Uranium Oxide Microspheres Prepared Under Hydrothermal Conditions As Reference Materials For Nuclear Safeguards

Particle analyses are a key tool for safeguards verification by the International Atomic Energy Agency. Indeed, detection and measurement of the isotopic composition of nanometer- to micrometer-sized particles collected during inspection of nuclear facilities provide precious information on their activities. Because of the low number and extremely small size of the particles of interest, particles analyses are always an analytical challenge. Moreover, laboratories must possess reference materials representative of the analyzed samples, i.e. actinide oxide particles with well-known sizes, densities and isotopic compositions that can be used for optimization and qualification of analytical methods and instruments. In order to prepare such materials, several protocols have already been tested, but frequently all of them require specific equipment and cannot be easily implemented in standard chemistry labs. On this basis, we developed from several years original wet chemistry routes aiming to precipitate directly morphology-controlled actinide oxides from mixtures of solutions under hydrothermal conditions. In the case of UOx samples, hydrochloric solution containing tetravalent uranium (natural isotopic composition) was mixed with aspartic acid, used both as complexing and shaping agent, to yield an amorphous uranium(IV) aspartate precipitate. Solid phase and supernatant were then placed under mild hydrothermal conditions (T = 160°C) for 30 hours. A multi-parametric study allowed evidencing the impact of pH, aspartic acid concentration, and mechanical stirring. Optimizing the synthesis conditions led to precipitate monodisperse spherical particles, with an accurately size control in the 400 nm - 2.5 µm range. For all the compounds prepared, further XRD analysis attested the formation of UO2.nH2O samples, while TG experiments revealed the presence of residual organics and water. Nevertheless, additional heat treatments performed up to 600°C led to produce anhydrous and carbon-free dense particles without altering their initial morphology. It also enabled to control the stoichiometry of the particles from UO2+x to U3O8 depending on the atmosphere. Finally, with the aim to store these samples as suspensions before use, alteration tests of the produced particles were undertaken in various media. Preliminary results revealed that only a negligible weight loss occurred after several months in ethanol, while no significant modification of the morphology was observed.