Searching for the optimum number of integral burnable absorber rods used in PWR assembly

Abstract

This article discusses the effect of different Integral Burnable Absorber numbers (IBAs) on the neutronic characteristics of a Pressurised Water Reactor (PWR) to provide a suitable safety level. MCNPX code version 2.6 was used to design a three dimensional model for PWR assembly. The designed model has been validated by comparing the output values of the infinity multiplication factor (Kinf) with a previously published value. The designed MCNPX model was used to analyse the radial distribution of thermal neutrons and the power through PWR assembly with and without IBA. Due to the high absorption cross-section of gadolinium, it has been used as a burnable absorber material in the IBA rods. The gadolinium isotopes suppressed the power in the regions where they were distributed. The existence of IBA rods has a large effect on the Kinf. This effect decreases gradually with burnup due to the degradation of gadolinium. The gadolinium isotopes degradation was analysed with burnup. Different numbers of IBA rods were investigated to optimise the suitable number that can be used in the PWR assembly. The reactivity has been investigated at different numbers of IBAs. The gadolinium effect on the concentration of 135Xe and 149Sm resulting from the fission process was analysed.