FuelSim desktop application or standalone executable
Python 3 with NumPy and Matplotlib
Operating System: macOS, Linux, or Windows
Disk Space: ~1 MB per sample case (a 500-sample run uses ~500 MB)
Runtime: Approximately 2–5 seconds per case (varies with problem size)

References

Bratton, R.N., et al., “Rod Internal Pressure Distribution and Uncertainty Analysis Using FRAPCON,” Nuclear Technology, 2017.
Geelhood, K.J., et al., “FRAPCON-4.0: A Computer Code for the Calculation of Steady-State, Thermal-Mechanical Behavior of Oxide Fuel Rods for High Burnup,” PNNL-19418, 2015.
IAEA-TECDOC-1912, “Backward and Forward Uncertainty Quantification for Nuclear Fuel Behavior and Safety Analysis,” IAEA, 2020.

LATEST RELEASE

What's New in FuelSim 2026?

Significant new solver capabilities that expand the scope of fuel rod analysis and uncertainty studies

Bias/Uncertainty Multipliers

Four new model bias multipliers for direct quantification of physics model uncertainty, ideal for UA sampling:

Fuel Conductivity Multiplier — scales fuel thermal conductivity
Fuel Swelling Multiplier — scales fuel volumetric swelling rate
Cladding Oxidation Multiplier — scales cladding oxide growth rate
Cladding Creep Multiplier — scales cladding creep strain rate

All default to 1.0 (nominal). These complement the existing FuelThermExpanFact, GapConductanceFactor, FissGasReleaseFactor, and PowerMultiplier.

Transient Analysis

After steady-state irradiation, an optional transient phase simulates power ramps, LOCA blowdowns, and RIA pulses with adaptive timestepping and DNB detection. UA templates with transient enabled run both phases for each sample.

ATF Materials

Accident Tolerant Fuel support with new fuel types (U3Si2, UN) and cladding types (FeCrAl, Cr-coated Zircaloy, SiC/SiC) using physically-based property correlations. UA studies on ATF designs use the same workflow.

MOX Pu Isotope Input

Explicit plutonium isotope vector specification (Pu-239 through Pu-242) for MOX fuel UA studies with varying Pu isotopic compositions instead of default reactor-grade distribution.

Spent Fuel Storage

Post-irradiation wet or dry storage modeling including decay heat, temperature evolution, cladding creep under internal pressure, and gas pressure changes during storage.

Refabrication

Mid-life rod refabrication at a specified timestep for UA studies on reconstituted or experimental fuel rods with modified geometry and fill gas conditions.

Interested in FuelSim UA?

Request more information about FuelSim UA for your uncertainty analysis and BEPU licensing needs.

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