Precipitation of morphology-controlled uranium(VI) peroxide in nitric acid media

Abstract

The precipitation of U(VI) peroxide (UO2(O2).4H2O, known as studtite) has been extensively studied in mildly acidic media (i.e. pH > 1). However, few studies have investigated the influence of highly acidic conditions (i.e. pH < 1) on the precipitation of studtite, particularly regarding the morphology of the final powder. The influence of high acidity (0.1 M < C(HNO3)ini < 2.0 M) and C(H2O2)ini/C(U)ini molar ratio (ranging from 10 to 70) on the precipitation kinetics, as well as the morphological and crystallographic properties of U(VI) peroxide was thus investigated in this study. Decreasing the acidity and increasing C(H2O2)ini/C(U)ini molar ratio significantly enhance both the final precipitation yield and the reaction kinetics. Furthermore, a correlation was established between the initial saturation index of the reaction mixture and the final morphology and crystallite size of the precipitated U(VI) peroxide, regardless of the initial precipitation conditions. These results demonstrate that in highly acidic nitric media, it is possible to control the morphology of the precipitated U(VI) peroxide by simply adjusting the initial saturation index. This correlation opens up interesting perspectives for the synthesis of powders with specific morphologies, particularly uranium peroxide or corresponding oxide powders obtained after thermal treatment. This morphology control is of great interest in the nuclear fuel fabrication field, as it allows obtaining oxide powders with optimized properties for sintering.