AnO2 nanocrystals via hydrothermal decomposition of actinide oxalates

Abstract

We have recently proposed the hydrothermal decomposition of AnIV-oxalates as a simple access to produce highly crystalline, reactive actinide oxide nanocrystals. The method could be easily applied at low temperature (95-250 °C) in order to produce highly crystalline nano-AnO2 (An= Th, U, Np, Pu). The size and shape of the crystals, together with their increased reactivity, enables the consolidation of homogeneous nanostructured mixed oxides as intermediates toward very dense nuclear fuels for advanced reactors. Spark plasma sintering studies indicates that such nanopowders have increased sinterability compared to powders obtained by conventional thermal decomposition of AnIV-oxalates. We have studied the formation conditions, stability, and thermal expansion of AnO2 (An= Th, U, Np, and Pu). U1-xThxO2 solid solutions have been produced under similar conditions by hydrothermal treatment of U1-xThx(C2O4)2 × n H2O obtained from oxalate co-precipitation. Ongoing experiments are aimed to show the validity of the method towards associate tri-, or even tetra-component associate solid solutions. The involvement of the water molecules in the oxalate hydrothermal decomposition mechanism has been proven by studying the isotopic exchange reaction during the thermal decomposition of An(C2O4)2 × n H2O in H217O through MAS-NMR and Raman techniques.