Abstract
Understanding the origin and the consequences of glass alteration regimes is necessary for the prediction of nuclear glass durability. The so-called “stage 3” or “resumption of alteration regime” of glasses used to sequester nuclear waste by vitrification, is characterized by a sudden acceleration of glass alteration rate arising from the precipitation of secondary minerals, mainly zeolites. To study this process, a promising approach is developed, based on seeding by synthesized zeolite seeds. This study quantitatively links the alteration of a six-oxide reference borosilicate glass (ISG) and the precipitation of zeolites that affects concentrations of key species—in particular aluminum—and thus the glass dissolution rate. The characterization of stage 3—easier at alkaline pH—can now be extended to pH conditions more representative of those found in a geological repository thanks to seeding that reduces, or even eliminates, the latency period preceding a resumption of glass alteration. The resumption occurrence and glass dissolution rate are related with temperature and pH. This study shows that the detrimental effect of zeolite precipitation decreases with decreasing pH and temperature, until it is no longer detectable at a pH around 9 imposed by the dissolution of the ISG glass. Even for both high temperature and high pH, the resumption rate is lower than the initial alteration rate, which remains the fastest kinetic regime.
Highlights
Like the United Kingdom, Japan, Russia and India, France has chosen to reprocess spent nuclear fuel
The altered glass fraction (AGf) during the latency period is higher for the free pH experiment than that achieved during the latency period of the higher pH tests
This study highlights the efficiency of seeding: by the faster kinetics it induces, seeding allows the characterization of resumptions of alteration at pH and temperature values lower than those classically investigated in the time scale of the laboratory
Summary
Like the United Kingdom, Japan, Russia and India, France has chosen to reprocess spent nuclear fuel. The vitrified product of waste, usually called “nuclear glass”, undergoes both dissolution and irreversible transformation into more stable phases; the rate of this transformation strongly depends on geochemical conditions. Formation of a passivating layer ( called “gel”) causes the reduction of the initial alteration rate r0—due to the hydrolysis of the vitreous network by nucleophilic substitution of hydroxide ions—until the persistence of a residual rate. For a glass, this gel can exhibit a great variability in composition—and in properties—depending on the environmental parameters, in particular pH, temperature and solution composition. A resumption of alteration (RA, called “stage 3” in the literature)—i.e., a sudden acceleration of the glass alteration rate—can occur.[1,2] Resumptions of alteration have been observed in specific experimental conditions, in alkaline environments—as a consequence of the dissolution of alkali-rich glasses or cement—at relatively high temperatures (typically above 90 °C) and high glass-surface-area-to-solutionvolume (S/V) ratios.[3,4,5,6,7] This phenomenon is associated with the precipitation of alumino–silicate minerals, mainly from the zeolite family.[8,9,10]
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