Incorporation of tetravalent actinides in three phosphated matrices: britholite, monazite/brabandite and thorium phosphate diphosphate (β-TPD)

Three phosphate based ceramics were studied for the immobilization of tri- and tetravalent actinides: britholite Ca9Nd1-xAnxIV(PO4)5-x(SiO4)1+xF2, monazite/brabantite solid solutions Ln1-2xIII CaxAnxIPO4 and Thorium Phosphate Diphosphate (β-TPD) Th4-xAnxIV(PO4)4P2O7. For each material, the incorporation of thorium and uranium (IV) was studied as a surrogate of plutonium. This work was the early beginning of the incorporation of 239Pu and/or 238Pu in order to evaluate the effects of α-decay on the three crystallographic structures. The incorporation of tetravalent cations was carried out by dry chemistry methods, using mechanical grinding to improve the reactivity of the initial mixture then the homogeneity of final solid prepared after calcination at high temperature (1200-1400 deg C). For britholites, the thorium incorporation was complete for weight loading up to 20 wt.%, leading to the preparation of homogeneous and single phase solid solutions when using the coupled substitution (Nd3+, PO43-) ↔ (Th4+, SiO44-). Due to redox problems, the incorporation of uranium was limited to 5 to 8 wt.% and always led to a two-phase mixture of U-britholite and CaU2O5+y. The preparation of homogeneous solid solutions of β-TUPD and of brabantites containing thorium and uranium samples was successfully obtained using three steps of mechanical grinding/calcination. For each matrix, dense pellets were prepared prior to the study of their chemical behaviour during leaching tests. The chemical durability of brabantites and β-TUPD were found to be close to that reported in literature. The formation of neo-formed phases was also evidenced onto the surface of Th-britholite samples. (author)