Application of GRAAL model to the resumption of International Simple Glass alteration

Abstract

The methodology developed for predicting nuclear waste behavior under disposal conditions combines experimental approaches and modeling. A waste glass canister placed in contact with water undergoes irreversible chemical processes leading to its degradation into more stable phases. This transformation occurs in three kinetic stages: the initial alteration rate (stage I), the residual rate (stage II), and, in some cases, a resumption of alteration (stage III) related to zeolites precipitation. Affinity effects based on the transition state theory are used to account for the rate drop from stage I to stage II. However, modeling of stage III has not been extensively studied. This study investigates the ability of the "glass reactivity with allowance for the alteration layer" (GRAAL) model to describe the effect of zeolite precipitation on the dissolution kinetics of the international simple glass (ISG). The GRAAL model-based description of the alteration layer was adapted to account for alkaline pH alteration mechanisms. A model describing the nucleation and growth kinetics of zeolites was proposed based on simple formalisms whose parameters can be inferred from previous studies’ results. These improvements give a description of the moment where a resumption of alteration occurs. As the predictive capacity of the GRAAL model is strongly dependent on the appropriateness of the alteration layers’ description, this work shows the need to develop new functions to describe the evolution of their compositions and solubilities with pH changes. Calculations also show the importance of Al and Ca activities and the effect of Al on the silica solubility.