Advanced passive design of small modular reactor cooled by heavy liquid metal natural circulation

Highlights: •Describe comprehensive design details of a LBE-cooled small modular reactor. •Describe design of core, heat transport, structural, material, seismic systems. •Conduct steady-state analysis of neutronics and thermal-hydraulic systems. •Develop fully natural circulation systems for normal and abnormal conditions. •Achieve long-burning cycle with less than $1 reactivity swing for 20 years or more. -- Abstract: We applied a passive cooling feature to both normal and abnormal operations of a small modular reactor (SMR) without external power sources and reactor coolant pumps. The SMR, named as URANUS, with a thermal power rating of 100 MW is well suited as a distributed power source because it has a refueling interval of 20 years without assembly reconfiguration. This reactor is a pool type fast reactor with an array of heterogeneous hexagonal core using UO₂ fuels. Material corrosion is limited by using corrosion-resistant materials and keeping acceptable low outlet temperature in combination with an oxygen control technique. The reactivity swing during the core lifetime is less than $1 without burnable poison rods to minimize excess reactivity. Three-dimensional seismic isolators are deployed underneath the entire reactor building allowing an earthquake of 0.5 g zero period acceleration for the safe shutdown earthquake. The adoption of the complete passive safety systems enhances the safety performance of the reactor, but simultaneously limits volume power density and discharge burn-up. Thus, the future study on this reactor will focus on the power maximization of the reactor while maintaining land transportable sizes and coolant natural circulation.