On steady state noncondensable gas buildup in Autonomous Reactivity Control (ARC) systems

The Autonomous Reactivity Control (ARC) system, a new engineered passive safety system for liquid metal reactors, has been the subject of many recent studies, having shown the potential to improve both safety and performance of liquid metal reactors while introducing only a minor impact on normal operations. This study investigates one area of ARC system performance which has not yet been examined — the production of internal noncondensable gases during normal operations. Using neutronic calculations, this study finds that a non-negligible quantity of gases will be produced through transmutation reactions of the internal ARC system fluids, and a lower bound on this quantity is calculated. Through a series of geometric arguments and considerations on mutual solubility and chemical reactivity, the volume and distribution of noncondensable gas buildup is examined for a wide range of ARC system designs. To demonstrate the impact of these noncondensable gases, a series of thermal-hydraulic and transient calculations are performed, and it is shown that the production of noncondensable gases from transmutation of the liquid potassium expander fluid will have negative impacts on the performance of the ARC system, causing an unsteady and restricted actuation which will lead to a reduction of benefits for a given reactor’s transient performance and an increased uncertainty on the state of the system during reactor operations. In the end, multiple mitigation strategies are discussed, but ultimately this study motivates a reevaluation of the fundamental ARC system design so that its benefits may be most fully realized.