UC Berkeley

An understanding of the relationship between the processing, properties, and performance of any potential fuel form is essential to define the limits of the design of a nuclear reactor. Thermal conductivity, which is highly dependent on temperature, chemistry, and crystal structure, is a key predictor of fuel performance under irradiation. Over ~80% of the energy produced during fission is deposited as heat in the fuel, and this energy must dissipate by thermal conduction. Heat retention near the centerline, a problem in oxide fuels which exhibit low thermal conductivity that decreases with increasing temperature, contributes to deformation and cracking of the fuel over time. Ultimately, the onset of melting at the centerline of the fuel pellet limits the power density of the core. Changes in chemistry from oxide to non-oxide ceramic fuels are associated with trade-offs in various intrinsic material properties. Generally, a departure from oxides results in a marginally lower melting point, but significantly higher thermal conductivity across all temperatures relevant to the normal operation of a nuclear reactor. For the case of fixed linear power density from fission across the fuel and for the same period of fuel burnup, such an increase in thermal conductivity results in reduced centerline temperatures and a reduction in the thermal gradient from centerline to surface. Fuel forms which may retain a higher thermal conductivity as a function of burnup will improve fuel performance by mitigating temperature dependent degradation by fuel swelling, grain growth, and fission gas release. In conjunction with measurements of thermomechanical properties, the thermal stress and thermal shock resistance of the fuel form may be characterized as a function of temperature and radius throughout the fuel pellet.

In what follows, a novel fabrication technique for the production of high purity thorium mononitride by carbothermic reduction to nitridation is presented. Greater than 90% dense fuel pellets were created by cold pressing and high temperature sintering. A parametric sintering study was conducted to determine the effect of a variety of pressure, temperature, time, and gaseous conditions on sintering kinetics. Thermophysical properties, including heat capacity, coefficient of thermal expansion, and thermal diffusivity were measured in a flowing high purity argon environment as a function of temperature up to 1700 K. Thermal conductivity was determined as a function of temperature from the thermal property measurements, and is discussed in the context of a mixed thorium and uranium nitride fuel architecture. Room temperature hardness and elastic properties of ThN and UN were determined by indentation and resonant ultrasound spectroscopy techniques. This data, in combination with thermophysical property measurements, was used to estimate hoop stress as a function of temperature and radius, thermal shock resistance, and the Grüneisen parameter for both ThN and UN. The favorable thermal conductivity in ThN yields a reduction in thermomechanical stress by up to a factor of five and results in an order of magnitude greater stress-normalized thermal shock resistance, compared to UN. The Grüneisen parameter, a necessary input in interatomic force constant (IFC) models concerning phononic contributions to thermal conduction, has been determined by estimation of the speed of sound in UN and ThN from measurement of the bulk modulus and density. Mixtures of (ThxU1-x)N were prepared and the thermophysical and thermomechanical properties were measured up to 1700 K. The measured experimental parameters of the pure mononitrides and the mixed nitrides were incorporated in MCNP models for a small compact reactor design which is currently under development at Los Alamos National Laboratory. This comprehensive dissertation on mixed (ThxU1-x)N fuel architecture supports modeling efforts of small modular reactors, micro-reactors, and other compact reactor concepts by providing essential thermophysical and mechanical property data not available in the literature and which are necessary inputs to such models.