Investigation of Cs-137(+), Sr-85(2+) and Am-241(3+) ion exchange on thorium phosphate hydrogenphosphate and their immobilization in the thorium phosphate diphosphate

Thorium phosphate diphosphate (TPD) has been proposed as a host matrix for the long-term storage of tetravalent actinides such as U(IV), Np(IV) and Pu (IV), which form solid solutions with TPD. Small amounts of di- or trivalen t cations can also be immobilized in this matrix. Another method of loading of TPD, using the ion-exchange properties of thorium phosphate hydrogenpho sphate (TPHP), a precursor of TPD, was applied to Cs-137(+), Sr-85(2+) and Am-241(3+). The adsorption of these cations on TPHP was studied in terms of pH in NaClO4 and NaNO3 media. The ion-exchange process on TPHP was found t o be reversible for Sr-85(2+) and Am-241(3+). Doped TPHP obtained in this w ay was heated at 1050 degreesC until TPD was formed. Several leaching tests were performed in acidic media on TPD doped with radionuclides with ionic strength equal to 0.1 M. The average partial order related to the proton wa s found to be equal to 0.24 +/- 0.10 and the normalized leaching rate const ant was in the range of 3 x 10(-6) to 2 x 10(-5) g m(-2) d(-1). These resul ts are similar to those obtained for thorium plutonium(IV) phosphate diphos phate solid solutions. On the basis of the results obtained from the adsorp tion and leaching studies, this adsorption-immobilization process, based on the TPHP matrix, can be proposed for the decontamination of low level radi oactive liquid wastes.