Immobilization of tetravalent actinides in phosphate ceramics

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 Ln1-2xIIICaxAnxIVPO4 and thorium phosphate diphosphate β-Th4-xAnxIV(PO4)4P2O7 (β-TPD). For each material, the incorporation of thorium and uranium (IV) was examined through dry chemistry routes, using mechanical grinding of the initial mixtures then heating at high temperature (1373–1673 K). The quantitative incorporation of thorium in the britholite structure was obtained up to 20 wt% through the coupled substitution (Nd3+,PO43-)⇔(Th4+,SiO44-). On the contrary, the incorporation of uranium was limited to 5–8 wt% and always led to a two-phase system composed by U-britholite and CaU2O5+y. The incorporation of Th and U(IV) was also examined in both matrices, β-TPD and monazite/brabantite solid solutions. Homogeneous and single phase samples of β-TUPD and (Th,U)-monazite/brabantite solid solutions were obtained using successive cycles of mechanical grinding/calcination. The three matrices were prepared in the pellet form then leached in 10−1 M or 10−4 M HNO3 at 363 K. The very low normalized dissolution rates confirmed the good resistance of the materials to aqueous alteration. Moreover, in over-saturation conditions, the formation of neoformed phases onto the surface of the pellets was evidenced for several sintered samples.