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 performed a systematic investigation of the incorporation of 99Tc into pyrochlore oxide structures, Ln2Tc2O7, where Ln represents trivalent lanthanide Ln3+ cations, while 99Tc is atetravalent, Tc4+, metal cation. Pyrochlore compounds are high-melting temperature oxides and are recognized for their durability. Our goal in this preliminary study is to characterize and quantify the range of stability of the lanthanum technetium pyrochlore oxide phase. Hereby, powder X-ray diffraction (XRD) and Rietveld analysis were used to determine and characterize the crystalline phase content with high accuracy, and scanning electron microscopy (SEM) was used to characterize the microstructure and homogeneity of the synthesized Ln-Tc pyrochlore specimens. The pyrochlore phases exhibited good crystallinity and the lattice parameters of the F d 3 m phases could be refined with remarkable accuracies, and low refinement residuals (RBragg) of 1.1 % to 3.1 % for the pyrochlore phases were achieved. The refined lattice parameter are ranging from 10.447156(83) Å for Pr2Tc2O7 to 10.13777(22) Å for Lu2Tc2O7. This technique offers the opportunity to stabilize and immobilize two major fission products, TcO2 and Nd2O3, we believe is a highly durable host phase. We anticipate good radiation tolerance and good chemical stability under typical repository storage conditions.