Conference Agenda

The development of Generation-IV (Gen-IV) reactors is accelerating globally to enhance safety and support diverse applications beyond electricity generation. Among these, the Molten Salt Reactor (MSR) stands out for its use of molten salt as fuel and coolant, enabling high operating temperatures and efficient heat transfer. This design offers inherent safety advantages, such as reduced meltdown risk and passive safety features. In Molten Salt Fast Reactors (MSFRs), the Gaseous Fission Products (GFPs) were removed by helium bubbles. Additionally, the helium bubbles could be used to control reactor reactivity. However, the complex interactions between helium bubbles and molten salt present challenges that traditional computational methods struggle to predict.

Understanding the dynamics of helium bubbles is essential to model these interactions accurately in MSFRs. Also, it could help to improve the efficiency of fission gas removal and the accuracy of the model that describes the physical phenomena in numerical simulation. Despite its importance, helium bubble dynamics have not been thoroughly explored. To address this, a multi-physics framework was implemented using the Volume of Fluid (VOF) method to track gas-liquid interfaces and the PoliMi neutronics model to simulate reactivity changes driven by helium bubbles.

Numerical simulations were conducted to study the impact of helium mass flow rates and injection points on bubble motion, deformation, and distribution. The results enhance our understanding of multi-phase flow dynamics in MSRs and provide critical insights for optimizing reactor performance. Moreover, the findings offer valuable data for AI-based analyses, aiding the design of safer, more efficient reactors.

Fission products in liquid-fueled molten salt reactors are often categorized as soluble salt-seekers, weakly soluble noble gases, and weakly soluble noble metals. Noble metal fission products include Mo, Tc, Nb, Ru, Te, Ag, etc. Based on the operation experience from the Molten Salt Reactor Experiment, these noble metals tend to separate from the salt phase and migrate to the interfaces presented in the reactor system, including heat exchangers, graphite moderator, entrained cover gas bubbles, liquid surface in the pump bowl, etc. The uncontrolled migration and deposition of noble metals negatively impacts the neutronics, radiation protection, and thermal-hydraulics of the reactor. In this study, molecular dynamics (MD) simulation is used to investigate the microscopic behavior of representative noble metal constituents in molten salts. The polarizable ion model is implemented in the LAMMPS code and open-sourced. The code implementation is verified against theoretical results and existing simulation study with CP2K. The model is validated against the experimental density, viscosity, and diffusivities of the base salt. After the verification and validation, the diffusivities of the noble metals in typical fuel salts are simulated, and comparison is made with the Stokes-Einstein correlation. Lastly, preliminary studies on the aggregation of noble metal molecules in molten salts are presented. This phenomenon is important as the formation of critical nucleus of noble metals from aggregation is the first step in the migration of noble metals in liquid-fueled molten salt reactors.

Pronghorn, a thermal-hydraulics code developed within Idaho National Laboratory's Multiphysics Object-Oriented Simulation Environment (MOOSE), has been adapted to model tritium production and transport in the molten salt tritium breeding blankets of fusion reactors. This work highlights recent developments in Pronghorn that enable detailed simulations of two-phase flows, mass transfer between phases, and the volatilization of tritium in molten salt systems—critical aspects for sustainable tritium production, fusion system safety, and tritium management strategies. At the core of Pronghorn's capabilities for this application are its two-phase mixture models, which allow for the simultaneous tracking of liquid and gas phases. These models incorporate mass transfer mechanisms that control tritium migration between the molten salt and gas phases. The integration of Thermochimica, a thermochemical equilibrium solver, provides accurate modeling of tritium volatilization and chemical speciation in molten salts, enabling a comprehensive understanding of tritium behavior under fusion system operating conditions. A case study is explored, focusing on tritium production in a molten salt blanket and its transport to the gas phase. The impact of key factors such as temperature gradients, flow patterns, and salt composition on tritium release and containment is examined. Finally, future development directions are discussed, aimed at further enhancing Pronghorn's predictive capabilities for tritium dynamics in molten salt tritium breeding blanket systems.

