Thermal Conductivity of Nuclear Fuel and its Degradation by Physical and Chemical Burnup

Abstract

Nuclear fuel performance during reactor operation has been studied using both atomic scale simulation and experimental procedure in order to investigate how nuclear fission process affects both the physical state and the chemistry of the fuel. Attention has been drawn to the consequences of nuclear exposure after Fukushima nuclear accident, as it relates to the impact of modern reactor design and nuclear fuel performance. With the recognition of the inherent risks associated with pure uranium oxide (UO2) fuel reactors, there is a need to study nuclear fuel with a view to highlighting their susceptibility to reactor accident, hence, developing an accident tolerant fuel. In this work, cerium oxide (CeO2) has been deployed as a surrogate material for UO2 fuel due to their uniquely similar physicochemical behaviors as fuel materials during operations of nuclear reactors. CeO2 is, however, non-radioactive. The nuclear reactor safety analysis revealed that thermal conductivity is an important property of nuclear fuel because it controls fuel operating temperature and therefore influences its safety. In line with this assertion, two key areas of focus have been identified in this investigation: i) degradation of thermal conductivity by structural and fission products in nuclear fuel and ii) the fuel microstructural evolution due to dissolved fission product. The former has been carried out using molecular dynamics (MD) simulations and analytical models over the full range of temperature of interest while the latter was carried out using both experimental procedure and MD simulations. MD simulations of the structural and thermal properties of CeO2 as a representative of UO2 fuel were carried out using Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) code. The thermal expansion, thermal conductivity, and oxygen ion diffusion were calculated using classical ionic potential models. During these processes, verification of methods was done to establish the best potential for CeO2. The many-body ionic potential in the Embedded Atom Method (EAM) and two-body force field potentials were used to predict lattice parameters and thermal conductivity. Nuclear fuel efficiency changes during reactor operation because of irradiation process. Fission products like fission gas bubble, pores, cracks, dissolved and precipitated fission product buildup in the fuel matrix. The effect of physical burnup such as porosity on the thermophysical properties of CeO2 was simulated using a large system with thousands of atoms. Pores were induced on the large CeO2 system by carefully removing an appropriate number of atoms in proper proportion to mimic porosity evolution. Lattice parameter and the thermal conductivity were calculated at a different percentage of porosity for CeO2. This calculation relates the degradation of thermal conductivity with a number of pores and increasing temperature. In irradiated oxide fuels (UO2 and PuO2), several fission products (FP) are produced and they take various chemical states depending on the conditions of the fuel. Some FPs dissolve as oxides in the fuel matrix. Structural and thermal properties of Ce3ZrO8, a solid solution material formed when Zirconium Oxide (ZrO2) dissolved in CeO2 matrix, were predicted. This calculation indicates the effect of chemical burnup in nuclear fuel as the thermal conductivity of CeO2 was degraded as it forms a solid solution with ZrO2. Xenon (Xe) and Krypton (Kr) are fission gases that collect in pores created in irradiation fuel matrix. The effect of fission gas bubbles like Xe and Kr, a component of chemical burnup, on the thermophysical properties of CeO2 was simulated by strategically placing Xe and Kr atoms separately in pores created in CeO2 to form CeO2-Xe and CeO2-Kr systems. The structure and thermal properties of CeO2-Xe and CeO2-Kr systems were investigated using Buckingham pair potentials for fission gases. At extremely high temperature, where the condition of the fuel becomes unstable, some activities will start to occur in the fuel lattice. These activities contribute to the failure of the fuel to recover quickly during the loss of coolant accident. To better understand the condition of the fuel at high temperatures, the diffusion of atoms in the CeO2 lattice is studied using molecular dynamics. This study clearly reveals the movement between the cerium (Ce) and oxygen (O) at high temperatures and suggest that O atoms break bond at a temperature relatively close to the melting temperature. Spark Plasma Sintering (SPS) is a novel sintering technology capable of sintering diverse materials near 100% theoretical densities. CeO2 and Ce3ZrO8 were synthesized using SPS for experimental analysis. The composition and the microstructure of the as-sintered pellets were characterized via X-ray Diffraction (XRD), Energy Dispersive Spectroscopy (EDS) and Electron Backscatter Diffraction (EBSD). These techniques were used on both CeO2 and Ce3ZrO8 pellet samples to understand the structure, crystal orientation and phase disparity of the two materials. The thermal conductivity of the pellets at different porosity was measured using Direct Laser Flash (DLF) method. The porosity of the samples was studied using the Archimedes principle.