Implementation of simultaneous mechanical- and chemical-spectral shift control mechanisms on the VVER-1200 assembly fueled with UO2 and MOX fuels

Abstract

In the present study, simultaneous mechanical- and chemical-spectral shift control mechanisms are implemented on the VVER-1200 assembly. The chemical regulation is applied by varying the volume fraction of heavy water in mixed $H_{2}O/D_{2}O$ moderator during the burnup. The mechanical spectral shift regulation is based on the burnup-dependent change of the volumetric ratio of the moderator to fuel. It also requires modifying the structural characteristics of the reference fuel assembly. For this aim, without any change in the total fissile mass, the spectral-shift control fuel assembly (SSC-FA) is introduced by reducing both the fuel rod pitch and fuel rod numbers. This modification, in turn, increases the unit-cell numbers, where some of the unit-cells are allocated to the movable spectral shift control rods (SSCRs) filled with ${\mathrm{ThO}}_2$. To implement the suggested simultaneous mechanisms, the burnup steps of a certain length are considered, and the optimum volume fraction of heavy water and optimum insertion heights of the SSCRs in each burnup step are calculated. The implementation of the suggested methodology shows a significant improvement in both cycle burnup and conversion ratio compared to those of the reference assembly. These improvements also increase by decreasing the burnup step length. The proposed method is also applied to the equivalent MOX-fueled SSC-FA. It is observed that the use of MOX reduces the heavy water requirement and also provides slightly greater improvement in both the cycle burnup and conversion ratio compared to ${\mathrm{UO}}_2$-fueled SSC-FA. Finally, for the considered cases, burnup-dependent variations of the fuel compositions, temperature reactivity coefficients, pin power peaking factors, and kinetics parameters are calculated and compared with each other.