Kevin T. Clarno

Liquid-fueled molten salt reactors present a complicated modeling problem due to the spatial distribution of radionuclides throughout the system. The mechanisms of transfer for species in the system vary widely depending on the location in the reactor. To accurately model the inventory in an off-gas system, all transfers throughout the system must be considered. In this document, following an introduction to fission products, a review of considerations for this modeling problem is presented. Modeling considerations are detailed, including discussion of the thermophysical properties such as volatility and solubility of species in molten salt. Mass transfer theory and applicable scenarios in molten salt, graphite, and helium bubbles is discussed following this. An extensive list of mass transfer correlations that may be applicable in molten salt reactor systems with explanations provided for each is also provided. The ability to detect species in an off-gas system is also discussed based on historical data. Lastly, a general framework for the numerical solution of the system is suggested along with future steps towards implementation of the methodology discussed.

This paper presents a reproducible reduced-order dynamic model of the Molten Salt Research Reactor (MSRR) in OpenModelica for three analyses: startup to criticality, nonlinear step transients, and power-dependent small-signal response. The model combines transport-aware modified point kinetics for circulating fuel, linear temperature-reactivity feedback, grouped decay-heat dynamics, and lumped thermal-hydraulic nodes in a shared framework with one-region and nine-region core options. The startup analysis explores a staged, source-assisted approach to criticality with progressive primary-flow increases and quantifies the associated delayed-neutron precursor drift compensation. The full startup sequence reaches approximately 1 MW during the final heat-removal ramp. The dynamics study explores step-response transients and multi-power sinusoidal frequency sweeps from 10^-3 to 10^1 rad/s, with accepted low-power reruns using extended settle times and reduced forcing amplitude to preserve small-signal behavior.Results show operating-point-dependent low-pass behavior and a modest core segmentation effect, with the nine-region model exhibiting about 3 dB lower low-frequency gain than the one-region model at nominal power. The primary contribution is an integrated and transparent workflow linking legacy reduced-order MSR dynamics methodology to an open-source, repository-based OpenModelica implementation suitable for design-stage operational and control studies.

Liquid-fueled molten salt reactors present a complicated modeling problem due to the spatial distribution of radionuclides throughout the system. The mechanisms of transfer for species in the system vary widely depending on the location in the reactor. To accurately model the inventory in an off-gas system, all transfers throughout the system must be considered. In this document, following an introduction to fission products, a review of considerations for this modeling problem is presented. Modeling considerations are detailed, including discussion of the thermophysical properties such as volatility and solubility of species in molten salt. Mass transfer theory and applicable scenarios in molten salt, graphite, and helium bubbles is discussed following this. An extensive list of mass transfer correlations that may be applicable in molten salt reactor systems with explanations provided for each is also provided. The ability to detect species in an off-gas system is also discussed based on historical data. Lastly, a general framework for the numerical solution of the system is suggested along with future steps towards implementation of the methodology discussed.

As the primary mechanism of passive heat removal in nuclear reactors, understanding natural circulation is paramount to their safe operation and shutdown conditions. Natural circulation in molten salt reactors is of particular relevance due to the fluids high Prandtl number as well as the phase of the fissile material in liquid-fueled reactors. Natural circulation loops are employed to study the thermal hydraulic behavior of fluids when subject to thermal gradients and small flow disturbances. The objective of this work is to introduce and analyze the experimental effects of local heating conditions on the velocity profile and near wall behavior (such as boundary layer thickness) of molten salts in a heated transparent test section. Particle image velocimetry (PIV) was performed, and the boundary layer was analyzed for three different heating conditions. Those conditions were applied to the transparent test section: a cooling test condition, a thermally isolated test condition, and a heated test condition. In addition, the other thermal conditions of the loop were held constant. As the test section is heated, the peak velocity and slope of the velocity profile increase with test section heater power.

