Fabrication and thermophysical properties of UO₂-UB₂ and UO₂-UB₄ composites sintered via spark plasma sintering

Uranium dioxide (UO₂) composites with uranium diboride (UB₂) and uranium tetraboride (UB₄) have been proposed as advanced fuel candidates due to their high thermal conductivity, high melting point, high fissile density and their ability to incorporate a built-in burnable poison by tailoring the targeted ¹⁰B/¹¹B ratio. As such, it is important to assess the fabrication, and thermal and micromechanical properties of such composites. In this work, UO₂-UB₂ and UO₂-UB₄ samples with boride phase fractions of 5, 15 and 30 wt% were fabricated to high densities (above 95% theoretical density) via spark plasma sintering (SPS). This enabled sintering at relatively low temperatures and short timescales. SPS also aided in maintaining the target phase fractions of the samples as reactions between the constituent phases were suppressed due to the short timescales and reducing environment during sintering. Thermal diffusivity measurements from 299 to 1273 K were conducted through laser flash analysis (LFA). The diffusivity increased as a function of boride weight fraction, and UB₂ additions increased the thermal diffusivity of the composites more than UB₄ additions. Assessment of the LFA results indicated that in-situ reactions between the UO₂ and boride phases that suppress the thermal diffusivity occur above 800 K for all samples. Oxidation of the boride phase was proposed as the underlying reaction. This was supported by thermodynamic assessments from the literature, as well as microstructural, crystallographic, and nanoindentation characterization performed on these samples.