The Influences of the Burnable Absorber (UO2+Gd2O3) on VVER-1200 Fuel Assembly

Abstract

Burnable absorber rods (UO2+Gd2O3) and chemical shim are the main control poisons that are used in the core for improving the reactor behavior and satisfying the safety criteria during the core life time. These absorbers (Gd2O3) have several constraints, criteria, and advantages and also disadvantages from the safety and operation points of view; and these characteristics depend on the concentration and distribution of mentioned poisons in the reactor core. In this study, the influences of the burnable absorbers (Gd2O3) on the main parameters of the reactor such as multiplication factor (Keff), neutron flux and burnup over the reactor life time are investigated. Monte Carlo Serpent code is used to design a three dimensional model for VVER-1200 assembly. The VVER-1200 assembly is loaded in different operation process with 8.0% Gd2O3 as integral burnable absorbers (IBAs). Burnable absorber rods (UO2+Gd2O3) are used in two rings in reactor core, one is central rings with low percentage of absorber (Gd2O3) due to low neutron flux in central part of core, and another is peripheral ring with high percentage of absorber (Gd2O3) because of high neutron flux in peripheral part of core. In this article, optimal position of absorbers (Gd2O3) and optimal amount of absorbers (Gd2O3) in VVER-1200 core are calculated. The strong influence of the burnable absorbers (BAs) composition and their location in the fuel elements on the dependence of the multiplication factor on fuel burnup is shown.