Actinide solubility-controlling phases during the dissolution of phosphate ceramics

Phosphate ceramics (britholites, monazite/brabantite solid solutions, thorium phosphate diphosphate, i.e. β-TPD, and associated β-TPD/monazite composites) are often considered as potential candidates to immobilize tri- and tetravalent actinides. In order to study the properties of such materials on the retention of actinides during aqueous alteration, phosphate-based neoformed phases were prepared using under- and over-saturation processes then extensively characterized (involving grazing XRD, EPMA, μ-Raman, IR or SEM). In over-saturation conditions, lanthanides (used as surrogates of trivalent actinides) are quickly precipitated as three hydrated forms (monazite, rhabdophane or xenotime) depending on the temperature, the heating time and the ionic radius of the element. Moreover, as already described for thorium, tetravalent actinides (Th, U, Np, Pu) are more often immobilized as phosphate hydrogenphosphate compounds. However, samples of (Ln,Ca,Th)-rhabdophane can also precipitate in the presence of large concentrations of calcium. Such neoformed phases were also precipitated at the surface of leached phosphate-based ceramics during under-saturation experiments. The associated thermodynamic solubility constants at infinite dilution were estimated. Due to their rapid precipitation and their very low solubility constants, these actinide phosphate solubility-controlling phases appear of significant interest in the field of the evaluation of the long-term performance of actinide-doped phosphate ceramics.