## 1 Introduction  
Due to their antifiltration properties, clay-based engineered safety barriers (ESB) are used for conservation of radiation-hazardous objects, including on-the-spot disposal [1]; as a matrix material for radioactive waste (RW) conditioning [2]; and at the sites of subsurface and deep geological RW disposal (DGR). Clays and clay materials are used for deep disposal of the waste of nuclear power plants in the USA, France, Sweden, etc. [3, 4, 5, 6]; it is also planned in Russia [7, 8]. The main characteristics of clay materials and of the barriers based on these materials, which provide for long-term storage safety are low filtration and diffusion coefficients, high coefficients of radionuclide interfacial distribution, high specific surface, significant swelling pressure, etc. [9]. Bentonite clays with high montmorillonite content are therefore the most promising ESB components. Polymineral materials with kaolin clay nowadays are used in barriers for nuclear legacy objects conservation [10]. The advantages of kaolin clays include their uniform fractional composition and resistance to physicochemical effects of high temperature and aggressive environments [11]. The long-term evolution of clay materials in DGR depends on a number of physicochemical factors, including the coevolution of other barriers in the system (steel or copper metal containers, cement materials, etc.) [12; 13]. Among the factors affecting clay properties during long-term DGR storage is the biological one [3]. Microbial processes may affect both the mineral composition and the properties of clay materials, especially their filtration and sorption properties. Microorganisms may promote phyllosilicate dissolution both directly by extraction of the mineral-forming elements by biofilms (for example by secretion of siderophores binding to Fe and Al), and indirectly, e.g., by acidification or alkalization [14; 15; 16; 17] of the medium. The main process initiating microbial transformations of clay minerals is the reductive leaching of iron, leading to a change in the layer charge. Smetization proceeds through a process of depletion and exchange of interlayer K and concomitant decrease of layer charge. Illitization is smectite transformation to illite by increased layer charge and by potassium or ammonium fixation in the interlayer. Illitization can be accompanied by a microbial iron reduction in former smectite [18; 19; 20]. Microbiological investigation constitutes a significant part of the research programs of RW disposal sites worldwide. Clays microbial diversity study in the Boom underground research laboratory (URL) HADES [21] revealed the community with the predominance of organotrophic members of the phyla _Proteobacteria_, _Actinobacteria_, _Bacteroidetes_, and _Firmicutes_, as well as aerobic and anaerobic bacteria capable of sulfate and iron reduction (genera _Acidithiobasillus_, _Desulfovibrio_, and _Desulfomaculum_). Since sulfide production enhances steel corrosion, it is considered one of the most negative biogeochemical factors in DGR. Investigation of Opalinus clays at Mont-Terri Rock Laboratory (Switzerland) [6] revealed bacteria of the genera _Pseudomonas_, _Pleomorphomonas_, _Desulfosprosinus_, and _Desulfo-capsa_, capable of sulfate and iron reduction, including the autotrophic processes with H\({}_{2}\) and CO\({}_{2}\) as an electron donor and carbon source, respectively. Activation of microbial processes by biophilic elements and organic matter of clays, as well as by external electron donors, such as molecular hydrogen produced by radiolysis or steel corrosion [21; 22], may affect the evolution of other safety barriers, causing, for example, corrosion of metal containers or promoting cement degradation [23]. Using compacted bentonite with a density above 1.5 g/cm\({}^{3}\) is one of the ways to suppress microbial activity [24].