## 1 Introduction  
Bentonite is a clay material proposed for use as an engineered barrier in deep geological repositories (DGRs). Deep geological disposal consists of isolating high-level radioactive waste (HLRW) from the biosphere by keeping it in underground galleries, normally at a depth of 300 to 1000 m depending on the specific location, within stable host geological formations. The low solubility and solid form of the waste is considered a barrier itself. The waste is encapsulated in a metallic container, a second barrier, that depending on each DGR concept can be made of copper-copper alloys (e.g., Swedish and Finnish concept) or carbon steel (e.g., as considered in Switzerland, the United Kingdom, France or Germany) [1]. The canisters are placed within the galleries and surrounded by barriers of compacted clay, typically composed of bentonite, until all of the galleries are completely sealed. In claye formations, it is essential to insert a concrete barrier between the bentonite layer and the geological formation to provide structural support for the galleries' construction. The combination of mechanical and physicochemical protection is referred to as the engineering barrier (third barrier), while the natural geological formation or bedrock is considered a natural barrier (fourth barrier). Ensuring safety requires ongoing and proper long-term maintenance, along with vigilant monitoring of the installation [2; 3; 4; 5; 6]. Carbon steel is the metal chosen by countries, such as Spain, Switzerland, Japan or Belgium to manufacture the canister that insulates the HLRW [7; 2; 8; 9]. This material has been studied, showing good properties for use in DGRs, such as a low corrosion rate or a long lifetime [10; 11]. Bentonite is a clay rock frequently produced by the hydrothermal alteration or diagenesis of acidic-to-intermediate chemistry volcanic glass [12]. Under these conditions, montmorillonite is the main clay mineral, a phylosilicate belonging to the smectite group [13]. Due to the properties of smectite, such as high swelling capacity, high cation exchange capacity and high chemical stability, bentonite is a competitive material used as an engineered barrier in HLRW management in DGRs [7; 14; 15]. HLRW will generate heat due to the decay of radionuclides. This heat will be transferred to the container for decades. The temperature is expected to rise to 100 \({}^{\circ}\)C at the surface of the canisters, although higher temperatures have recently been considered to reduce galleries spacing [16; 17]. Due to the elevated temperature, geochemical interactions between the metal vessel and the bentonite will occur [10; 18]. The canister should keep its structural integrity without cracks for at least 1000 years. The corrosion of the canister will depend on the environmental conditions. Temperature, groundwater, bentonite's porewater chemistry, especially the pH and the concentrations of chlorides, sulfur species and carbonates, the redox conditions, including the effect of gamma radiation, microbial activity and the degree of bentonite saturation are factors that will influence the formation of corrosion products as well as the corrosion rate [1; 14]. The bentonite barrier will be subjected to hydration conditions in contact with the bedrock. Therefore, the location of the DRG is also a variable to consider, since the hydrogeochemical conditions of the host rock will affect the bentonite barrier, providing hydration with variable salt content. Low saline host rocks (e.g., Grimsel granite formations in Switzerland), high saline rocks (e.g., clay formations in Switzerland, France or Spain) or even marine-type porewater formations (e.g., granite formations in the Forsmark area, Sweden) have been studied [19; 20; 21; 22]. It is predictable that full saturation of the bentonite barrier will be reached before dissipation of the thermal gradient, which will take place between 100 and 1000 years after deposition, depending on the particular characteristics of the repository [7; 23; 24; 25; 26; 27].