ISIL-NÖTRONİK ETKİLEŞİMLERİN URANYUM-TORYUM İÇEREN NÜKLEER YAKIT ELEMANLARININ TESİR KESİTLERİNE ETKİSİ

Abstract

Neutron effective cross sections those define the interactions of neutrons with fuel elements are important parameters in defining operational and safety limitations of nuclear reactors. Hence, generation of these cross sections is very delicate and demanding calculations. These effective cross sections are related to geometry of fuel elements, isotopes within the fuel, amounts of these isotopes, operational reactor temperature and reactor type. In order to minimize both the costs and calculation time, some simplifications and assumptions are made during these calculations. In general, geometry is simplified by unit fuel cell approach and radial isothermal temperature is assumed within the fuel element. However, there will be a radial parabolic temperature distribution within the fuel element, so this assumption will lead to uncertainties in calculated reactor parameters. Since such temperature assumption is used, effective neutron cross sections, which are directly affected by the temperature, will show variations, especially in the resonance region. Effective cross sections those are generated this way, will also affect the self-shielding calculations. Thereby, by calculating the radial temperature distribution within the fuel element and using that temperature distribution in the calculations will lead to more accurate results. The purpose of this thesis is to determine the uncertainties caused by isothermal temperature assumption by calculating temperature distribution within the fuel by solving the heat equation and coupling that with selected reactor physics software. The aim is to generate effective neutron cross sections of uranium-thorium fuel elements by taking the interactions defined in the previous paragraph into consideration, and to determine the uncertainties caused by ignoring the temperature distribution within the fuel. In the calculations performed in this thesis, unit fuel cells of Three Mile Island Unit-1 (PWR), Peach Bottom Unit-2 (BWR) and Kozloduy Unit-6 (VVER-1000), which are investigated in the “Uncertainty Analysis in Modeling” (UAM) project, are used as reference. To perform spent fuel uncertainty analysis, a typical Westinghouse PWR unit fuel cell of a 17x17 fuel assembly is used, but content of fuel element is changed from UO2 to (Th-U)O2. The reference case used in this work will be given in the following chapters. In the performed work, results of temperature distributed case and isothermal temperature assumed case is compared. Effect of using the temperature profile on multiplication factor, Doppler coefficient, temperature distribution and radial heat generation rates are investigated.