Potential Matrices for Immobilization of the Rare Earth-Actinide Fraction of High-Level Waste in the REE_2Zr_2O_7-REE_2Ti_2O_7 System

摘要

Phase compositions, structural features, and element distributions were studied for samples of the compositions REE_2(Zr_(2-x)Ti_x)O_7, which are potential matrices for immobilization of the rare earth-actinide fraction of high-level waste from spent nuclear fuel reprocessing. Samples with x up to 0.8 consist of pyrochlore, and at higher titanium content, of pyrochlore and monoclinic REE titanate with perovskite-type structure. The monoclinic phase becomes prevalent at x > 1.2. With respect to the content of the incorporated waste, it surpasses pyrochlore matrices by 10 wt %. The radiation resistance of this phase is close to that of titanate pyrochlore, but its amorphization dose is lower than for zirconate and titanate-zirconate pyrochlore. To check the suitability of monoclinic titanate for immobilization of the REE-actinide waste fraction, it is necessary to study its behavior in solutions and the effect of amorphization on the actinide leaching. The matrix can be prepared by the cold pressing-sintering method suggested in the United States for the synthesis of pyrochlore matrices with plutonium. High rate of solid-phase reactions in titanate systems allows the equilibrium to be attained at a moderate temperature (1400℃) within short sintering time (the first hours). One more possible procedure for matrix fabrication is cold crucible induction melting followed by the melt crystallization.