Modeling of transport and reaction in an engineered barrier for radioactive waste confinement

Bentonites have been proposed as buffer materials for barriers in geological disposal systems for radioactive waste. In these conditions the bentonite barriers may be submitted to changes of humidity, temperature variation, fluid interaction, mass transport, etc. This could modify the physico-chemical performance of the barrier, mainly on the interface with the steel container and with the geological barrier. The engineered barrier development necessitates thus the study of the physico-chemical stability of its mineral constituents as a function of time under the conditions of the repository in the long term. The aim of this paper is to simulate the chemical transformations of the engineered barrier and chemical-elements diffusion impact resulting from the temperature increase (disintegration reactions of wastes), the presence of fluids coming from the geological barrier, and from the liberation of iron (degradation of the containers). The current study was based on the laboratory experiments and mineral characterization achieved by LEM and GREGU research laboratories. The simulations to predict the chemical transformations of argillaceous barrier (MX80 bentonite) and chemical-elements diffusion impact (water-saturated medium) were carried out by using a thermo-kinetic hydrochemical code (KIRMAT: Kinetic Reactions and Mass Transport). This code considers a 1D multisolute mass transport system. Here, all simulations were made in reducing initial conditions ( P _{O₂}≅0; E _h =−200 mV) and the reaction temperature was considered as constant at 100 °C during 1000 years. The results show that the more significant chemicals transformations were (1) the Na/Ca–montmorillonite-to-Ca–montmorillonite conversion, (2) the montmorillonite-to-chlorite conversion and (3) the dissolution/precipitation of accessory minerals. The first chemical transformation represents about 22% in the worst case, i.e., near of the engineered barrier interfaces while the second chemical transformation only represents about 3% (near the container), i.e., a high concentration of iron favours the chlorite–FeAl precipitation. Concerning the dissolution/precipitation of accessory minerals, in general, it was observed that the quartz, microcline and albite were re-precipitated in the system; the calcite and biotite were partially dissolved, and the pyrite was kept inactive.