Effects of various spacer grid modeling on the neutronic parameters of the VVER-1000 reactor

Abstract

The main objective of this paper is to study the effects of various spacer grid models on the neutronic parameters of a VVER-1000 reactor. Specifically, the data of the nuclear power plant at the Bushehr site, which is of a VVER-1000 type, will be studied. Three models, representing the spacer grids along the fuel assemblies are presented. These three models are the homogeneous and the heterogeneous local spacer grid models and the shroud spacer grid model. In the homogeneous and the heterogeneous models, the spacer grids are considered at their actual locations in the axial direction. The only difference between the two models is that in the homogeneous model, the spacer grids are homogenized with the coolant while in the heterogeneous model, the spacer grids are modeled around the fuel cells at their exact axial positions. In the shroud model, the spacer grids are modeled in the shroud region containing the coolant and are not necessarily placed at their appropriate axial positions. The aforementioned models and the core geometry are modeled in 3-D hexagonal geometry and solved by the CITATION code. The required cross sections are obtained from the WIMS code using the associated ENDF/B-7 based library. The visual FORTRAN programming 90 is used for analysis of the whole calculation process and provides a sophisticated link between the WIMS and the CITATION codes during core cycle burnup. The core excess reactivity and the critical boron concentration of the reactor are calculated at normal operating state. In addition, the changes in the critical boron concentration and power peaking factors during the first core cycle, for the homogeneous and shroud models are also predicted. The core calculation analysis is done for the core with fully withdrawn control rods. As will be shown, the heterogeneous model, which is a close representation of the actual case, provides a more accurate result than the homogeneous and the shroud models. However, the heterogeneous model is complicated as compared to the other two models. Therefore, the choice of which model to use depends on the accuracy required by the user.