Preparation and characterisation of Pu$_{0.5}$Am$_{0.5}$O$_{2-x}$-MgO Ceramic/Ceramic composites

Abstract : This study describes the preparation and characterization of Pu$_{0.5}$Am$_{0.5}$O$_{2-x}$-MgO ceramic/ceramic (cercer) composites with 20 and 30 vol% of Pu$_{0.5}$Am$_{0.5}$O$_{2-x}$. The sintered materials demonstrated very different reduction behavior when exposed to a reducing sintering cycle. The composites were studied by combined X-ray diffraction (XRD) and oxygen-to-metal ratio measurements and exhibited various amounts of body-centered-cubic (bcc) and face-centered-cubic (fcc) phases corresponding to different reduction states of the mixed actinide oxide. The fcc phases correspond to a near stoichiometry phase while the bcc phases are attributed to most reduced phases, which demonstrate a greater similarity with the Am$_2$O$_3$ bcc phase. The XRD results suggest a reduction of Am prior to Pu, which explains this greater similarity. In addition, the 30 vol% composite contains 65 wt% of the bcc phase while the 20 vol% composite exhibits only 29 wt%. This result can be explained by the percolation theory when applied to the oxygen diffusivity and indicates that a threshold value for Pu$_{0.5}$Am$_{0.5}$O$_{2-x}$ content in the cercer composite exists where the reduction of the mixed oxide significantly increases.