Synthesis and characterization of homogeneous (U,Am)O₂ and (U,Pu,Am)O₂ nanopowders

This paper details the first dedicated production of homogeneous nanocrystalline particles of mixed actinide oxide solid solutions containing americium. The target compositions were $U_{0.75}Pu_{0.20}Am_{0.05}O_{2}$, $U_{0.90}Am_{0.10}O_{2}$ and $U_{0.80}Am_{0.20}O_{2}$. After successful hydrothermal synthesis and chemical characterisation, the nanocrystals were sintered and their structure and behaviour under self-irradiation were studied by powder XRD. Cationic charge distribution of the as-prepared nanocrystalline and sintered $U_{0.80}Am_{0.20}O_{2}$ materials was investigated applying $U M_{4}$ and $Am M_{5}$ edge high energy resolution XANES (HR-XANES). Typical oxidation states detected for the cations are $U_{(IV)}/U_{(V)}$ and $Am_{(III)}/Am_{(IV)}$. The measured crystallographic swelling was systematically smaller for the as-synthesised nanoparticles than the sintered products. For sintered pellets, the maximal volumetric swelling was about 0.8% at saturation, in line with literature data for $PuO_{2}$, $AmO_{2}$, $(U,Pu)O_{2}$ or $(U,Am)O_{2}$.