Dry-Cooled Supercritical CO₂ Power for Advanced Nuclear Reactors

Currently, waste heat rejection from electrical power systems accounts for the largest fraction of water withdrawals from the U.S. fresh water table. Siting of nuclear power plants is limited to areas with access to a large natural supply of fresh or sea water. Due to a rise in energy needs and increased concern over environmental impact, dry air cooling systems are poised to play a large role in the future energy economy. In practice, the implementation of dry air-cooled condensing systems at steam plants has proven to be capital-intensive and requires the power cycle to take a significant efficiency penalty. These shortcomings are fundamental to dry-air steam condensation, which must occur at a fixed temperature. Closed-cycle gas turbines are an alternative to the conventional steam Rankine plant that allows for much improved dry heat rejection compatibility. Recent research into advanced nuclear energy systems has identified the supercritical CO₂ (s-CO₂) Brayton cycle in particular as a viable candidate for many proposed reactor types. The s-CO₂ Brayton cycle can maintain superior thermal efficiency over a wide range of ambient temperatures, making these power systems ideally suited for dry air cooling, even in warm climates. For a sodium fast reactor (SFR) operating at 550 °C, thermal efficiency is calculated to be 43% with a 50 °C compressor inlet temperature. This is achieved by raising CO₂ compressor inlet pressure in response to rising ambient temperatures. Preliminary design studies have shown that s-CO₂ power cycle hardware will be compact and therefore well-matched to near-term and advanced integral small modular reactor (SMR) designs. These advantages also extend to the cooling plant, where it is estimated that dry cooling towers for an SFR-coupled s-CO₂ power cycle will be similar in cost and scale to the evaporative cooling tower for a light-water reactor (LWR). The projected benefits of the s-CO₂ power cycle coupled to dry air heat rejection may enable the long-awaited rise of next-generation nuclear energy systems, while redrawing the map for siting of small and large nuclear energy systems.