Phase Distribution of Uranium in Matrices for Immobilization of the Rare Earth-Actinide Fraction of High-Level Waste

摘要

The uranium distribution in candidate host materials for the rare earth-actinide fraction was studied. The compositions of the materials were set so as to obtain artificial phases with the structures of pyrochlore Ln_2(Ti,Zr)_2O_7 (space group Fd3m, Z = 8) and of monoclinic (P2_1, Z = 4) and rhombic (Fddd, Z = 16) titanates with the formulas Ln_2Ti_2O_7 and Ln_4Ti_9O_(24), respectively, where Ln is a mixture of lanthanides simulating the wastes. The uranium content was varied from 3.5 to 5.5 wt %. In some cases, an equiatomic amount of calcium was introduced into the charge to check the possibility of the replacement of two Ln~(3+) cations by the Ca~(2+) + U~(4+) pair. Samples were prepared by sintering of the oxide charge or by its melting and crystallization of the melt. Along with the prevalent target compounds, the products contained uranium and lanthanide oxides, titanate of Ln and U with brannerite structure, Ln titanosilicates, and other phases. The main uranium concentrators in the samples are pyrochlore, oxides, and brannerite; small amounts of uranium (1-2 wt %) are incorporated in titanosilicates and rhombic titanate. The addition of calcium does not influence the capacity of monoclinic and rhombic titanates for uranium. Based on these data and the results of previous studies, a conclusion is made that rhombic titanate Ln_4Ti_9O_(24) and brannerite (Ln,U)Ti_2O_6 deserve an additional study as candidate host materials for the rare earth-actinide fraction.