Thorium Phosphate-Diphosphate (TPD) can be considered as a potential matrix for the immobilization of actinides and especially for the plutonium coming from dismantled nuclear weapons. The capability of this solid to form solid solutions by substitution of thorium by tetravalent uranium, neptunium or plutonium is rather high. The synthesis of sintered pellets of thorium-uranium(IV) phosphate-diphosphate was successfully performed. Their density reached 95–99% of the value calculated from XRD data. The open and closed porosities were equal to 2–3% each. The resistance of the TPD and derivative solid solutions to aqueous corrosion are also very good. Leaching tests of powdered samples containing tetravalent plutonium or uranium showed that the normalized dissolution rate remained low (between 10−5 and 10−6 g/(m2.d)) even in acidic media. For tetravalent actinides, the saturation of the leachate seems to be controlled by the precipitation of the thorium phosphate-hydrogenphosphate hydrate which solubility product was found to be KS,0°* = 10−66.6 ± 1. This very low value allows a supplementary delay of the actinides migration in the hypothesis of their release in the leachate from the matrix.

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