Thermophysical and mechanical property assessment of UB2 and UB4 sintered via spark plasma sintering

Abstract

Uranium diboride (UB2) and uranium tetraboride (UB4) are candidate constituents for multi-phase accident tolerant fuel due to their anticipated high thermal conductivity. These fuels have high uranium density that contributes to fission, and by tailoring the ratio of 10B/11B, can also act as an integrated burnable poison. Understanding the thermophysical and mechanical properties of uranium borides, for which only limited data are available in the literature, is of importance to determine their accident tolerance. In this work UB2 and UB4 have been synthesized via arc melting and sintered to high densities via spark plasma sintering (SPS). High density samples, >90% theoretical density, were used to measure the thermal diffusivity and thermal expansion of UB2 and UB4 and, in conjunction with specific heat literature data, their thermal conductivities were calculated from 298 to 1773 K. Additionally, resonance ultrasound spectroscopy (RUS) and nanoindentation were performed to investigate the mechanical properties of the uranium borides. Our results are discussed in the context of available literature. Both UB2 and UB4 exhibit thermal conductivities higher than that of UO2, with UB2 having the highest. The thermal conductivity of UB2 increases with temperature above 874 K, while for UB4 there is a linear increase over the entire measured range. X-ray diffraction (XRD) results indicate that impurity phases were present in the fabricated materials, which could explain why literature density functional theory (DFT) results predict higher values. This suggests that if impurity phases or any microstructural defects can be eliminated then the thermal conductivity can be further increased.