Design concept for a small pebble bed reactor with ROX fuel

The conceptual design of a small rock-like oxide fuel pebble bed reactor with once-though-then-out (OTTO) cycle is proposed here. TRISO-coated particles based on AGR-1 design were used to achieve a target burnup larger than 100 GWd/t-HM without any failure of spent fuel. In the first step, optimization of fuel composition was implemented by cell calculations. After that, whole core calculations were performed with and without movement of the fuel pebbles. With a heavy metal amount of 2 g per pebble and 20% uranium enrichment, the pebble bed reactor with OTTO cycle could achieve maximum burnup of about 145 GWd/t-HM and fissions per initial fissile atom (FIFA) of 75%. The results show that the core height can be reduced due to the fact that the impact of bottom core on burnup performance is insignificant. Also, the peak power density of the reactor exceeded the limit of that for the PBMR design. Therefore, subsequent optimizations of the core design were carried out by decreasing the core height and reactor power to reduce the construction cost as well as the peak power density. A reactor with 6-m core height and 120-MWth reactor power was ultimately determined as the optimal design for a pebble bed reactor with ROX fuel. This optimal design also has a negative temperature coefficient, and the peak power density was less than the limit of 10 W/cm3.