Nuclear safety is one of the most important tasks when analyzing the operation of nuclear power plants. In order to predict the behavior of thermohydraulic processes inside the reactor, computer software such as the ASYST program is used. Considering the complexity of a real nuclear plant, ASYST provides many additional features included in the program. With them, better quality and more precise final results can be achieved. One such feature is uncertainty analysis. Uncertainties are caused by the random variability of the parameters that describe the system and the lack of precision of computer physical models. Uncertainty analysis is carried out for the purpose of determining the acceptance criteria, since the fulfillment of the acceptance criteria and obtaining the operating license represent the main task of nuclear facilities. In this thesis, an uncertainty analysis was made based on the Krško Nuclear Power Plant model written in the form of code for the ASYST program. In order to carry out the uncertainty calculation, input uncertainty parameters were selected whose values vary according to the defined probability density functions. In addition, the confidence level and interval and the number of important factors were determined with the aim of obtaining the number of necessary calculations. After the simulation of 22 calculations, graphs of all the most important variables of the primary system in the nuclear power plant (pressures, temperatures, flows, produced hydrogen, the core damage) are presented and uncertainty limits are determined. Statistical analysis was used to calculate the dispersion of the results, to determine the importance measures used to assess the connections between input parameters and output results and to perform a sensitivity analysis.