Modeling of the dynamics of radioactive impurities in the conditions of annual runoff in coastal areas

As a result of the increase in the scope of use of radioactive substances in anthropogenic activities - nuclear power industry, etc. their pollution of the environment increases and, as a result, water pollution occurs. The transfer of radionuclides by water currents is one of the main factors in the spread of radioactive contamination in the environment. The study of the migration of radionuclide pollution in marine areas is complicated by sea currents of gravitational origin and associated with climatic and seasonal changes. Therefore, the issues discussed in the article are relevant. The purpose of the work is to identify the mechanisms of physical processes that determine the migration of radionuclide pollution as a result of annual runoff to the coastal areas of the northwestern part of the Black Sea. Radiation pollution can be dissolved in water, or it can be associated with the presence of dispersed phase substances in it, including radioactive ones. The mechanisms of the spread of radiation pollution may differ depending on the aggregate state of the pollution. The mechanisms of physical processes accompanying the migration of radionuclides in the aquatic environment are considered. These are the processes of gravitational sedimentation of dispersed particles or their floating as a result of the Archimedean force, the influence of the Coriolis force, as well as the processes of Brownian diffusion of dispersed particles that can cause their coagulation. An advective model of the transfer of radionuclides in the aquatic environment is proposed. Approbation of the model when processing the radioactivity measurements of the water of the Dnieper basin and the Black Sea shows that this model well describes the decrease in concentrations of radionuclides coming from polluted areas and, spreading in the Dnieper basin of rivers and reservoirs, falling into the northwestern part of the Black Sea. At the same time, the value of the diffusion coefficient for aqueous solutions in water is 10-9 m2/s, and for Brownian diffusion - 10-10÷10-11 m2/s and less. This testifies to the essential role of diffusional dilution in the process of water migration and needs to be taken into account when modeling the water migration of radionuclides, and also confirms that the rate of diffusion of dispersed particles in water is 1-2 orders of magnitude lower than in solutions. The convective-diffusion model of the migration of radionuclides in water is considered. All obtained results of calculations based on this model indicate that the presence of convection contributes to an increase in the scale of the distribution of impurities (in other words, it contributes to a wider distribution of impurities). Using the obtained exact solutions, the evolution of the radionuclide concentration distribution in the presence of a convective flow is shown. It can be seen that, in contrast to the case of diffusion, the maximum of the distribution shifts in the direction of the convective flow. The self-similarity factor indicates the dependence of the concentration (or other value) on the spatial and temporal distribution of the convection speed, spatial coordinate, and time. The considered approach can be used for the analysis and modeling of transport and diffusion processes in physical systems with a moving medium. By applying the self-similarity factor to the corresponding equation, it is possible to obtain analytical solutions that will reflect the transfer and distribution of impurities or other physical quantities in the system under conditions of large convection movements and diffusion processes. The conclusions of the work and the practical significance of the work consist in the development of algorithms for the water migration model of radionuclides, suitable for predicting critical regimes of physical processes, which can serve as a tool for preparing a response to environmental accidents associated with radioactive contamination.