The analysis of advanced reactor concepts such as the Molten Salt Reactor (MSR) requires the development of new modelling and simulation tools to deal with the characteristic features brought by the innovative design. One of the peculiar aspects of liquid-fuel reactors such as the MSR is the mobility of fission products (FPs) in the reactor circuit. Some FP species appear in the form of solid precipitates carried by the fuel flow and can deposit on reactor boundaries (e.g., heat exchangers, fuel containment walls), potentially representing design issues related to the degradation of heat exchange performance or radioactive hotspots. The solid FPs tracking is therefore relevant for the prediction of these phenomena. For this problem, both the Eulerian-Eulerian (E-E) and Eulerian-Lagrangian (E-L) approaches can be used, however, while the former can only track a scalar field representing the average concentration of FPs, the latter allows to individually track the behaviour of solid particles inside the reactor domain. Treating the particles as physical bodies instead of scalar fields allows for a proper introduction of the phenomena influencing its behaviour, especially for deposition. For this reason, an E-L based solver is verified against an analytical case. This case was previously developed for the verification of an E-E multiphysics solver developed at Politecnico di Milano. The benchmark case was adapted for an E-L approach in OpenFOAM with the modification of a pre-existing solver. The verification was done by comparing the solid FPs concentration profiles obtained by the CFD simulation and the analytical case.

Insoluble fission products, including noble metals and noble gases, can significantly impact the operation of molten salt reactors. For example, noble metals tend to deposit on structural surfaces, potentially altering local heat transfer capabilities and, in severe cases, clogging narrow tubes like those found in heat exchanger. Meanwhile, noble gases like Xe-135, which has a high neutron absorption cross-section, must be efficiently removed from primary loop to minimize reactivity effects. The removal of these insoluble fission products from the primary loop is typically achieved through a gas sparging process, where the characteristics of the bubbles play a crucial role in determining the efficiency of insoluble fission products mass transfer to circulating bubbles. Research indicates that noble metals form surfactants at the bubble interface, making the bubble interfaces more rigid and thereby decreasing the efficiency of mass transfer. However, the precise impact on fissional products removal due to surfactants has not yet been fully explored. This study addresses this gap by modeling a time-dependent mass transfer coefficient, mimicking the gradual surface contamination process on cover gas bubbles. It further examines how this varying coefficient influences the distribution of noble metals throughout the MSRE loop. This approach enables a quantitative analysis of how surface surfactants affect the efficiency of mass transfer to circulating bubbles. The findings can provide valuable insights into optimizing fresh cover gas injection frequency, ultimately improving the removal of insoluble fission products from MSR primary loop.

The decay product of molybdenum-99 (Mo-99), technetium-99 m (Tc-99m), is a common, short-lived radioisotope used in medical imaging. Several technologies are being investigated as alternative means for producing Mo-99. Online extraction from Molten Salt Reactors (MSRs) through electrochemical deposition is one such technology that is used as the motivation for this paper.

The purpose of this paper is to perform a validation, verification, and uncertainty qualification study on a flow past a cylinder model acting as a simplified model of online Mo-99 deposition in an MSR. One reason for this study is to examine the feasibility of using Reynolds Averaged Navier Stokes (RANS) models to accurately simulate mass transfer on a cylinder in external flow. The RANS results are compared to experimental and Large Eddy Simulation (LES) heat transfer results to determine their accuracy because mass and heat transfer are analogous. Another reason is to determine the mesh complexity needed to produce accurate results. An extensive mesh independence and input uncertainty study is performed on each baseline mesh.

From the validation, verification, and uncertainty quantification study, RANS models are determined to be accurate before the separation angle of the cylinder but overestimate the mass transfer in the wake region. LES is needed to estimate this turbulent recirculation region. A fully complex mesh usually used in flow past cylinder simulations is not needed for mass transfer simulations with RANS models. Simpler meshes are sufficient and reproduce similar results while reducing the computational time.