Supercritical CO2 Cycle for Fast Gas-Cooled Reactors

Abstract

Brayton cycles are currently being extensively investigated for possible use with nuclear reactors in order to reduce capital cost, shorten construction period and increase nuclear power plant efficiency. The main candidates are the well-known helium Brayton cycle and the less familiar supercritical CO2 cycle, which has been given increased attention in the past several years. The main advantage of the supercritical CO2 cycle is comparable efficiency with the helium Brayton cycle at significantly lower temperature (550°C/823K), but higher pressure (20MPa/200 normal atmospheres). By taking advantage of the abrupt property changes near the critical point of CO2 the compression work can be reduced, which results in a significant efficiency improvement. Among the surveyed compound cycles the recompression cycle offers the highest efficiency, while still retaining simplicity. The turbomachinery is highly compact and achieves efficiencies of more than 90%. Preliminary assessment of the control scheme has been performed as well. It was found that conventional inventory control could not be applied to the supercritical CO2 recompression cycle. The conventional bypass control is applicable. The reference cycle achieves 46% thermal efficiency at the compressor outlet pressure of 20MPa and turbine inlet temperature of 550°C. The sizing of the heat exchangers and turbomachinery has been performed. The recuperator specific volume is 0.39m3/MWe and pre-cooler specific volume 0.08m3/MWe. For the reference 600MWth reactor this translates to ∼ 99m3 heat exchanger core for the recuperator and ∼ 21m3 for the pre-cooler. Overall the cycle offers an attractive alternative to the steam cycle. The supercritical CO2 cycle is well suited to any type of nuclear reactor with core outlet temperature above ∼ 500°C.