Production of Spherical UO2-UC2 for Nuclear Fuel Applications Using Thermochemical Principles

A process for the fabrication of uranium dioxide‐uranium dicarbide microspheres for use as an advanced nuclear fuel is described. The uranium‐carbon‐oxygen phase diagram was used extensively in applying thermochemical principles to the combined process of uranium carbide synthesis and kernel sintering. Variation of the partial pressure of carbon monoxide during the carbothermic reduction of urania plus carbon allowed the kernel composition and density to be controlled. X‐ray diffraction, microstructural examination, and detailed chemical analyses were used to identify the kernel composition. A procedure was developed to convert urania plus carbon microspheres produced by a wet‐chemical gelation process to a highly dense UO2−UC2 product at 1550°C. Kernels were first treated at 1550°C in Ar‐1% CO for 4 h to produce high‐density microspheres with a composition of UO2+UCx:Oy, (x +y≤1.1). These kernels were then processed at 1550°C for an additonal 4 h in Ar‐3% CO to shift the thermodynamic equilibrium from UO2+UCxOy, to the desired UO2+UC2. Batches of material containing different initial amounts of carbon were processed to produce high‐density microspheres having specific UC2 contents.