Neutronic characteristics of the VVER-1000 fuel assembly with mechanical spectral shift regulation

A self-sustaining reactor system with both a high conversion ratio (CR) and fuel discharge burnup (B d ) is essential for enhancing the utilization of nuclear fuel and improving its natural resources. The mechanical spectral shift control (SSC) method can improve the efficiency of nuclear fuel utilization by removing, or at least greatly lowering, soluble and burnable neutron poisons during reactor operation. The implementation of the proposed mechanical SSC method involved the on-power variation of the moderator-to-fuel volume ratio (V M /V F ). This was achieved by means of movable natural uranium spectral shift control rods (NU-SSCRs) distributed uniformly within the fuel assembly. In this work, the neutronic characteristics of the VVER-1000 fuel assembly benchmark with mechanical SSC regulation have been investigated, compared to the traditional poison method described in the OECD/NEA benchmark model. We adopted three different sequences of NU-SSCRs withdrawal strategies at various burnup points, each associated with different V M /V F , during one batch fuel loading pattern. The findings revealed a notable improvement of approximate to 40 % in the discharge burnup across the three different withdrawal strategies, alongside the attainment of a CR value of 0.82 compared to the benchmark poisoned model. Additionally, the mechanical SSC regulation has effectively suppressed the excess reactivity at the beginning of the cycle compared to boric acid (H 3 BO 3 ) and gadolinium oxide (Gd 2 O 3 ), which are incorporated in the benchmark model by 754 pcm. The results showed a higher accumulated mass of plutonium in the fuel assembly of 3.3, 5.2, and 6 kg for the three different sequences of withdrawing strategies, respectively, at the end of the cycle. Furthermore, the void reactivity coefficient has been investigated for the three different withdrawal strategies in both partially voided (20 % void) and completely voided (100 % void) systems.