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 an indirect Rankine cycle is inevitably subjected to a sodium-water reaction. To prevent hazardous situation caused by sodium-water reaction, the SFR with Brayton cycle using Supercritical Carbon dioxide
($S-CO_2$ cycle) as a working fluid can be an alternative approach.
The $S-CO_2$ Brayton cycle is more sensitive to the critical point of working fluids than other Brayton cycles. This is because compressor work significantly decreases at slightly above the critical point due to high density near the boundary between the supercritical state and the subcritical state. For this reason, the minimum temperature and pressure of cycle are just above the $CO_2$ critical point. The critical point acts as a limitation of the lowest operating condition of the cycle. In general, lowering the rejection temperature of a thermodynamic cycle increases the efficiency and thus, changing the critical point of $CO_2$ can result in an improvement of the total cycle efficiency with the same cycle layout.
Modifying the critical point of the working fluid can be done by adding other gases to $CO_2$. The direction and range of the $CO_2$ critical point variation depends on the mixed component and its amount. In particular, chemical reactivity of the gas mixture itself and the gas mixture with sodium at high temperatures are of interest. To modify the critical point of the working fluid, several gases were chosen as candidates by which chemical stability with sodium within the interested range of cycle operating condition was assured; $CO_2$ was mixed with $N_2$, $O_2$, He, Ar and Xe.
To evaluate the effect of shifting the critical point and changes in the properties of the
$S-CO_2$ Brayton cycle, a supercritical Brayton cycle analysis code connected with the REFPROP program from the NIST was developed. The developed co...