The capital cost of the basic design compared to a helium Brayton cycle is about the same, but the supercritical COB2B cycle operates at significantly lower temperature.
The thermal efficiency of the advanced design is close to 50% and the reactor system with the direct supercritical COB2B cycle is ~ 24% less expensive than the steam indirect cycle and 7% less expensive than a helium direct Brayton cycle. It is expected in the future that high temperature materials will become available and a high performance design with turbine inlet temperatures of 700P o PC will be possible. This high performance design achieves a thermal efficiency approaching 53%, which yields additional cost savings. [Current nuclear reactors are at about 35% thermal efficiency, and some newer designs will have 40-45%]The turbomachinery is highly compact and achieves efficiencies of more than 90%. For the 600 MWBthB/246 MWBeB power plant the turbine body is 1.2 m in diameter and 0.55 m long, which translates into an extremely high power density of 395 MWBeB/mP 3P. The compressors are even more compact as they operate close to the critical point where the density of the fluid is higher than in the turbine. The power conversion unit that houses these components and the generator is 18 m tall and 7.6 m in diameter. Its power density (MWBeB/mP 3 P) is about ~ 46% higher than that of the helium GT-MHR (Gas Turbine Modular Helium Reactor).
Information from a 2004 MIT study that is 6.6 megabytes and 326 pages.
H/T Kirk Sorenson at thoriumenergy for his article "Supercritical CO2 is dense like water".
More information on supercritical carbon dioxide at wikipedia