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

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, only a 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. Therefore, the influence of high acidity (0.1 M < C(H⁺)_ini < 2.0 M) and the 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 investigated in this study. Decreasing the acidity and increasing the C(H₂O₂)_ini/C(U)_ini molar ratio have significantly enhanced both the final precipitation yield and the reaction kinetics. Furthermore, a correlation was found between the initial saturation index of the reaction mixture and the final morphology and crystallite size of the precipitated U(VI) peroxide, independent 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, in particular uranium peroxide or corresponding oxide powders obtained after thermal treatment. This morphology control is of great interest in the field of nuclear fuel fabrication, as the characteristics of oxide powder, such as flowability or sintering reactivity, have a strong impact on the manufacturing process and the properties of the final ceramic.