Background Validation of neutron transport calculations for nuclear criticality safety requires selecting relevant benchmarks and consolidating multiple uncertainty sources to estimate application bias and uncertainty. Existing practices rely on sensitivity/uncertainty (S/U) techniques and heuristic similarity thresholds, but they lack a simple, uncertainty-aware metric for comparing validation outcomes across consolidation methods and benchmark sets. Methods We introduce a concept of score that quantifies validation performance as the posterior residual normalized by its associated uncertainty. The score is evaluated for four consolidation approaches—parametric, nonparametric, Whisper, and generalized least-squares methodology (GLLSM)—using S/U-based indices. The analysis considers keff predictions for representative sets of International Handbook of Evaluated Criticality Safety Benchmark Experiments benchmarks and corresponding applications. Results Score distributions reveal distinct behaviors across methods. Nonparametric and Whisper generally yield positively shifted means, reflecting conservative bias estimation by design. Parametric and GLLSM produce means closer to zero but can exhibit wider score spreads when selection is based solely on conventional similarity thresholds. Incorporating the scaled similarity index reduces spread and mean shift for the parametric method by accounting for sensitivity magnitude. For GLLSM, acknowledging additional prior sources of uncertainty not captured by nuclear data alone brings the empirical score spread closer to unity across relevance levels, indicating improved statistical consistency. Conclusions The score provides uncertainty-aware lens for comparing consolidation methods and guiding benchmark selection. Results support using scaled similarity with the parametric method and accounting for unmodeled prior uncertainty within GLLSM. The framework helps analysts select benchmarks and consolidation strategies that yield statistically consistent posterior keff predictions while maintaining defensible safety margins.

Background Validation of neutron transport calculations for nuclear criticality safety requires selecting relevant benchmarks and consolidating multiple uncertainty sources to estimate application bias and uncertainty. Existing practices rely on sensitivity/uncertainty (S/U) techniques and heuristic similarity thresholds, but they lack a simple, uncertainty-aware metric for comparing validation outcomes across consolidation methods and benchmark sets. Methods We introduce a concept of score that quantifies validation performance as the posterior residual normalized by its associated uncertainty. The score is evaluated for four consolidation approaches—parametric, nonparametric, Whisper, and generalized least-squares methodology (GLLSM)—using S/U-based indices. The analysis considers k eff predictions for representative sets of International Handbook of Evaluated Criticality Safety Benchmark Experiments benchmarks and corresponding applications. Results Score distributions reveal distinct behaviors across methods. Nonparametric and Whisper generally yield positively shifted means, reflecting conservative bias estimation by design. Parametric and GLLSM produce means closer to zero but can exhibit wider score spreads when selection is based solely on conventional similarity thresholds. Incorporating the scaled similarity index reduces spread and mean shift for the parametric method by accounting for sensitivity magnitude. For GLLSM, acknowledging additional prior sources of uncertainty not captured by nuclear data alone brings the empirical score spread closer to unity across relevance levels, indicating improved statistical consistency. Conclusions The score provides uncertainty-aware lens for comparing consolidation methods and guiding benchmark selection. Results support using scaled similarity with the parametric method and accounting for unmodeled prior uncertainty within GLLSM. The framework helps analysts select benchmarks and consolidation strategies that yield statistically consistent posterior k eff predictions while maintaining defensible safety margins.

This technical report aims to provide energy developers and policymakers with information and preliminary analyses on the potential for Texas coal plant sites to be repurposed for nuclear power. Investigation into coal-to-nuclear (C2N) has shown that constructing a nuclear reactor on the site of a retired coal plant has both economic and environmental benefits. The data presented includes operational details of the coal power plants, the presence of nearby hazards, geological and hydrological data, and population statistics. This information was gathered for 19 coal powered electricity generation sites in Texas. Thirteen of the sites assessed have no hazards or other factors that would disqualify them from hosting a Small Modular Reactor (SMR). Of these, 11 sites are also suitable for a Light Water Reactor (LWR). The smaller size and power output of SMRs makes these additional 2 sites possible, even near a population center. The remaining 6 coal plant sites would require more specific on-site analysis or potential adjustments to the reactor design to be considered for licensing. Information supporting these findings are presented here in detail and are intended to give interested parties easy access to relevant C2N conversion information. These results can be utilized to make informed decisions on this strategy for nuclear development and its benefits to grid stability and baseload power in Texas.