Actinide nitride ceramic transmutation fuels for the Futurix-FTA irradiation experiment

Full text of publication follows. The transmutation of plutonium and other minor actinides is an important component of an advanced nuclear fuel cycle. The Advanced Fuel Cycle Initiative (AFCI) is currently considering mono-nitrides as potential transmutation fuel material on account of the mutual solubility of actinide mono-nitrides as well as their desirable thermal characteristics. The feedstock is most commonly produced by a carbothermic reduction/nitridisation process, as it is for this programme. Fuel pellet fabrication is accomplished via a cold press/sinter approach. In order to allow for easier investigation of the synthesis and fabrication processes, surrogate material studies are used to compliment the actinide activities. Fuel compositions of particular interest denoted as low fertile (i.e. containing uranium) and non-fertile (i.e. not containing uranium) are (PuAmNp)0.5U0.5N and (PuAm)0.42Zr0.58N, respectively. The AFCI programme is investigating the validity of these fuel forms via Advanced Test Reactor (ATR) and Phenix irradiations. Here, we report on the recent progress of actinide-nitride transmutation fuel development and production for the Futurix-FTA irradiation experiment. Furthermore, we highlight specific cases where the complimentary approach of surrogate studies and actinide development aid in the understanding complex material issues. In order to allow for easier investigation of the fundamental materials properties, surrogate materials have been used. The amount of surrogate in each compound was determined by comparing both molar concentration and lattice parameter mismatch via Vegard Law. Cerium was chosen to simultaneously substitute for Pu, Am and Np, while depleted U was chosen to substitute for enriched U. Another goal of this work was the optimisation of added graphite during carbothermic reduction in order to minimise the duration of the carbon removal step (i.e. heat treatment under H2 containing gas). One proposed solution is to switch to an atmosphere containing hydrogen buffered by either N2 or an inert gas. Preliminary experiments have suggested that carbo thermically reducing and nitriding material in powder form more effectively removes carbon than in pellet form. The fabrication of actinide nitride fuel pellets at the Los Alamos National Laboratory (LANL) for the Futurix-FTA irradiation experiment in the Phenix reactor was performed using improved parameters for solutionisation and carbothermic reduction developed using surrogate materials. Improved powder processing techniques identified in previous work were used as the foundation for the process parameter development for the Futurix-FTA fabrication; however, several differences exist between the Futurix-FTA and previous actinide nitride fabrications. These differences are discussed along with the resolution path leading to fuel pellet production. (authors)