Improvement of core design of small pebble bed reactor with accumulative fuel loading scheme

Innovative nuclear power plant designs and high-efficiency utilization of nuclear fuel are important issues in the field of nuclear power. Pebble bed reactors with an accumulative fuel loading scheme have been introduced to obtain high burnup and efficient uranium utilization. Monte Carlo codes, MVP/MVP-BURN, were used to perform the neutron transport and burnup calculation. Optimum fuel composition was obtained in the finite geometry using 6-g HM of uranium per pebble ball with 20% 235U enrichment. The results show that the maximum burnup was 223 GWd/t with 10.2 years of operation. However, a large amount of excess reactivity occurred in the initial condition. One of the options for minimizing this was to reduce the enrichment of 235U from 20% to 3.42%, only for the initial condition. The result showed a relatively small amount of excess reactivity during the operation period. However, the maximum burnup decreased to 199 GWd/t with 8 years of operation.