Conference Agenda

Tritium extraction is a universal challenge in advancing all types of nuclear power: fission plants must dispose of, and fusion plants must fabricate fuel from it. Gas-liquid contactors (GLCs) which leverage two-phase dynamic mixing to supercharge the mass transfer of tritium are an untapped concept in the field. In theory GLCs should be capable of achieving more surface area than static liquid-solid interface extractors with sufficiently small & many bubbles, but bubble coalescence has been a major challenge. By building on Volume of Fluid (VoF) multiphase simulation support in the computational fluid dynamics tool OpenFOAM, we developed an extensible Continuum Species Transport (CST) solver for the loosely coupled concentration field of a dilute miscible species. This establishes a new framework for the design and pre-experimental evaluation of novel GLC concepts in mass transfer applications for molten salt. To demonstrate the utility of our new solver, a simple evaluation of a bubble column with one large Ar inlet against four small Ar inlets is performed, showing a straightforward correlation between bubble size and tritium extraction efficiency.

Molten salt reactors (MSRs) have gained much interest in the nuclear community over the past few years, and efforts are currently being made in the design and deployment of a molten salt research reactor (MSRR) at Abilene Christian University. Multiple experimental salt loops are being designed to test various aspects of MSRs and understand the fluid dynamics of molten salts at a higher level. One such experiment is a bubble flow salt loop built at Texas A&M. High fidelity models of this experiment have been constructed, utilizing the computational fluid dynamics (CFD) modules of the MOOSE software. These CFD models, being at a high fidelity, may provide important information and a more wholistic view of the mechanics of molten salts, and may be included as a digital twin (DT) component in the experimental loop, as well as provide useful information to the MSRR. However, these CFD models require computationally expensive runs to provide reasonable and usable data. To combat this, a reduced order model (ROM) algorithm will be developed for these CFD models, solving these high fidelity and high dimensional problems in a significantly lower dimensional latent space, reducing cost with acceptable losses in accuracy. For these models, various ROM algorithms are created, using methods such as proper orthogonal decomposition (POD), neural networks (NN), and convolutional neural networks (CNN). These algorithms are then tested in an offline mode, comparing the forward propagation of the lower dimensional problem against the propagation of the full dimensional model.

The Stable Salt Reactor (SSR) integrates features of molten salt reactor technology with conventional light water reactor fuel assembly designs to achieve enhanced safety and economic benefits. Utilizing a fast reactor configuration, the SSR employs recycled nuclear waste as fuel, contained within salt-filled fuel pins submerged in a liquid salt coolant. Effective heat transfer between the molten fuel salt and coolant salt is critical to the reactor's core safety and operational reliability.

This study investigates conjugate heat transfer (CHT) processes in the SSR's narrow salt-filled fuel pins using the high-fidelity spectral element computational fluid dynamics (CFD) code, NekRS. The analysis encompasses internal natural convection within the molten fuel salt and external forced convection in the liquid salt coolant under both normal and transient conditions. Parametric studies are conducted to assess the influence of reactor power and coolant flow rates on heat transfer performance. The resulting data are leveraged to develop and validate heat transfer models for integration into the SAM system code, facilitating efficient transient safety analyses. The findings of this work refine safety system models, enhance the predictive accuracy of SSR core designs, and quantify uncertainties in molten salt CHT simulations. This study highlights the critical role of advanced CFD technologies in expediting the engineering design and licensing of next-generation nuclear reactors like the SSR.

Molten Salt Reactor (MSR) is currently one of the most promising Generation IV reactors, actively being developed internationally due to its high economic efficiency and safety. While evaluating the economic feasibility of developing MSR is important, assessment of various accident scenarios is also required for safety evaluation. One of the most anticipated scenarios in MSR is a salt spill accident caused by the crack or rupture of reactor pipes and the reactor vessel. In such cases, it is necessary to effectively cool the spilled molten salt and contain it within the desired location such as drain tank. To design an efficient molten salt transport structure, a detailed analysis of the molten salt behavior is essential. Furthermore, there is a possibility of releasing fission products into the atmosphere in the form of aerosols from the molten salt, for which boundary conditions can be provided. Lagrangian-based Computational Fluid Dynamics (CFD) calculations are considered a more advantageous numerical method for analyzing solidification behavior compared to Eulerian CFD methods. This is due to its meshless analysis characteristics, which allow free changes of boundaries between fluid and wall according to phase changes. In this study, the Moving Particle Semi-implicit (MPS) method was used to analyze salt spill behavior in MSR. To accurately simulate behavior accompanied by solidification, a model was developed to account for heat transfer and wall adhesion based on phase changes. A comparative analysis was conducted with results from other numerical methods, including Eulerian-based analysis.

Two-phase flow in Molten Salt Reactors (MSRs) is important as it impacts reactivity evolution, reactor transient response, and the removal of species dissolved in the molten salt through gas phase transfer. Therefore, accurately predicting the gas distribution and the associated liquid-gas interface area in MSRs is essential for their design and operation.

Recently, we integrated two new models into Idaho National Laboratory (INL)’s Multiphysics Object-Oriented Simulation Environment (MOOSE): a multi-D generalization of a mixture drift-flux model and a Euler-Euler model. The Euler-Euler model offers higher fidelity, while the mixture drift-flux model provides greater computational efficiency, which is typically preferred for modeling reactor transients. However, the mixture model's accuracy in capturing void distribution and interfacial area in MSRs still needs to be assessed.

This article begins with a description of the mathematical framework for the two-phase models implemented in MOOSE. It then presents validation of these models against relevant experimental data. Finally, both models are applied to the Molten Salt Reactor Experiment case study, analyzing various operational conditions such as different rates of fission product volatilization and diverse cover gas entrainment scenarios at the reactor pump. The article concludes by assessing the suitability of both models for capturing the two-phase flow dynamics critical to MSR operations.