The advanced nuclear concept group at Sandia National Laboratories has been investigating two advance right size reactors (RSR); this paper will discuss one of these two systems. The supercritical carbon dioxide (S-CO2), direct cycle gas fast reactor (SC-GFR) concept was developed to determine the feasibility of a RSR type concept using S-CO2 as the working fluid in a direct cycle fast reactor. Although a significant amount of work is still required, this type of reactor concept maintains some potentially significant advantages over ideal gas-cooled systems and liquid metal-cooled systems. The analyses presented in this paper show that a relatively small long-life reactor core could be developed that maintains decay heat removal by natural circulation. The SC-GFR concept is a relatively small (200 MWth) fast reactor that is cooled with CO2 at a pressure of 20 MPa. The CO2 flows out of the reactor vessel at ∼650°C directly into a turbine-generator unit to produce electrical power. The thermodynamic cycle that is used for the power conversion is a supercritical gas Brayton cycle with CO2 as the working fluid. With the CO2 gas near the critical point after the heat rejection portion of the cycle, it can be compressed with less power as compared to a standard gas Brayton cycle, thereby allowing for a higher thermal efficiency at the same turbine inlet temperature. A cycle efficiency of 45–50% is theoretically achievable for an optimized configuration. The major advantages of the concept include the following: • High thermal efficiency at relatively low reactor outlet temperatures; • Compact, cost-effective, power conversion system; • Non-flammable, stable, inert, non-toxic, inexpensive, and well-characterized coolant; • Potential long-life core and closed fuel cycle; • Small void reactivity worth from loss of coolant; • Natural convection decay heat removal; • Feasible design using today’s technologies. The goal of this work was to develop a SC-GFR concept and perform scoping analyses, including a review of other concepts that are similar in nature, to determine concept feasibility, advantages, disadvantages, and issues requiring further investigation. Overall, the SC-GFR concept as described in this paper appears feasible and warrants further study.

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