Burnup Performance of a PBR with an Accumulative Fuel Loading Scheme Utilizing Burnable Poison Particles in UO₂ and ROX Fuels

The accumulative fuel loading scheme is a promising design for pebble bed reactors (PBRs). In a previous study, burnup analysis for such a reactor was performed using UO₂ fuel and the results showed that the maximum burnup and operation period were 223 GWD/t and 10.2 years, respectively. In this study, rock-like oxide (ROX) fuel was analyzed in PBR with an accumulative fuel loading scheme. The results showed that a maximum burnup and operation period of 218 GWd/t and 8.4 years, respectively. However, high excess reactivity occurred in the initial condition for both UO₂ and ROX fuels. In this study, burnable poison (BP) particles were used to suppress the initial excess reactivity. Several BP materials such as B₄C, Gd₂O₃, Er₂O₃ and CdO were investigated. In case of UO₂ fuel, the combination of B₄C and Gd₂O₃ was found to be optimal for flattening the reactivity swing in the initial condition. However, due to the fissile density of ROX fuel being about five times lower than that of UO₂ fuel, single-material BP particles (B₄C) seemed to be optimal for flattening the reactivity swing. By utilizing the BP particles at the initial condition, the results of UO₂ and ROX fuels showed that maximum keff was decreased from 1.423 to 1.068 and from 1.357 to 1.033, respectively. On the other hand, the maximum discharge burnup value and core lifetime were quite similar to the results for the reactor that did not use BP particles at the initial condition.