Influence of the pH on the dissolution of TPD and associated solid solutions

The dissolution of thorium phosphate diphosphate (TPD) doped or not with trivalent actinides and that of associated solid solutions with tetravalent plutonium was studied from a kinetic point of view as a function of the acidity or the basicity of the leachate. From the evolution of the normalized mass losses, the dissolution rates were determined. For all the solids considered, the values were found between 1.2×10−5 and 4.4×10−9 g m−2 d−1 which confirms the very good durability of TPD to aqueous corrosion. The expression of the dissolution rate was given in acidic and in basic media (10−1–10−4 M HNO3 or HClO4 and 10−1–10−4 M NaOH). The partial orders related to the proton and hydroxide ion concentrations were found to be equal to n=0.31–0.40 and to m=0.37, respectively. The associated dissolution rate constant at pH=0 and pH=14 were found to k298 K,0.1 M′=1.2×10−5 to 2.4×10−5 g m−2 d−1 and to k298 K,0.1 M″, (7.8±1.9)×10−5 g m−2 d−1, respectively. In these conditions, the dissolution rate value extrapolated in neutral medium was evaluated to 2.4×10−7 to 3.6×10−7 g m−2 d−1 at room temperature and to 5.0×10−6 to 7.5×10−6 g m−2 d−1 at 90 °C which remains very low by comparison to the other ceramics studied for the same applications.