Kinetics of oxyapatite [Ca2Nd8(SiO4)6O2] and powellite [(Ca,Sr,Ba)MoO4] dissolution in glass-ceramic nuclear waste forms in acidic, neutral, and alkaline conditions

Abstract

Immobilization of chemically complex aqueous waste streams from used nuclear fuel reprocessing is achievable at higher waste loadings with glass ceramics as compared to borosilicate glasses. Additionally, crystalline phases with similar chemistry are more durable than their amorphous counterparts. However, during glass ceramic fabrication, mechanical stresses at crystal-glass interfaces, which are caused by thermal expansion mismatching during cooling, create locales where water is capable of accessing and reacting with the various phases in the glass ceramic, thus releasing radionuclides into the aqueous phase. In the present work, we build on previous chemical durability investigations of a glass-ceramic containing crystalline oxyapatite [Ca2Nd8(SiO4)6O2] and powellite [(Ca,Sr,Ba)MoO4] secondary phases. The individual crystalline and bulk glass phases have been fabricated separately and the corrosion behavior has been investigated with single-pass flow-through (SPFT) testing at 90 °C in buffered pH(RT) 4, 7, 9 and pH 11 solutions and with the static product consistency test (PCT). The results demonstrate the varying dissolution kinetics of the individual phases in the range of pH studies. The consequence of the varying dissolution kinetics are described with a conceptual model of glass-ceramic dissolution mechanisms.