Dissolution of simplified nuclear waste glass and formation of secondary phases

Immobilization of high-level and intermediate-level nuclear wastes by vitrification in borosilicate glass is a well-established process. There is a consensus between the waste management agencies of many countries and many experts that vitrified nuclear waste should be disposed of in a deep geological waste repository and therefore its long-term behavior needs to be taken into account in safety assessments. In contact with water, borosilicate glass is metastable and dissolves. In static dissolution experiments, often a surface alteration layer (SAL) forms on the dissolving glass, and later sometimes secondary phases form. Based on boron or lithium release rates, commonly three stages of glass dissolution are defined as a function of the reaction progress: (I) initial dissolution, described by a congruent glass dissolution at the highest rate, (II) residual dissolution, characterized by a glass dissolution rate several orders of magnitude lower than the initial one, and (III) resumption of glass alteration with initial rates. Microscopically, the formation of a complex SAL has been identified as a prerequisite for the slower dissolution kinetics of stage II. Stage III is typically observed under specific conditions, i.e., high temperature and/or high pH driven by the uptake of Si and Al into secondary phases. Different glass dissolution models explaining the mechanisms of the SAL formation and rate-limiting steps have been proposed and are still under debate.

In this article different aspects of glass dissolution from recent studies in the literature and our own work are discussed with a focus on the microscopic aspects of SAL formation, secondary phase formation and the resumption of glass dissolution. Most of the experiments in the literature were performed under near-neutral pH conditions and at 90 ^∘C, following standard procedures, to understand the fundamental mechanisms of glass dissolution. The example of interaction of glass and cementitious materials as discussed here is relevant for safety assessments because most international concepts include cement e.g., as lining, for plugs, or as part of the general construction of the repository. The aim of the investigations presented in this paper was to study the combined effect of hyperalkaline conditions and very high surface area/volume ratios ($\text{SA}/\mathrm{V}=\mathrm{264}\mathrm{000}{\mathrm{m}}^{-\mathrm{1}}$) on the dissolution of international simplified glass (ISG) and the formation of secondary phases at 70 ^∘C in a synthetic young cement water containing Ca (YCWCa). The new results show that the SA$/$V ratio is a key parameter for the dissolution rate and for the formation of the altered glass surface and secondary phases. A comparison with similar studies in the literature shows that especially on the microscopic and nanoscale, different SA$/$V ratios lead to different features on the dissolving glass surface, even though the SA-normalized element release rates appear similar. Zeolite and Ca-silicate-hydrate phases (CSH) were identified and play a key role for the evolution of the solution chemistry. A kinetic dissolution model coupled with precipitation of secondary phases can be applied to relate the amount of dissolved glass to the evolution of the solution's pH.