Neutron excess method for performance assessment of thorium-based fuel in a breed-and-burn reactor with various coolants

Fertile fuel, such as thorium or depleted uranium, can be bred into fissile fuel and burnt in a breed-and-burn (B&B) reactor. Modeling a full core with fertile fuel can assess the performance of a B&B reactor with exact quantitative estimates, but costs too much computation time. For simplicity, performing the recently developed neutron balance method with a zero-dimensional (0-D) model can also provide a reasonable result. Based on the 0-D model, the feasibility of the B&B mode for thorium fuel in a fast reactor cooled by sodium was investigated by considering the (n, 2n) and (n, 3n) reaction rates of fuel and coolant in this work, and compared with that of depleted uranium fuel. Afterward, the performance of the same thorium-based fuel core, but cooled by helium, lead-bismuth, and FLiBe, respectively, is discussed. It is found that the (n, 2n) and (n, 3n) reactions should not be neglected for the neutron balance calculation for thorium-based fuel to sustain the B&B mode of operation.