Study of Th{sub 4}(PO{sub 4}){sub 4}P{sub 2}O{sub 7} and solid solutions with U(IV), Np(IV) and Pu(IV): Synthesis, characterization, sintering and leaching tests

In the framework of radwaste storage, the thorium phosphate-diphosphate Th{sub 4}(PO{sub 4}){sub 4}P{sub 2}O{sub 7} (TPD) has been synthesized in the ThO{sub 2}-P{sub 2}O{sub 5} system. This material is very resistant to aqueous corrosion. In this paper the authors report the conditions of synthesis of TPD doped with small amounts of trivalent actinides and of solid solutions with uranium (IV), neptunium (IV) and plutonium (IV). Sintered samples of TPD and Th{sub 4{minus}x}U{sub x}(PO{sub 4}){sub 4}P{sub 2}O{sub 7} (TUPD) have been prepared after pressing at 100--800 MPa at room temperature then heating at 1250 C for 10 hours. The corresponding density varies from 87 to 93% of the calculated value from crystal data. Leaching tests of Th{sub 4}(PO{sub 4}){sub 4}P{sub 2}O{sub 7} (TPPD) have been performed in water and in several acidic media at room temperature and at 90 C. The dependence of the leaching rate on the pH has been determined: the partial order related to [H{sub 3}O{sup +}] is in the 0.35--0.44 range while the apparent leaching rate is equal to about 1 x 10{sup {minus}5}-4x10{sup {minus}5} g(m{sup 2}.d).