Weathering of ancient and medieval glasses—potential proxy for nuclear fuel waste glasses. A perennial challenge revisited

Abstract

Ancient and medieval glasses that have survived the deleterious attack of the environment for millennia have long since proposed as proxy to estimate and predict the corrosion mechanism of nuclear waste glasses. However, because both composition and environmental burial conditions vastly differ between hydrolytically less stable ancient glasses and modern advanced nuclear waste glasses, only semiquantitative conclusions can be drawn about the likely performance of the latter as longterm stable immobilization matrices for high-level radioactive nuclear waste. In this contribution, special emphasis has been devoted to the behavior of manganese, present as both iron decolorant and coloring ions in ancient Roman and medieval glasses. Study of the behavior of manganese in ancient glasses during weathering may provide some limited clues to the behavior of long-lived radioactive technetium-99. Knowledge of the corrosion kinetics of ancient glasses will allow, eventually, a reasonable prediction of the long-term performance of glassy nuclear waste forms as function of their composition and environmental parameters, i.e. groundwater composition, flow rate, pH, solution volume, and surface area.corrosion, manganese, medieval glasses, nuclear waste glass, technetium | INTRODUCTIONGlass is a supercooled melt, a state that retains the statistical short-range order (SRO) of a liquid during cooling in lieu of the 3D-periodical long-range order (LRO) of a crystal.1 Key distinguishing properties of glass as opposed to crystalline ceramics include the following features:1. Glass shows isotropic properties, i.e. there are no direction-dependent variations of physical properties such as thermal expansion, index of refraction, hardness, modulus, electrical polarization, and others. The reason for this can be sought in the statistical distribution of structural elements such as tetrahedral glass-forming SiO 4 groups. 2. Glass shows reversible softening and solidification during transformation from brittle-solid to liquid state and vice versa, without formation of a new phase. Softening and solidification respectively occur within a more or less wide temperature interval, dependent on the composition, in particular on the silica content. Hence, in a thermodynamical sense glass has no fixed 'melting point'.In addition, the thermodynamic disequilibrium of glass leads to its composition-dependent leachability in the presence of water as well as minerals contained, for example in burial environment. Many ancient glasses buried under humic climatic conditions or exposed to deleterious environment such as external medieval church windows show surfaces depleted of leachable alkaline and/or alkaline earth ions. However, frequently those surface layers are strongly enriched in amorphous silica as well as silicate minerals such as clays or, rarely, zeolites.2

A review of glass corrosion: the unique contribution of studying ancient glass to validate glass alteration models

“…Although all types of alkaline silicate glass are susceptible to weathering degradation, from a thermodynamic point of view stability increases as in the following: K 2 SiO 3 < Na 2 SiO 3 < Li 2 SiO 3 38 . Silica rich glass, such as Roman SSL glass, is more durable than poor silica glass, like medieval glass 69 . In addition, K + , which is contained in medieval glass as a monovalent cation modifier, is more susceptible to leaching out from the deeper region of the glass network during weathering alteration than bivalent cations such as Ca 2+ , which are present in Roman glass 70,71 .…”