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 $U_{0.75}P{u}_{0.20}A{m}_{0.05}{O}_2$, $U_{0.90}A{m}_{0.10}{O}_2$ and $U_{0.80}A{m}_{0.20}{O}_2$. 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 $U_{0.80}A{m}_{0.20}{O}_2$ 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 $A{m}_{(III)}/A{m}_{(IV)}$. 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 $Pu{O}_2$, $Am{O}_2$, $(U,Pu)O_2$ or $(U,Am)O_2$.