Production of High-Density Uranium Carbide Compacts for Use in Composite Nuclear Fuels

Abstract

Composite nuclear fuel consisting of uranium carbide (UC) fuel microspheres dispersed in an inert matrix is one of the fuel forms being actively considered for use in gas-cooled fast reactors (GFRs). High-density UC electrodes were required for the production of fuel microspheres by the rotating electrode method as an alternate method to the sol-gel particle production route. These compacts (to serve as electrodes) were fabricated by the exothermic combustion synthesis reaction of uranium hydride and graphite powders. Ignition of combustion synthesis was then followed by solid-state sintering at different temperatures of 1521, 1779, and 1929°C. During the course of testing the electrodes for microsphere production, it was found that the structural integrity of the electrodes and thus their suitability for microsphere production depended on the microstructural characteristics of the electrodes. Those produced at higher temperatures (1929°C) had higher densities (86.6% theoretical density) and lower open porosities (2.3%) and were shown to withstand the mechanical forces and thermal stresses imposed by this microsphere production method. The processing conditions were chosen to evaluate sintering characteristics of UC and to the extent possible to find the lowest possible process temperature. Here it is understood that the intended future GFR fuel form should involve recycled fuels including minor actinides (MAs). Concern over MA volatility in high-temperature processes thus motivated investigating the effects of lower processing temperatures. It was deduced from this study that a delicate balance exists between the processing parameters, the microstructural characteristics of the electrodes, and microsphere production.