Study of migration of radioactive elements in clay layers during the burial of radioactive waste

In 2015, Kazakhstan and the International Atomic Energy Agency (IAEA) signed an agreement to host a low-enriched uranium bank in Ust-Kamenogorsk. In 2019, several batches of enriched uranium were delivered to Kazakhstan and the bank began operations at the Ulba Metallurgical Plant. When transporting and disposing of radioactive elements, there is a need to reduce this possibility by limiting the transfer of uranium from underground storage to underground water. Therefore, in this article, a study was conducted on the migration of radioactive elements in clay layers during the disposal of radioactive uranium waste. There are now many underground repositories (for some types of radioactive waste). These systems are based on different underground container structures for different geological formations. For underground repositories located in geological environments where enriched uranium can migrate, other system components must reduce this possibility by preventing or limiting uranium mobility. This work investigates the process of convective transport of radioactive elements, in a moist soil layer through the installation of an additional natural clay barrier layer, the migration of radioactive elements during safe disposal, the effect of diffusion and convection through the solid waste layer.

Stability of clay buffer systems in the conditions of deep geological disposal of radioactive waste in the presence of microorganisms

<p>Clay minerals are the main promising materials for engineering safety barriers in the disposal of radioactive waste in geological formations. Clays have high chemical stability, good sorption properties, and low diffusion coefficients. Bentonite clays combine the most optimal properties - high swelling pressure, low diffusion coefficients. At the moment, there is no unified international concept of the clay barrier density and its composition. Also, the parameters of the influence of biogenic processes on the properties of clay materials have not been correctly determined. It is planned to use of bentonite barrier between the metal container and the external environment in the design of the supercontainer for the new disposal of radioactive waste in the Nizhnekanskiy gneiss massif.</p><p>Within the studies of microbiological processes in the Yeniseisky disposal site, big attention will be paid to clay barriers as sources of biogenic elements in the system and microflora and organic and inorganic carbon.</p><p>Special attention will be paid to thermophilic microorganisms characterized by high growth rates and high levels of metabolic processes, which, along with the extreme impact of radioactive waste (temperature, gas release) on a site in the mountain range, can lead to the destruction of safety barriers.</p><p>Based on the data of phylogenetic analysis of the 16S rRNA gene sequences in clay materials, which are planned to be used as a barrier material, bacteria of the fermentative type of metabolism, capable of forming biogenic gases and organic acids, sulfate-reducing microflora, and a wide variety of microorganisms of the iron cycle were found. We investigating the processes under conditions corresponding to both the internal and external conditions of the clay barrier. As a result of our studies, in model experiments, the effect of microflora activation by radiolysis products, carbon steel corrosion products, hydrogen, and carbon dioxide was found. A thermophilic microbiota was found in samples with bentonite clays of the Khakass and Dinosaur deposits cultivated at temperatures of 50, 70, 90° C. High content of aluminum and silicon amorphous oxide phases was found in the liquid phase after cultivation, and an increase in bioleaching was observed with increasing temperature. Screening of biocidal additives was performed to suppress microbial activity, primarily sulfate reduction. The most effective, thermally stable biocide with prolonged action was polyhexamethylguanidine at a concentration of 0.5 wt. %.</p>

Biogenic safety of clay barrier materials for radioactive waste repository database

<p>Clay minerals are widely used as materials for construction of engineered barriers for nuclear waste and spent fuel repositories all over the world due to perfect isolation properties and high sorption capacity. Unwanted microbiological processes that occur in geological repository can cause deterioration of clay barrier materials, which may significantly affect long-term safety of the repository. It is important to note that such unwanted processes could be caused both by native microbial population and bacteria brought in from outside during the construction of the repository.</p><p>This paper aims to develop a general concept that could be used to prove the risk of unwanted microbial processes’ occurrence in clay materials.  </p><p>Some features of mineral composition of clay materials, including the content of iron, sulphur, phosphorus, organic and mineral carbon, provide the basis for the concept. The ratios of free mono- and di-valent cations present in the solution (Na-K-Ca-Mg) are also taken into account. Another approach presumes microflora composition analysis by means of high-efficient 16S rRNA sequencing method. In addition, the results of several tests dedicated to microbial communities’ stimulation are discussed. These include tests on hydrogen or organic substance addition as electron donors with subsequent standard tests on metabolic activity evaluation, MTT test and respiration assessment of microbial population, which is represented by both planktonic cells and cells incorporated into biofilms. The developed concept was used to assess clay materials found in Russian Federation that could potentially be used to construct engineered safety barriers. These data formed the basis for the formation of a database of microbial safety of engineering barrier materials for radioactive waste storage.</p>

Performance of Clay Materials Constituting to Safety Barriers in Radiation Hazardous Facilities

The paper elaborates on the characteristics of clays and clay materials governing functional properties (performance) of clay barriers. It considers methods applied to identify these characteristics with relevant examples being provided Criteria were proposed to select the required and sufficient numerical values of these characteristics (requirements) considered appropriate for the barrier materials. The paper discusses operating and envisaged test installations designed for mock-up and field tests of clay barrier materials.

Impact of dry density, saturation, and confinement on gas permeability of clay-barrier materials

The safe isolation of high-level radioactive waste from the human environment is a key requisite in the nuclear fuel cycle. Deep geological repositories (i.e. >500m) in a suitable rock mass, involving both engineered and natural barriers is currently the most favoured option for the disposal of high-level nuclear waste. Complex Thermal-Hydro-Mechanical and Chemical (THMC) phenomena are anticipated during the repository lifetime. The formation of gasses (e.g., as a result of oxidation processes) and their possible migration through the engineered barrier system (EBS) could jeopardise the main barrier functions. This paper focuses on the experimental study of the gas flow process through clay-barrier materials, particularly on the effect of degree of saturation, clay initial dry density, and confinement stress on gas permeability. It is shown that the initial dry density of the clay has a significant impact on gas flow, especially at low saturations. As expected, lower gas permeability values were obtained near saturation. Confinement has a minor impact on gas flow, at least for the range of stresses investigated in this paper.

Use of Clay Materials in the Construction of Protective Barriers at Radiation Hazardous Facilities

The article discusses the types of mineral raw materials that can be used to manufacture clay barrier materials. The paper evaluates the characteristics of materials governing the performance of clay barriers: grain size, mineral and chemical composition, physical, mechanical (in dry state) and colloidal properties, stability in the environment. It considers the methods used to identify these characteristics and provides relevant examples.