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Previous pre-conceptual core neutronics and system thermal hydraulics calculations initiated the investigation of viability of a Small Secure Transportable Autonomous Reactor (SSTAR) lead-cooled small modular fast reactor concept.1 The calculations indicated that a single-phase natural circulation SSTAR reactor concept with good core reactor physics performance, good system thermal hydraulics performance, and a high Supercritical Carbon Dioxide (S-CO{sub 2}) Brayton cycle efficiency of 40 % may be viable at an electrical power of 18 MWe (45 MWt). Pre-conceptual studies of SSTAR viability have continued with the objective of improving the system thermal hydraulic performance and raising the plant efficiency as well as extending the neutronics analysis. This effort has been motivated by several considerations. First, the initial Pre-conceptual studies were focused upon a ''pancake'' core having a height-to-diameter of 0.5. It was found that a compact core with high average burn up could be realized with a height-to-diameter ratio of 0.8. Second, the initial assumed reactor vessel height of 12.2 meters limited the height of the Pb-to-CO{sub 2} in-reactor heat exchangers (HXs) which reduced the efficiency of supercritical carbon dioxide (S-CO2) Brayton cycle power converter. It was found that by increasing the reactor vessel height to 18 meters, the greater driving head for single-phase natural circulation would offset both the greater pressure drop of the 0.8 height-to-diameter ratio core as well as the pressure drop of taller HXs. This has enabled the plant efficiency to be increased from 40 to 43 % and the plant electrical power to be raised from 18 to 20 MWe. Third, reactivity feedback coefficients, which had previously not been generated for SSTAR, have now been calculated for the core. The reactivity feedback coefficients provide a basis for future investigation of the autonomous load following and passive shutdown behavior of the reactor. The current status of SSTAR and the Pre-conceptual viability studies are described below

Gen IV Nuclear Energy Systems Interim Status Report on Pre-conceptual LFR Design Studies and Evaluations