Design Optimization and Performance Analysis of a Dry-Cooled Helium-Xenon Brayton Cycle for Nuclear Microreactor Based on Detailed Models of the Cycle Components

Nuclear microreactors offer reliable, low-carbon dispatchable power and heat for various end use and applications. The gas-cooled reactor coupled with the dry-cooled closed Brayton cycle has a simple plant layout and can be deployed in water-deficient areas. To evaluate the system performance more precisely and optimize the mole fraction of the He-Xe mixture for the gas-cooled microreactor, an integrated design method based on detailed models of the cycle components was developed. Three designs using 4 g/mol pure helium, 15 g/mol, and 40 g/mol He-Xe were first established as demonstration examples, and performances of the cycle and components were compared. Furthermore, the effects of the He-Xe's molecular weight on the system's size and efficiency were discussed, and a parametric analysis of crucial design parameters like specific speed and pressure ratio was conducted. In contrast to the previous results obtained from simplified component models, the current results revealed that a Brayton cycle using 40 g/mol He-Xe could achieve a higher thermal efficiency of 44.42 % compared with 37.65 % of pure helium, which resulted from the improved efficiency in turbomachinery and reduced pressure loss in heat exchangers. Concerning the system size, it was recommended to use the 20-30 g/mol He-Xe for reduced volume as the stage number of the turbomachinery decreased from 5 of pure helium to a single stage of the 20 g/mol He-Xe. Although the turbomachinery occupied a relatively small volume, increasing the compressor inlet temperature could reduce the volume of the precooler, thus significantly reducing the system volume.