Mitigating Localized Overheating Using Flow Optimization and Magnetohydrodynamic Power Augmentation in HALEU-Fueled NTP Systems
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hoak-shoak-phd-nse-2026-thesis.pdf
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Author(s)
Hoak, Steven M.
Advisor(s)
Shirvan, Koroush
Date Issued
February 2026
Publisher
Massachusetts Institute of Technology
Abstract
This dissertation examines the ongoing challenge of alleviating localized overheating in fuel elements within HALEU-fueled nuclear thermal propulsion (NTP) reactors. Drawing on a detailed characterization of the thermal response of the fuel–coolant system, the work investigates a series of advanced strategies to address these limitations. The first is an evolutionary strategy that intentionally alters the mass flow distribution within the fuel element to lower the peak fuel temperatures by strategically adjusting the geometry of the fuel elements. To assess these changes, one- and three-dimensional thermal models are developed, enabling analysis of internal heat transfer and associated reductions in peak temperatures, thereby demonstrating that thermal management via flow optimization is achievable. This finding establishes tailored mass flow as an effective first-order design strategy to prolong the lifetime of fuel elements and to broaden the operational range of thermally moderated HALEU-fueled NTP systems. The second is a revolutionary approach. Rather than relying solely on mass flow modification through minor changes to fuel element geometry, the revolutionary approach is to deliberately operate the reactor at a lower thermal outlet temperature and compensate for the loss in exhaust performance using external power conversion and magnetohydrodynamics (MHD) enhancement. To evaluate this approach, a He/Xe closed loop Brayton MHD power cycle is constructed and parameterized across reactor power, MHD generator performance, and geometric constraints to determine the practical limits of extracting electrical power from the designed reactor while maintaining acceptable core performance and criticality. The results show that only a narrow design space provides sufficient thermal margin for heat extraction without redesigning the reactor and even then only with tightly constrained pipe diameters, pressure drops, and radial placements within the reactor that respect criticality. Finally, the electrical power from the closed loop MHD generator system is coupled to an MHD accelerator to determine whether the lost exhaust performance can be restored. For reactor powers of 15-30 MWt , all modeled enthalpy extraction fractions provide more than enough electrical power to overcome the degraded Isp. At 5 MWt only the highest extraction fractions are marginally sufficient. Furthermore, while the hybrid NTP–MHD system is technically capable of recovering performance, it does so with substantial mass penalties. Mission-level analysis comparing the base-NTP architecture with SpaceX’s Starship concept indicates that, although the base-NTP design can achieve Starship’s proposed Mars mission parameters, it does so at a significantly higher cost. Overall, this study provides the first integrated assessment of flow-based thermal mitigation, closed loop MHD power generation, and MHD accelerator thrust augmentation for HALEU-fueled NTP reactors.
MIT Department
Massachusetts Institute of Technology. Department of Nuclear Science and Engineering
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