Résumé
Plutonium dioxide is a dense and hard material. It is particularly inert towards most chemical substances and is very refractive to dissolution especially when fired at high temperature. Acidic non complexing media cannot dissolve this material in agreement with the high positive free energy related to this process (ΔrG°= 32-41 kJ.mol$^{-1}$). The most famous media for dissolution of PuO$_2$ requires the use of catalytic amount of fluoride ions F- in boiling and concentrated nitric acid. This complexing dissolution route however suffers from some drawbacks related to the relatively low rate of dissolution, the equipment corrosion, and the effluent management. Due to the existence of four relatively stable oxidation states of Pu in acidic media, redox dissolution of PuO$_2$ can be considered. Oxidative dissolution of PuO$_2$ can be enabled with oxidants having redox potentials higher than ~1.2 V/SHE for PuO$_2$$^{2+}$/PuO2 couple, or 1.4-1.6 V/SHE for PuO$_2$$^{2+}$/PuO$_2$ couple. Strong oxidant cations such as Ce(IV), Ag(II), or Am(V,VI) allow to successfully dissolve PuO$_2$ with the possibility of regenerating the oxidant ions with ozone or electrochemistry. Although very fast dissolutions are observed at almost room temperature for such processes, the overconsumption of oxidant ions in the presence of organic materials or fission products in the dissolver can be problematic. The reductive dissolution route can be performed with redox couples having potentials below ~0.5 V/SHE which may solve the corrosion problems and propose milder reaction conditions. However, reported studies remain scarce and do not allow satisfactory dissolution kinetics and yields, in addition to the use of non-compatible reagents or solvents for nuclear industry.