PNNL

Abstract

A glass-ceramic waste form is being developed for immobilization of waste streams of alkali (A), alkaline-earth (AE), rare earth (RE), and transition metals generated by transuranic extraction for reprocessing used nuclear fuel. Benefits over an alkali borosilicate waste form are realized by the partitioning of the insoluble fission product fraction into a suite of crystalline phases through controlled cooling, including (AE,A,RE)MoO4 (powellite) and (RE,A,AE)10Si6O26 (oxyapatite). In this study, a simplified 8-oxide system (SiO2-Nd2O3-CaO-Na2O-B2O3-Al2O3-MoO3-ZrO2) was melted then soaked at various temperatures from 1450 to 1150°C and subsequently quenched, in order to obtain snapshots into the phase distribution at these temperatures. For these samples, small angle X-ray and neutron scattering, quantitative X-ray diffraction, electron microscopy, 23Na nuclear magnetic resonance, Nd3+ visible absorption, and temperature dependent viscosity were characterized. In this composition, soak temperatures ~1250°C or below appear necessary to nucleate calcium molybdate phase (~10 – 20 nm in diameter). Further cooling produces oxyapatite and total crystallization increases with lower soak temperatures. Both Na and Nd enter the crystalline phases with lower temperature soak conditions. Slow cooling or long isothermal treatments ~975°C produces significantly higher crystal fractions.