A Supercritical CO₂ Gas Turbine Power Cycle for Next-Generation Nuclear Reactors Available to Purchase

Although proposed more than 35 years ago, the use of supercritical CO₂ as the working fluid in a closed circuit Brayton cycle has so far not been implemented in practice. Industrial experience in several other relevant applications has improved prospects, and its good efficiency at modest temperatures (e.g., ∼45% at 550°C) make this cycle attractive for a variety of advanced nuclear reactor concepts. The version described here is for a gas-cooled, modular fast reactor. In the proposed gas-cooled fast breeder reactor design of present interest, CO₂ is also especially attractive because it allows the use of metal fuel and core structures. The principal advantage of a supercritical CO₂ Brayton cycle is its reduced compression work compared to an ideal gas such as helium: about 15% of gross power turbine output vs. 40% or so. This also permits the simplification of use of a single compressor stage without intercooling. The requisite high pressure (∼20 MPa) also has the benefit of more compact heat exchangers and turbines. Finally, CO₂ requires significantly fewer turbine stages than He, its principal competitor for nuclear gas turbine service. One disadvantage of CO₂ in a direct cycle application is the production of N-16, which will require turbine plant shielding (albeit much less than in a BWR). The cycle efficiency is also very sensitive to recuperator effectiveness and compressor inlet temperature. It was found necessary to split the recuperator into separate high- and low-temperature components, and to employ intermediate recompression, to avoid having a pinch-point in the cold end of the recuperator. Over the past several decades developments have taken place that make the acceptance of supercritical CO₂ systems more likely: supercritical CO₂ pipelines are in use in the western US in oil-recovery operations; 14 advanced gas-cooled reactors (AGR) are employed in the UK at CO₂ temperatures up to 650°C; and utilities now have experience with Rankine cycle power plants at pressures as high as 25 MPa. Furthermore, CO₂ is the subject of R&D as the working fluid in schemes to sequester CO₂ from fossil fuel combustion and for refrigeration service as a replacement for CFCs.