Thermo-hydro-chemical (THC) behaviour of a Spanish bentonite after dismantling of Heater#1 and Heater#2 of the FEBEX in situ test at the Grimsel Test Site

Abstract

Bentonites are an essential component of the multi-barrier system securing the long-term safety of the final disposal of nuclear wastes. The efficiency of such engineering clay barrier relies on its physical and chemical confinement properties: low permeability, low diffusivity, high retention and swelling capacity. Therefore, an important issue for performance assessment purposes is to have confidence and demonstrate the long-term preservation of these properties over the long term, i.e., hundreds of thousands of years under real conditions of a repository. The challenging scientific approach used to tackle the problem of predicting long-term clay barrier behaviour is by analysing the results from experiments conducted in underground research laboratories (URL) at real scale and at real conditions. The FEBEX in situ test project provided the opportunity to understand and quantify the processes taking place in the near-field and to evaluate the behaviour of the compacted FEBEX bentonite after 18 years of experiment. From the geochemical point of view, specific long-term requirements for the function of a bentonite to isolate the canisters from water and retard the migration of radionuclides is to maintain a suitable chemical environment for the canister integrity and radionuclide retention over time, buffering possible alteration/deterioration processes of the bentonite. The dismantling of the second part of the FEBEX in situ test allowed to quantify the modifications of the bentonite after reacting due to changes in the initial physico-chemical conditions and different types of perturbations: a) interactions with granitic groundwater (saturation phase), b) heat (desaturation phase due to heating), c) perturbations linked to the interactions between the bentonite and the allochthonous engineered solid materials (concrete, iron, C-steels and other metals, organics, etc.), and d) interactions with different gases produced and consumed in the system (CO2, H2, CH4, etc.) due to mineral dissolution/precipitation, microbiological reactions and corrosion processes. In this report the impact of different perturbations were described in terms of: 1) modifications in the pore water chemistry and redox conditions, which may affect the smectite structural stability and speciation of dissolved ions, 2) dissolution and redistribution of primary mineral phases and precipitation/neoformation of secondary minerals, 3) clay mineral alteration or modifications of the clay mineral properties (especially cation exchange capacity, cation exchange population, crystallochemical structure, layer charge and swelling ability), and 4) transport properties through modifications in porosity, permeability and tortuosity due to cementation processes. Some specific reactions observed in laboratory and large scale experiments over time were also detected in the FEBEX in situ experiment. These reactions are: dissolution/precipitation, cation exchange, metal corrosion, gas production, hydrocarbons oxidation and Mg accumulation at the heater interface. However, specific results were obtained from this FEBEX in situ experiment, which is the longest real scale in situ experiment up to date. Nevertheless, the global data analysed from samples obtained after the dismantling operations of the Heater #1 in 2002 and Heater#2 in 2015, indicated that no significant structural changes in the smectite were produced, the whole bentonite barrier preserving its main physico-chemical properties unaltered (CEC, layer charge, water adsorption capacity and total and external surface area). The alterations observed were restricted to localized zones, mainly at the bentonite/shotcrete (< 6.25-12.5 cm) and bentonite/heater (< 5-10 cm) interfaces.