Résumé
Phosphate matrices are considered as potential candidates for the specific immobilization of tri- and tetravalent actinides. One of the main properties of interest of these matrices concerns their chemical durability. It was studied through their behavior regarding to the retention of actinides and developped using under- and over-saturation experiments. In this field, neoformed phases were precipitated. Lanthanides were used as surrogates for trivalent actinides while uranium and thorium were used for tetravalent actinides. These phases were extensively characterized through several analytical and spectroscopic techniques (SEM, EPMA, XRD, µ - Raman, TRLFS…). New phosphate form such as Nd1-2xCaxThx-yUy(PO4,F). ½ H2O rhabdophane were identified during the dissolution of britholites. The precipitation of Nd1-2xCaxThxPO4. ½ H2O rhabdophane was obtained and it appeared that there was a segregation between neodymium, calcium and thorium after a few days to several months depending on the thorium weight loadings and the precipitation time, thorium precipitating as TPHPH and neodymium as NdPO4 ½ H2O. The determination of the solubility constants showed very low values which means that these phases exhibit some interesting properties for the retention of radionuclides.