Structure studies on lanthanide technetium pyrochlores as prospective host phases to immobilize ~(99)technetium and fission lanthanides from effluents of reprocessed used nuclear fuels

摘要

We report here results of a systematic investigation regarding the incorporation of ~(99)Tc into pyrochlore oxide structures, Ln _2Tc_2O_7, where Ln represents trivalent lanthanide Ln~(3+) cations, while ~(99)Tc is a tetravalent, Tc~(4+), metal cation. In this study, we used the following Ln cations: Pr, Nd, Sm, Gd and Lu. The goal in this preliminary study was to characterize and quantify the range of stability of the lanthanum technetium pyrochlore oxide phase. Powder X-ray diffraction (XRD) and Rietveld analysis was used to characterize the crystalline phase content, while scanning electron microscopy (SEM) was used to characterize the microstructure and homogeneity of the Ln-Tc pyrochlore specimens. All of the pyrochlore samples exhibited good crystallinity and their lattice parameters could be refined with remarkable accuracy. Low refinement residuals (R_(Bragg)) of 1.1-3.1% were achieved. The refined, cubic lattice parameters ranged from 1.0447156(83) nm for Pr_2Tc _2O_7 to 1.013777(22) nm for Lu_2Tc _2O_7, with a linear trend relative to the Ln~(3+) ionic radius. We also demonstrated here the successful synthesis of Nd _2Tc_2O_7, using a simple, scalable synthesis route.