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. The chemical durability of the phosphate phases was studied with respect to the retention of actinides and developed using under- and over-saturation experiments. In this field, neo-formed 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, micro - Raman, TRLFS). Secondary phosphate phases such as Nd{sub 1-2x}Ca{sub x}Th{sub x-y}U{sub y}(PO{sub 4},F) . 1/2 H{sub 2}O rhabdophane were identified during the dissolution of britholites. The precipitation of Nd{sub 1-2x}Ca{sub x}Th{sub x}PO{sub 4} . 1/2 H{sub 2}O 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 NdPO{sub 4} . 1/2 H{sub 2}O. Quantification of the solubility constants showed very low values that means that these phases exhibit some beneficial properties for the retention of radionuclides. (authors)