Although most of the discussed Gas Turbine Modular Helium Reactor power plants are based on the Rankine cycle, the use of the complex Brayton cycle can be an advantage. However, scientific papers published in recent years do not evaluate this combination in more detail. This paper provides a thermodynamic analysis of the modular nuclear plant with thermal power of 250 MW, consisting of a helium-cooled nuclear reactor and the energy conversion unit, using the vertical gas turbine operating according to the complex Brayton cycle. Two modes of nuclear plant operation were considered, mainly electricity generation and combined electricity and heat production. The energy conversions unit parameters, such as the electrical efficiency, electrical power, and thermal power of heat regenerator and heat exchangers were obtained and analyzed. The results have confirmed that high cycle efficiency in the electricity production mode can be obtained if the best parameters of all in-plant elements currently achieved in the modern gas turbine and power engineering industries are used in the design. As found, the temperature coefficient of helium intercooling demonstrates a great impact on the nuclear plant's electrical efficiency and electrical power. The sensitive analysis was carried out to assess the reduction of GT-MHR-250 performance due to deterioration of the in-plant elements (turbine, compressor, heat exchangers) during operation. In this case decrease in the plant's electrical efficiency and electrical power is more noticeable in the combined mode rather than in the electricity generation.
Currently, approximately 80 % of the world’s energy demand comes from fossil fuels, while only about 30 % of the energy is used to produce electricity. Climate warming and environmental issues are accompanied with fossil energy generation. There are many approaches have developed and continue to develop to decrease the environmental load from fossil fuels combustion that focused research interests on the mixing biofuels, syngas, and hydrocarbon fuels [1,2]; application of combustible and non-combustible additives to fossil fuels, for example, water (coal-water fuel)  or glycerol ; on the specific treatment of the fuel composition before or during the combustion , and on the organizing of the combustion in some more efficient way [6–8]. Much research was also devoted to the utilization of coal and oil waste , ammonia combustion for power engineering purposes , and hydrogen energy utilization [11,12]. However, none of them do implement on the industrial level, up to date the main part of electricity and heat generation is based on the combustion processes. On the other hand, the deep energy crisis that is developed in the world due to natural gas market price manipulation by Russia clearly shows that mankind needs another energy source, which would be sustainable and environmentally friendly.
Nuclear energy is now being employed for about 14 % of the world’s electricity production, moreover, in some countries (France, and Belgium) nuclear power dominates by producing over 60 % of the electrical energy. Nuclear power could be the option for reducing carbon dioxide emissions; for example, nuclear power plants in Europe reduce annually up to 700 million tons of CO2 discharged into the atmosphere. From this point of view, nuclear energy is considered “green energy”.
As of the year 2021, more than 440 nuclear power reactors are in the operation throughout the world, over 100 of them are in the United States, while about 200 are in France, China, and Russia. In addition, more than 50 nuclear reactors are now under construction in different countries. Currently, the park of nuclear power stations is based on reactors from 1000 to 1600 MW of thermal power. However, they are difficult to operate and control, occupy a large area, and require many maintenance personnel (1 person per 1 MW of plant capacity). In recent years, the movement towards more flexible and low-power units of (200 – 600) MW occurred, operating independently or as a part of larger energy systems.
The thermodynamic parameters of the GT-MHR modular nuclear plant of 250 MW thermal power with helium reactor and vertical gas turbine, operating according to the complex Brayton cycle were obtained and discussed in this paper. Two operating modes of the nuclear plant, namely the electricity generation mode and the combined mode with electricity and heat production were considered. The analysis was carried out to define the GT-MHR-250 plant sensitivity concern in performance deterioration of the in-plant elements during their operation. Based on the results obtained, the following main conclusions are drawn as follows:
• In the electricity production mode, quite a high electrical efficiency of the plant was achieved (46.3 %) if the best parameters of in-plant elements currently achieved in the gas turbine and power engineering industries are used.
• In the combined mode the plant’s thermal power is 182.13 MW, while the electrical power is only 69.66 MW with an electrical efficiency of 27.9 %.
• The scheme of the GT-MHR-250 nuclear plant is low sensitive concerning performance deterioration of in-plant elements however reduction in the helium intercooling between compressor stages affects greatly the plant’s electrical efficiency and electrical power.
• A decrease in the plant’s electrical efficiency and electrical power due to performance deterioration of in-plant elements is more noticeable in the combined mode rather than in the electricity production mode.