This paper deals with the issue of increasing efficiency of nuclear power plants with the modular high-temperature gas reactor (HTGR) and direct gas-turbine cycle. It should be noted that only this combination can highlight the advantages of the HTGR, namely, the ability to heat helium to about $1000°C$, in comparison with other reactor plants for electricity generation. The HTGR has never been used in the direct gas-turbine cycle. At present, several designs of such commercial plants are at the stage of experimental validation of main technical features. In Russia, OKB Mechanical Engineering together with General Atomics (United States) are developing the GT-MHR project with a reactor power of 600 MW, a reactor outlet helium temperature of $850°C$, and an efficiency of about 45.2%; the South African Republic is developing the РBMR project with a reactor power of 400 MW, a reactor outlet helium temperature of $900°C$, and an efficiency of about 42%; and Japan is developing the GTHTR-300 project with a reactor power of 600 MW, a reactor outlet helium temperature of $850°C$, and an efficiency of about 45.6%. As it has been proven by technical and economic estimations, one of the most important factors for successful promotion of reactor designs is their net efficiency, which must not be lower than 47%. A significant advantage of a reactor plant with the HTGR and gas-turbine power conversion unit over the steam cycle is considerable simplification of the power unit layout and reduction in the required equipment and systems (no steam generators, no turbine hall including steam lines, condenser, deaerator, etc.), which makes the gas-turbine power conversion unit more compact and less costly in production, operation, and maintenance. However, in spite of this advantage, it seems that in the projects currently being developed, the potential of the gas-turbine cycle and high-temperature reactor to more efficiently generate electricity is not fully used. For example, in modern reactor plants with highly recuperative steam cycle with supercritical heat parameters, the net efficiency of electricity generation reaches 50–55%. There are three methods of the Brayton cycle carnotization: regeneration, helium cooldown during compression, and heat supply during expansion. These methods can be used both separately and in combination, which gives a total of seven complex heat flow diagrams. Besides, there are ways to increase helium temperature at the reactor inlet and outlet, to reduce hydraulic losses in the helium path, to increase the turbomachine rotation speed in order to improve the turbine and compressor efficiency, to reduce helium leaks in the circulation path, etc. The analysis of GT-MHR, PBMR, and GTHTR-300 development experience allows identification of the main ways of increasing the efficiency by selecting optimal parameters and design solutions for the reactor and power conversion unit. This paper estimates the probability of reaching the maximum electricity generation efficiency in reactor plants with the HTGR and gas-turbine cycle with account of the up-to-date development status of major reactor plant components (reactor, vessels, turbocompressor, generator, heat exchange equipment, and structural materials).

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