Fabrication and properties of technetium-bearing pyrochlores and perovskites as potential waste forms

摘要

Technetium-99 is one of the most abundant, long-lived radiotoxic isotopes in used nuclear fuel (UNF). As such, it is targeted in UNF separation strategies such as UREX+, for isolation and encapsulation in solid waste forms for storage in a nuclear repository. We report here results regarding the incorporation of Tc-99 into ternary oxides of different structure types: pyrochlore (Nd_2Tc_2O_7), perovskite (SrTcO_3), and layered perovskite (Sr_2TcO_4). The goal was to determine synthesis conditions of these potential waste forms to immobilize Tc-99 and to harvest crystallographic, thermophysical and hydrodynamic data. Within these studies, Rietveld structure refinement was applied to allow for crystallographic characterization, while a physical property measurement system (PPMS) was used to determine thermophysical properties. The ternary oxides exhibited good crystallinity and their lattice parameters and atomic coordinates could be refined to high accuracy. Low refinement residuals (R_(Bragg)) of 2.0,2.4, and 3.9 % were achieved for Nd_2Tc_2O_7, SrTcO_3, and Sr_2TcO_4, respectively. The strontium technetates, SrTcO_3 and Sr_2TcO_4, show superconductivity at rather high critical temperatures of T_c =7.8 K and 7 K, respectively. Nd_2Tc_2O_7 did not show any changes in magnetic properties above 3 K.