Contribution of zeolite-seeded experiments to the understanding of resumption of glass alteration

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.npj Materials Degradation (2017) 1:17 ; doi:10.1038/s41529-017-0018-x INTRODUCTION Like the United Kingdom, Japan, Russia and India, France has chosen to reprocess spent nuclear fuel. Ultimate waste arising from this recycling process-consisting of fission products and minor actinides-are confined by vitrification. The reference solution for the management of these waste packages over geological time scales is their storage in a deep, low-permeability and stable geological formation.In contact with water, 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 (also called "gel") causes the reduction of the initial alteration rate r 0 -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 thus in properties-depending on the environmental parameters, in particular pH, temperature and solution composition. Gel stability and its passivation properties allow-under the most favorable conditions-a glass package lifetime of hundreds of thousands to millions of years. However, a resumption of alteration (RA, also 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, particularly in alkaline environments-as a conseque...

zeo19: A thermodynamic database for assessing zeolite stability during the corrosion of nuclear waste immobilization glasses

“…[2][3][4] This is significant since the formation of zeolitic secondary phases has been suggested to result in a resumption of glass corrosion, which can significantly reduce the long-term durability of nuclear wasteforms. [5][6][7][8] Hence, accurate predictions of the propensity for formation and the stability of zeolitic phases are important to assess the long-term integrity of nuclear wasteforms in sub-surface conditions, e.g., < 250-m depth. 9 But, assessing such propensities experimentally is challenging, especially since there is a myriad of factors that affect the formation of zeolites; which themselves feature a multitude of compositions.…”

A comparative review of the aqueous corrosion of glasses, crystalline ceramics, and metals

“…51,52 It is also observed that the extent of the induction period preceding the acceleration (Stage II) decreases with increasing pH and with seeding of some zeolites, but not others. 53 Conversely, in the nearneutral pH and lower temperature conditions representative of most repository environments, it is not known if, over times exceeding laboratory experiments, zeolite formation would disrupt the passivating mechanisms, and increase the glass dissolution rate. This important question was partly addressed in the case of International Simple Glass (ISG) 53 but requires more work to be generalized.…”

“…53 Conversely, in the nearneutral pH and lower temperature conditions representative of most repository environments, it is not known if, over times exceeding laboratory experiments, zeolite formation would disrupt the passivating mechanisms, and increase the glass dissolution rate. This important question was partly addressed in the case of International Simple Glass (ISG) 53 but requires more work to be generalized. Similar accelerated dissolution has been observed to be concurrent with the precipitation of certain phases resulting from the interaction with near-field ions.…”

“…76 Recent studies have focused on the effects of composition on the triggering of Stage III corrosion. 22,53,[77][78][79][80] Clearly, high concentration of alkali elements, which increase pH, sufficient concentrations of Ca (to form CSH), Al (to form zeolites), Mg (to form magnesium silicates), or Fe (to form iron silicates) can promote the acceleration of corrosion rate to Stage III behavior, or at least will maintain a high Stage II rate.…”

Parameterizing a borosilicate waste glass degradation model

“…Most rate equations were developed based on experimental observations of Stage 1 and Stage 2 behaviors and Stage 3 behavior has been modeled using ad hoc modifications of those equations. 9,17 However, experimental observations indicate the Stage 3 rate remains nearly constant (within experimental uncertainty limits) as the small amounts of glass used in most laboratory tests are completely dissolved; 14,18,19 neither the diffusion-based nor the reaction affinity-based models developed to represent Stage 2 are consistent with a constant Stage 3 rate. Furthermore, a constant Stage 3 rate is counter-intuitive because the surface areas of glass and secondary phase(s) change throughout the transformation.…”