<p>This paper presents a numerical simulation of the distribution of thermal runoff generated by cooling nuclear power plant generators in a natural reservoir. The aim of the study was to assess the thermal impact on the waters of Lake Balkhash near the town of Ulken, where a nuclear power plant site has been prepared. To obtain realistic results using computer modeling, the calculations took into account the actual shoreline contour of the study area. One of the main objectives of the study was to evaluate the effectiveness of numerical modeling of thermal pollution during the discharge of heated water into a limited area of Lake Balkhash using the Navier-Stokes and energy equations. To close the problem, the SST k-ω turbulent model was used. To numerically solve the problem, the original equations were approximated using the finite volume method. To verify the mathematical model and numerical algorithm, a comparative analysis of the obtained results was conducted with experimental data and numerical solutions presented in the works of other authors. Additionally, a study of the solution’s sensitivity to spatial discretization parameters was conducted, including an analysis of the effect of different computational grid sizes on the accuracy and stability of the numerical solution. According to the obtained results, numerical modeling provided a complete picture of the development of thermal pollution over time. The dynamics of heat propagation are clearly visible, and the applied numerical scheme demonstrated stability, accuracy, and the ability to adequately reproduce the physical properties of the process. Thus, the developed model allows for forecasting changes in the lake’s temperature regimes due to thermal pollution and selecting the optimal volume of discharged water to minimize the negative impact.</p>