Nitrate is a substance which influences the prevailing redox conditions in groundwater, and in turn the behaviour of U. The study of groundwater in an area with low-level radioactive sludge storage facilities has shown their contamination with sulphate and nitrate anions, uranium, and some associated metals. The uranyl ion content in the most contaminated NO₃–Cl–SO₄–Na borehole is 2000 times higher (1.58 mg/L) than that in the background water. At the same time, assessment of the main physiological groups of microorganisms showed a maximum number of denitrifying and sulphate-reducing bacteria (e.g., Sulfurimonas) in the water from the same borehole. Biogenic factors of radionuclide immobilization on sandy rocks of upper aquifers have been experimentally investigated. Different reduction rates of NO₃⁻, SO₄²⁻, Fe(III) and U(VI) with stimulated microbial activity were dependent on the pollution degree. Moreover, 16S rRNA gene analysis of the microbial community after whey addition revealed a significant decrease in microbial diversity and the activation of nonspecific nitrate-reducing bacteria (genera Rhodococcus and Rhodobacter). The second influential factor can be identified as the formation of microbial biofilms on the sandy loam samples, which has a positive effect on U sorption (an increase in Kd value is up to 35%). As PHREEQC physicochemical modelling numerically confirmed, the third most influential factor that drives U mobility is the biogenic-mediated formation of a sulphide redox buffer. This study brings important information, which helps to assess the long-term stability of U in the environment of radioactive sludge storage facilities.

Bentonite clays have unique properties that determine their use as the main component of engineered barrier systems (EBS) for the isolation of radioactive waste. At present, the Russian Federation is elaborating the concept of deep geological disposal of radioactive waste in crystalline rocks of the Yeniseisky site, where bentonite clays play an important role in ensuring the safety of the repository for a long period of time. This work demonstrates the first results of short-term laboratory experiments (1 and 3 months) on the thermochemical interaction of bentonite and concrete in the presence of synthetic water at an elevated temperature. These experiments will help predict the effect of EBS materials on montmorillonite. Bentonite from the 10th Khutor deposit (Russia) and Portland cement were used in the experiments. At the first stage of the experiments, solutions were obtained after leaching the concrete with a synthetic groundwater solution for 1 month at 90 °C. At the second stage, the interactions of the obtained solutions with bentonite at 90 °C were studied. As a result of the experiments, the processes of concrete leaching were revealed, which changed the composition and acidity (an increase in the pH from 6.1 to 12.1) of the synthetic water and led to an increase in the porosity of the material in contact with the solution. However, no dissolution of montmorillonite was observed, and the changes were quite small. The research results show the high stability of bentonite from the 10th Khutor deposit under model conditions, which was confirmed by modeling. Thus, we can say that at pH ≈ 12 and at elevated temperatures, montmorillonite retains a stable structure for a long time, which is important for ensuring the safety of disposal in general.

Materials based on Portland cement and concrete are widely used for radioactive waste solidification and construction of engineered safety barriers at radioactive waste repositories. As compared to other candidate matrices, cement also exhibits the potential for chemical immobilization by reacting with many radionuclides and binding them. The water permeation through the bulk of concrete leads to its degradation, to a change in the composition of its pore water, and to transformations of solid mineral phases constituting the concrete. The long-term degradation of concrete can be estimated using geochemical modeling. A procedure for forecasting the degradation of materials based on Portland cement concrete is described. The model takes into account the stages of cement hydration and chemical degradation. The geochemical changes in the course of concrete degradation were calculated using numerical modeling with the PhreeqC code. Schemes of the stepwise concrete degradation are considered for estimating the radionuclide transport. Relevant radionuclide distribution coefficients are applied to each stage. The duration of concrete degradation stages largely depends on the filtering water composition.

The article presents a model of aluminophosphate glass leaching, an analogue of the RW glass matrix, in static mode in the presence of bentonite. Parameterization of the model is based on experimental data. The model takes into account the kinetics of the glass matrix leaching and the transformation of bentonite mineral phases; sorption processes are also taken into account, as well as the inhibition of leaching by the precipitation of secondary mineral phases formed. The model demonstrates a small transformation of clay phases, as well as the formation of secondary phosphate phases and gibbsite upon leaching of glass in the contact zone with bentonite. The simulation was carried out in the PhreeqC software.

We propose a transport model for fractured porous medium that takes into account the dissolution and precipitation of mineral phases. It is based on the DFM approach and includes the influence of mineral phase content on the hydraulic and transport properties of both fractures and porous matrix. The model is tested on typical problems of concrete decalcification.