Award Date
12-15-2025
Degree Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Mechanical Engineering
First Committee Member
Yi-Tung Chen
Second Committee Member
Hui Zhao
Third Committee Member
Melissa Morris
Fourth Committee Member
Alexander Barzilov
Fifth Committee Member
Monika Neda
Number of Pages
153
Abstract
The inclusion of Molten Salt Reactors (MSRs) as one of the six advanced nuclear reactor concepts in the Generation IV International Forum (GIF) program underscores their growing importance in the future of nuclear energy [1]. MSRs offer several key advantages over traditional solid-fueled reactors, such as atmospheric pressure operation, higher operational temperatures, and continuous online fuel reprocessing [2]. These characteristics enhance the thermodynamic efficiency and safety of the nuclear power plant, while reducing risks associated with high pressure systems and solid fuel degradation. Originating from the Molten Salt Reactor Experiment (MSRE) conducted at Oak Ridge National Laboratory (ORNL) in the 1960s [3, 4], MSRs have since evolved significantly. The MSRE demonstrated the feasibility of using liquid fuel, validated thermal-hydraulic behavior, and provided a valuable foundation for further research. However, accurately modeling the unique behavior of MSRs, particularly during transients, remains a substantial challenge due to the continuous motion of the liquid fuel, which impacts reactor kinetics [5].
This dissertation presents a methodology for modeling MSR kinetics by extending the Monte Carlo NParticle Transport Code (MCNP) to account for fuel circulation effects. The liquid nature of MSR fuel causes delayed neutron precursors (DNPs) to migrate from their birth locations, leading to the decay of a substantial portion of DNPs outside of the core region. This behavior directly impacts the effective delayed neutron fraction (βeff), a critical parameter influencing nuclear reactor control and safety. Conventional modeling approaches, such as multigroup diffusion (MGD) methods, are limited by high computational costs, sensitivity to energy group structures, and reduced spatial accuracy near material boundaries. Additionally, they are not well suited to the complex reactor geometries of MSRs.
To overcome these limitations, a point reactor kinetics (PRK) model was employed and coupled with computational fluid dynamics (CFD) simulations in ANSYS FLUENT. The PRK model, which simplifies neutron behavior using ordinary differential equations, provides a cost-effective means of simulating transients without the need for spatial resolution. This approximation is particularly suitable for MSRs, where flow driven DNP transport plays a critical role. By integrating PRK into FLUENT through userdefined functions (UDFs), a dynamic coupling between neutron kinetics and thermal-hydraulics is achieved, enabling real-time feedback modeling during transient events. This integration allows for the simulation of Doppler and temperature feedback effects, offering enhanced fidelity for safety and design evaluations.
This work benchmarks the developed methodology against the Molten Salt Fast Reactor (MSFR) concept from the Evaluation and Viability of Liquid Fuel Fast Reactor Systems (EVOL) project and validates it using the experimental data from the MSRE. CFD simulations provide detailed velocity fields used by MCNP to evaluate βeff. This allows for detailed tracking of how DNPs are transported throughout the reactor system, including phenomena such as vortex recirculation and boundary layer effects. These fluid dynamic behaviors significantly impact where and when DNPs decay, which in turn affects the reactor's transient response. By using separate, optimized meshes for thermal-hydraulics and neutronics, the method avoids the issues associated with a single mesh, thereby improving both computational efficiency and accuracy. One of the key insights gained from this study is the influence of recirculation zones on DNP behavior. The vortex recirculation of the fluid flow traps DNPs in the core at high recirculation velocity and increases removal of DNPs from zones of high neutron importance at low recirculation velocity, which significantly affects βeff. These insights offer valuable guidance for reactor designers seeking to optimize the flow patterns within an MSR to achieve desired control characteristics.
Furthermore, this research explores the potential for using variations in fuel salt velocity as a control mechanism. The study examines how variations in flow rate influence the residence time of fuel in both the reactor core and heat exchangers, thereby affecting fission rates, temperature distribution, and heat removal efficiency. At lower flow rates, fuel salt remains longer in the core, resulting in greater energy deposition and higher temperature increases, while also spending more time in heat exchangers, enhancing heat transfer. Conversely, higher flow rates reduce core residence time and moderate temperature rises. These thermal effects directly impact reactivity through MSRs’ inherent negative temperature feedback, enabling passive and responsive reactivity control. This self-regulating behavior can be strategically enhanced by adjusting fuel salt velocity, offering a flexible, safety enhancing operational approach. The findings underscore the unique advantages of MSRs in achieving stable, efficient power generation under varying load conditions through thermally coupled flow control.
Controlled Subject
Nuclear reactors; Thermodynamics; Dynamics
Disciplines
Engineering | Mechanical Engineering | Nuclear Engineering
File Format
File Size
2600 KB
Degree Grantor
University of Nevada, Las Vegas
Language
English
Repository Citation
Nguyen, Thanh Hung, "Modeling of Neutronics and Thermal-Hydraulics of Molten Salt Reactors Based on The Modified Point Reactor Kinetics Model" (2025). UNLV Theses, Dissertations, Professional Papers, and Capstones. 5451.
https://oasis.library.unlv.edu/thesesdissertations/5451
Rights
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