CHEMICAL CONDITIONS OF SYNTHESIS OF TH-4(PO4)(4)P2O7 - PREPARATION OFTHORIUM PHOSPHATE HYDROGENPHOSPHATE AS PRECURSOR

Thorium phosphate-diphosphate, Th-4(PO4)(4)P2O7, is obtained, at high temperature (800-1250 degrees C), for a given ratio r = PO4/Th equal t o 3/2 from a thorium compound and a phosphating reactant whatever the chemical conditions. This compound, whose solubility is very low, can be loaded in situ and used as a host matrix for radwaste storage. One of the methods for its synthesis involves the precipitation of a precu rsor, thorium phosphate-hydrogenphosphate. This precursor is used as a n adsorber for low concentrations of radionuclides and, after heating in air in the range 800-1200 degrees C, it is transformed into doped T h-4(PO4)(4)P2O7. The optimum conditions for the precipitation process have been determined. Starting from 0.5-2 NI solutions of thorium nitr ate or chloride and (NH4)(2)HPO4 at pH 9-9.5 at the ratio r = 3/2, a c ompound of general formula Th-2(PO4)(2)HPO4 . nH(2)O (n = 3-7) is synt hesized. Its physicochemical properties have also been studied.