Conceptual design of a supercritical water reactor with double-row-rod assembly

A novel type of fuel assemblies with double rows of fuel rods between water rods is proposed and optimized for a supercritical light water reactor design. It brings improved neutron moderation and lower local power peak. Gadolinium is introduced as burnable poison to reduce excess reactivity at the beginning of the fuel cycle. The optimizations for the fuel rods with gadolinium are performed in the present paper. SS316L is used for fuel rod cladding and structural material. In order to reduce the amount of SS316L because of its high thermal neutron absorption, honeycomb structure filled with thermal isolation is introduced to replace the solid stainless steel. The two-pass water flow scheme is chosen with more fuel assemblies for downward flow. Fuel in-core loading pattern and control rod clusters pattern are designed to flatten power distribution at inner regions to enhance coolant outlet temperature. Axial fuel enrichment is zoned into three regions to control axial power peak, which might affect maximum cladding surface temperature. An equilibrium core is then analyzed based on neutronics/thermal-hydraulics coupling model. The numerical results indicate that a high average coolant outlet temperature of 500 °C is achieved with a maximum cladding surface temperature less than 650 °C. The void reactivity effects of moderator and coolant are negative throughout the cycle.