Plutonium dioxide dissolution in nitric/hydrofluoric acid mixtures: kinetics and mechanism.

The kinetics of the fluoride-catalyzed dissolution of powdered plutonium oxide in nitric acid have been investigated over the concentration ranges of 2.0 to 12.0 molar nitric acid and 0.01 to 0.5 molar fluoride.The dissolution rate is controlled by diffusion of fluoride through a product layer on the surface of the solid PuOg.The thickness of this layer increases as the reaction progresses and rapidly slows dissolution.The kinetics follow the parabolic rate law, C=Kt%, where C is the concentration of dissolved Pu.The constant, K, is dependent on HF and HNO3 concentrations, surface area, the diffusion coefficient, and temperature.The kinetic data can be explained on the premise that two parallel reactions occur, one forming the unreactive surface layer and inhibiting the reaction and the other forming the soluble plutonium species.