Saponite as a New Generation Engineered Buffer Material for High Level Nuclear Waste Disposal: Its Chemical Stability in High-pH and High-Temperature Environments

Abstract

Abstract Saponite, a tri-octahedral smectite with an ideal formula of Mg3(Si,Al)4O10(OH)2·4H2O for an Mg-endmember (saponite-15Å), has similar swelling properties as montmorillonite and therefore can be considered as a potential buffer material for nuclear waste disposal. In this experimental study, we investigated the chemical stability of this material under alkaline conditions (pH ∼ 12) at high temperatures (up to 150 °C) and saturated vapor pressures over an experimental duration of about three weeks. Alkaline conditions in a geologic repository can be induced by the interaction of groundwater with borosilicate waste forms or by the leaching of alkaline porewaters from cementitious materials introduced as structural components, such as seals, liners, and plugs. The interactions of saponite with portlandite [Ca(OH)2]-saturated solutions at 60, 125, and 150 °C were experimentally simulated. The results indicated that saponite is chemically more stable in these environments than montmorillonite and remained unchanged after the interactions. The stability is attributed to its low solubility, with dissolved-magnesium concentrations in the portlandite-saturated solutions estimated to be less than 4×10−7 mol/kg or 2×10−7 mol/kg at 60 or 150 °C, respectively. For comparison, we also investigated the interactions of montmorillonite (SWy-2) with a portlandite-saturated solution at 150 °C. Our results indicate that montmorillonite is not stable, leading to the formation of zeolite (phillipsite) after 9 days of reaction. This study thus demonstrates the feasibility of using saponite as a potential buffer material (as an alternative to montmorillonite currently being considered in many disposal concepts) in an alkaline and elevated-temperature disposal environment.