Synthesis Route for the Safe Production of Targets for Transmutation of Plutonium and Minor Actinides

Abstract

Use of composites of actinide oxides dispersed in a Mo metal matrix is a recent inert matrix fuel concept for the transmutation of Pu and the minor actinides (Np, Am, and Cm). These elements are present in spent nuclear fuel, and their long-term radiotoxicity can be minimized if they are recovered from the fuel and irradiated in dedicated targets in nuclear reactors. The synthesis of such highly radioactive fuels is not simple, and given the high radiotoxicity of Am, the safety of operation of such a process must be examined for production of small-scale analytical batches. Infiltration of americium nitrate into porous PuO₂ beads has potential safety bonuses. The beads are produced by a sol-gel external gelation route. Tests have been developed here with CeO₂, as a surrogate for PuO₂, and have been optimized for both bead production and pelletization of a blend of calcined beads and Mo powder. Addition of carbon to the sol-gel feed solution and its subsequent pyrolysis provides a means to optimize the porosity of the oxide beads. The carbon acts as a pore former. The highest-quality product meeting typical fuel specifications required addition of 20 g/l carbon in the sol-gel feed and calcination of the CeO₂ beads at 800°C. Subsequent Mo cermet composites with 20 or 40 vol% of ceramic reached densities in excess of 90% of the theoretical value as is required for nuclear reactor applications. Finally, the step from CeO₂ surrogates to (Pu, Am)O₂ targets has been made and pellets of excellent quality produced.