Performance of Supercritical CO2 Brayton Cycle with Additive Gases for SFR Application

by Woo Seok Jeong, Yong Hoon Jeong, & Jeong Ik Lee
Korea Advanced Institute of Science & Technology (KAIST)

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A Sodium-cooled Fast Reactor (SFR) is one of the strongest candidates for the next generation nuclear reactor. However, the conventional design of a SFR concept with a Rankine cycle is subjected to a sodium-water reaction. To prevent any hazards from sodium-water reaction, a SFR with the Brayton cycle using Supercritical Carbon dioxide (S-CO2) as working fluid can be an alternative approach to improve the current SFR design. However, the S-CO2 Brayton cycle is more sensitive to the critical point of working fluids than other Brayton cycles. This is because compressor work is significantly decreased slightly above the critical point. For this reason, the minimum temperature and pressure of cycle are just above the CO2 critical point. In general, lowering rejection temperature of a thermodynamic cycle can increase the efficiency. Therefore, changing the critical point of CO2 can result in an improvement of the total cycle efficiency. Other gases can be added in order to change the critical point of CO2. Several gases that show chemical stability within the interested range of cycle operating condition were chosen as candidates for the mixture; CO2 mixing with Ar, Xe, N2 and O2. To evaluate the effect of shifting the critical point and changes of properties on the S-CO2 Brayton cycle, a supercritical Brayton cycle analysis code was developed. As a result of the simulation, CO2-Xe binary mixture shows the highest cycle efficiency increase. Contrary to CO2-Xe binary mixture, the cycle efficiencies of CO2-Ar, CO2-N2, and CO2-O2 binary mixtures are decreased compared to the pure S-CO2 cycle. It is found that the increment of critical pressure leads to decrease of cycle operating pressure ratio which results in negative effect on total cycle efficiency. In addition, the effects from changed minimum operation condition and property variations of multi-component working fluid changed the recuperated heat in the cycle which closely related to cycle performances.