PNNL

Abstract

Solidification of chemically complex aqueous waste streams from used nuclear fuel reprocessing is achievable with glass ceramics. Glass ceramics are a desirable waste form because higher waste loading is achievable compared to borosilicate glasses. Additionally, crystalline phases with similar chemistry are more durable than their amorphous counterpart. However, during glass ceramic fabrication, mechanical stresses at crystal-glass interfaces, which are caused by thermal expansion mismatch during cooling, create locales where water is capable of accessing and reacting with the various phases in the glass ceramic and releasing radionuclides into the aqueous phase. In the present work, we build on previous chemical durability investigations of a glass-ceramic containing crystalline powellite [(Ca,Sr,Ba)MoO4] and oxyapatite [Ca2Nd8(SiO4)6O2] 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 – B (PCT-B). 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.