Improvement of the VVER-1200 Fuel Cycle by Introducing Thorium with Different Fissile Material in Blanket-Seed Assembly

This work presents a comparison between the homogeneous and heterogeneous [seed-blanket (SB)] fuel assembly used in the VVER-1200 core. The MCNPX 2.7 code with the ENDF/B-VII.0 data library was used to investigate the possible advantages that can be achieved when the SB assembly is used instead of homogeneous assembly. Thorium-232 was used as a fertile material in the blanket region and different fissile materials were investigated in the seed region. The neutronic characteristics of the presented designs were investigated by comparing four different combinations of fissile materials with (Th,U)O₂ that were distributed uniformly through the whole assembly. The radial power distribution was investigated in both homogeneous and SB assemblies. The power distribution is flatter in the homogeneous assembly than the heterogeneous assembly. The suggested fuels in the SB assembly achieved a longer fuel cycle than the homogeneous assembly. Neutronic parameters related to reactor safety operation, such as control rod worth, Doppler reactivity coefficient, and effective delayed neutron fraction β_eff have been investigated for the suggested fuel types. The SB assembly achieved a higher conversion ratio than the homogeneous assembly. Therefore, the fissile inventory ratio decreased more slowly with burnup in the case of SB than in the homogeneous assembly. Using ²³²Th instead of ²³⁸U reduced the production of the plutonium and the transuranic atoms.