Chemical interaction between a simulated nuclear waste glass and different backfill materials under a thermal gradient

Abstract

The initial stage of a HLW disposal will be dominated by a strong thermal gradient that will exist between the hot canister and the “cold” geological medium. In the case of hot wastes (MOx, direct disposal of used spent fuel, short interim storage), nuclear glass can begin to dissolve in groundwater and to interact with other materials in the presence of a thermal gradient. Moreover, it has already been demonstrated that thermal gradients strongly influence the hydrothermal corrosion of nuclear glass by creating large elemental segregation and subsequent mass transport processes. The aim of this article is to experimentally test whether the presence of a simulated engineered clay barrier affects these mass transport processes. In parallel, experiments were performed with different clays of various Si/Al ratios (smectite, kaolinite and allophane) as Si and Al are often invoked as possible rate limiting species for nuclear glass aging. A simulated nuclear glass was disposed between two layers of various clays, enclosed in a sealed gold tube filled with deionised water. The gold tube was put in a cold-seal vessel and submitted to a thermal field under high temperatures (300–200°C, 140 bars, 100 days). High temperatures were used in order to enhance transformation kinetics as mass transport is supposed to be independent of the temperature. At the end of the experiments, the newly formed phases along the tube as well as the mineralogical transformations of the reactants were studied by integrated in-situ analyses (SEM, TEM, EDS and XRD). Precise descriptions of the mineralogical transformations are presented. The major result is a systematic re-crystallization of the initial clay under the influence of the leached elements from the glass (in particular Si and alkalies). The nuclear glass aging is strongly affected by the composition of the surrounding clay and consumes Al from the clay. The aluminous-rich medium seems to limit the glass corrosion and the crystallinity of the newly formed phases. Furthermore, the cold end of the system is devoted to almost pure Al–Si bearing minerals associated with borate. This finding attests to (i) the migration capacities of Si, Al and B through the clay barrier and (ii) the confining properties of the clay barrier for the heaviest elements (Fe, Zn, Zr) which were mineralogically trapped at the clay–glass interface. In conclusion, the study reveals that interactions between the nuclear glass and the backfill materials are important. Thus, these interactions have to be taken into account in order to predict the long term evolution of the system. Furthermore, backfill materials are an efficient chemical barrier which favors the immobilization of the heaviest elements (Fe, Zn and Zr in our experiments).