EFECTO DE LA FRACCIÓN DE VACÍOS EN EL QUEMADO DE COMBUSTIBLE NUCLEAR

Abstract

The design of fuel assemblies for a boiling water reactor (BWR) has evolved in recent years due to the addition of certain features seeking a more efficient burning of the fuel assemblies such as internal water channels regions, designed to flatten the power distribution within the assembly, and the use of some partial fuel rods that increase locally the flow in the channel area, however these characteristics has introduced large uncertainties regarding the distribution of voids and their impact on physical parameters of reactor. This void distribution has an important effect over the coolant moderation properties with a significant effect on nuclear parameters such as: the infinite multiplication factor (k∞), migration area (M2), the local peak factor (LPF) and the void coefficient of reactivity (αv). In this Master Thesis, a study has been performed of the variation of nuclear parameters of the assembly, caused by the presence of voids in the moderator assembly. The fuel assembly design SVEA-96 for a BWR has been chosen for the study, and it has been assessed using the codes INTERPIN-3 and CASMO-4. The geometry of the SVEA-96 is characterized by subdivision assembly in four sub-bundles, through an internal bypass cross-shaped gap to allow a more uniform distribution of the moderator, providing a better distribution in the flow of neutrons, and thus a better distribution of the energy generated and burnup. Furthermore, this work analyzes the uncertainties associated with the heterogeneous coolant distribution and the effects caused by the moderator void fraction distribution through the assembly burnup. The INTERPIN-3 code was used to determine the fuel assembly operation temperature, which is one of the parameters used by CASMO-4. This latter code performs two dimensional fuel assembly calculations for the burnup steps considered, and it reports results of nuclear parameters for the design of fuel assemblies. The study was conducted for a wide range of void fractions, from 0% (pure liquid) to 80% and covered: 1) The effects caused by the presence of voids during the burning of nuclear fuel, 2) the effects of the structure of energy groups including libraries of cross sections of the code CASMO-4, 3) the impact of the presence of control rod, and 4) consideration of a radial distribution homogeneous and heterogeneous empty.