Radiation chemical synthesis and characterization of UO2 nanoparticles

In a deep repository for spent nuclear fuel, U(VI)(aq) released upon dissolution of the fuel matrix could, in reducing parts of the system, be converted to U(IV) species which might coalesce and form nanometer-sized UO2 particles. This type of particles is expected to have different properties compared to bulk UO2(s). Hence, their properties, in particular the capacity for oxidant consumption, must be investigated in order to assess the effects of formation of such particles in a deep repository. In this work, methods for radiation chemical synthesis of nanometer-sized UO2 particles, by electron- and γ-irradiation of U(VI) solutions, are presented. Electron-irradiation proved to be the most efficient method, showing high conversions of U(VI) and yielding small particles with a narrow size distribution (22-35 nm). Stable colloidal suspensions were obtained at low pH and ionic strength (pH 3, I = 0.03). Furthermore, the reactivity of the produced UO2 particles towards H2O2 is investigated. The U(IV) fraction in the produced particles was found to be ∼20% of the total uranium content, and the results show that the UO2 nanoparticles are significantly more reactive than micrometer-sized UO2 when it comes to H2O2 consumption, the major part of the H2O2 being catalytically decomposed on the particle surface