Date of Award

Summer 2023

Document Type

Open Access Dissertation


Nuclear Engineering

First Advisor

Travis W. Knight


Conceptual design and preliminary safety analysis of a proposed helium-cooled Micro Nuclear reactor In ONe megawatt (MINION) thermal output is done through three-dimensional computational analysis. Compared with the conventional high temperature gas-cooled reactors (HTGR) that use tri-structural or bi-structural isotropic (TRISO or BISO) fuel particles, MINION uses traditional clad fuel pellets. This greatly simplifies the core design and allows for a substantially more compact design thanks to the fuel’s signif-icantly higher U-235 atom density. Uranium carbide (UC) enriched up to 19.75% in U-235 is used as fuel. In the power-flattened design, U-235 enrichment varies radially between 19.75% and 10.5% in four concentric rings. An additional design is investigated that uses 4.95% enriched U-235. A variety of reactor materials have been investigated. The refer-ence design uses a combination of UC as fuel, SiC as cladding, BeO as moderator and reflector, and He as coolant. The Pareto optimization methodology is used to optimize re-actor core design in a multi-dimensional trade space of competing parameters. The final reactor is 2 m tall, including the reactor pressure vessel (RPV). The reactor core is a 0.96 cm3 right circular cylinder with the axial and radial reflector components. Six control drums and four control rods ensure redundant reactivity control to satisfy the single failure criterion (SFC) and to provide defense-in-depth (DiD). The core has a lifetime of at least 10 years with enough excess reactivity. Full-core neutronic and thermal-hydraulic simula-tions done using nuclear safety analysis and design modeling suite SCALE and computa-tional fluid dynamics (CFD) code STAR-CCM+ modeled fuel depletion, heat generation, and heat transfer performance in a soft-coupled iterative manner. Thermal energy genera-tion is modeled for individual fuel rods to account for the hot channel factor (HCF) as well as the power peaking factor (PPF). MINION operates at the lower end of the typical HTGR operating pressure (3 MPa) to ensure increased safety and portability for mobile applica-tions. During nominal operation, an average coolant inlet mass flow rate of 0.557 kg/s provides an average coolant exit temperature of 670 °C. At the steady state, the maximum allowable cladding temperature is maintained at ~1000 °C, resulting in a peak fuel center-line temperature of ~1250 °C. An additional design is investigated using annular fuel pel-lets to improve the bulk coolant exit temperature. This modification led to a 15% increase in coolant exit temperature (760 °C). Failure modes and effects analysis (FMEA) is carried out by investigating pressurized and depressurized loss of forced cooling (PLOFC and DLOFC) and air ingress accident scenarios under the influence of decay heat. The low power density (6 W/cc) of MINION, along with the large thermal inertia provided by the beryllia moderator and reflector, as well as the large surface-to-volume ratio of the core ensure passive removal of decay heat through conduction and radiation. The peak fuel and cladding temperature do not exceed their nominal operating values at any point during the transient events. Thus, no external cooling mechanism is required for MINION under pos-tulated accident scenarios, equipment failures, or malfunctions.