The purpose of the study was to quantify the reduction of carbon dioxide emissions in the energy sector of Ukraine under the scenarios of replacing gas and coal-fired power plants with small modular nuclear reactors and renewable energy sources. The methodology was based on the analysis of international regulatory approaches to assessing the carbon intensity of generation, the examination of the actual technical and economic characteristics of Ukrainian power-generating facilities for 2020-2024, and scenario modelling of changes in total emissions within three options for the transformation of the energy system. The results showed that coal-fired power plants have the highest carbon dioxide intensity in the range of 750-1050 gCO₂/kWh, gas stations form 380-510 gCO₂/kWh, while nuclear power plants and small modular reactors provide 8-16 gCO₂/kWh. Calculations have shown that replacing 20-30% of thermal generation provides an 18-27% reduction in emissions, while the scenario of replacing more than 50% of thermal capacity gives more than 45%. Small modular nuclear reactors have been found to stabilise the operating mode of the power grid and reduce the variability of emission indicators, while solar and wind power plants provide an additional potential for reducing thermal generation emissions in the range of 330-1050 gCO₂/kWh, depending on which thermal technology they replace. The results show that the combined use of small modular nuclear reactors, renewable energy sources, and traditional nuclear power plants allows achieving an integral level of carbon intensity of 100-120 gCO₂/kWh, which corresponds to the parameters of low-carbon energy systems of the European Union. The practical value of the study lies in the formation of scientifically based scenarios for the modernisation of the Ukrainian energy system, which can be used for planning investment decisions and strategies for deep decarbonisation.