ATOMIC SCALE SIMULATION OF ACCIDENT TOLERANT FUEL MATERIALS FOR FUTURE NUCLEAR REACTORS

The 2011 accident at the Fukushima-Daiichi power station following the earthquake and tsunami in Japan put renewed emphasis on increasing the accident tolerance of nuclear fuels. Although the main concern in this incident was the loss of coolant and the Zr cladding reacting with water to form hydrogen, the fuel element is an integral part of any accident tolerant fuel (ATF) concept. Therefore, to license a new commercial nuclear fuel, the prediction of fuel behavior during operation becomes a necessity. This requires knowledge of its properties as a function of temperature, pressure, initial fuel microstructure and irradiation history, or more precisely the changes in microstructure due to irradiation and/or oxidation. Amongst other nuclear fuels, uranium diboride (UB2) and uranium silicide (U3Si2) are considered as potential fuels for the next generation of nuclear reactors due to their high uranium density and high thermal conductivity compared to uranium dioxide (UO2). However, the thermophysical properties and behavior of these fuels under extreme conditions are not well known, neither are they readily available in the literature. Therefore, in this thesis, density functional theory (DFT) and classical molecular dynamic (MD) simulations were used to investigate the thermophysical properties, radiation tolerance and oxidation behavior of UB2 and U3Si2 as potential fuels or burnable absorbers for the next generation of nuclear reactors. UB2 was studied in order to understand its thermophysical properties as a function of temperature. The phonon-assisted thermal conductivity (kph) exhibits large directional anisotropy with larger thermal conductivity parallel to the crystal direction. This has implications for the even dissipation of heat. The increase in thermal conductivity with temperature is justified by the electronic contribution to the thermal transport, especially at high temperatures. This shows that UB2 is a potential ATF candidate. In terms of radiation tolerance, Zr is more soluble in UB2 than Xe, while uranium vacancy is the most stable solution site. Furthermore, as the concentration of Zr fission product (FP) increases, there is a contraction in the volume of UB2, while an increase in Xe results in swelling of the fuel matrix. In terms of diffusion, the presence of an FP in the neighboring U site increases the migration of U in UB2, making U migrate more readily than B as observed in the ideal system. The thermophysical properties of U3Si2 as a possible ATF were studied and discussed considering the neutronic penalty of using a SiC cladding in a reactor. The calculated molar heat capacity and experimental data are in reasonable agreement. Due to the anisotropy in lattice expansion, a directional dependence in the linear thermal expansion coefficient was noticed, which has also been experimentally observed. The thermal conductivity of U3Si2 increases with temperature due to the electronic contribution while the phonon contribution decreases with increasing temperature. A comparison of the thermal conductivity in two different crystallographic directions sheds light on the spatial anisotropy in U3Si2 fuel material. The inherent anisotropic thermophysical properties can be used to parametrize phase field models by incorporating anisotropic thermal conductivity and thermal expansion. This allows for a more accurate description of microstructural changes under variable temperature and irradiation conditions. Due to the metallic nature of U3Si2, the oxidation mechanism is of special interest and has to be investigated. Oxidation in O2 and H2O was investigated using experimental and theoretical methods. The presence of oxide signatures was established from X-ray diffraction (XRD) and Raman spectroscopy after oxidation of the solid U3Si2 sample in oxygen. Surface oxidation of U3Si2 can be linked to the significant charge transfer from surface uranium ions to water and/or oxygen molecules. Detailed charge transfer and bond length analysis revealed the preferential formation of mixed oxides of U-O and Si-O on the U3Si2 (001) surface as well as UO2 alone on the U3Si2 (110) and (111) surfaces. Formation of elongated O−O bonds (peroxo) confirmed the dissociation of molecular oxygen before U3Si2 oxidation. Experimental analysis by Raman spectroscopy and XRD of the oxidized U3Si2 samples has revealed the formation of higher uranium oxides such as UO3 and U3O8. Overall, this work serves as a step towards understanding the complex anisotropic behavior of the thermophysical properties of metallic UB2 and U3Si2 considered as potential accident tolerant nuclear fuel. The calculated anisotropy of thermophysical properties can be used to parametrize phase field model and to incorporate in it anisotropic thermal conductivity and thermal expansion

DFT + U study of the adsorption and dissociation of water on clean, defective, and oxygen-covered U3Si2{001}, {110}, and {111} surfaces

The interfacial interaction of U3Si2 with water leads to corrosion of nuclear fuels, which affects various processes in the nuclear fuel cycle. However, the mechanism and molecular-level insights into the early oxidation process of U3Si2 surfaces in the presence of water and oxygen are not fully understood. In this work, we present Hubbard-corrected density functional theory (DFT + U) calculations of the adsorption behavior of water on the low Miller indices of the pristine and defective surfaces as well as water dissociation and accompanied H2 formation mechanisms. The adsorption strength decreases in the order U3Si2{001} > U3Si2{110} > U3Si2{111} for both molecular and dissociative H2O adsorption. Consistent with the superior reactivity, dissociative water adsorption is most stable. We also explored the adsorption of H2O on the oxygen-covered U3Si2 surface and showed that the preadsorbed oxygen could activate the OH bond and speed up the dissociation of H2O. Generally, we found that during adsorption on the oxygen-covered, defective surface, multiple water molecules are thermodynamically more stable on the surface than the water monomer on the pristine surface. Mixed molecular and dissociative water adsorption modes are also noted to be stable on the {111} surface, whereas fully dissociative water adsorption is most stable on the {110} and {001} surfaces

Oxidation behaviour of U3Si2: an experimental and first principles investigation

Uranium-containing metallic systems such as U3Si2 are potential Accident Tolerant Fuels (ATFs) for Light Water Reactors (LWRs) and the next generation of nuclear reactors.</p

Characterization and mechanical property measurements by instrumented indentation testing of Niger Delta oil shale cuttings

Oil shales have unstable mechanical and chemical properties, which makes their extraction for characterization and conventional mechanical testing uneasy and complex. Most often, mechanical property measurements are usually taken from core samples that are costly to extract and test using conventional testing methods. This paper presents a focused study carried out on oil shale cuttings obtained from the sidewalls of two different wellbore depths in the Niger Delta area of Nigeria. Using the X-ray Diffraction (XRD) and Scanning Electron Microscope (SEM) characterization techniques, the morphology of these shales was studied. The results obtained clearly showed the composition, bonding and variations in the morphology of the studied shale samples. Furthermore, the heterogeneity associated with these shales across varied depths were revealed. An efficient and less expensive technique compared to conventional testing methods, instrumented indentation testing (IIT) was carried out to obtain essential mechanical parameters of the shale specimen. These properties are important parameters in determining the hydrocarbon storage space of shale formations, wellbore stability, and optimization of hydraulic fracturing which is necessary for efficient drilling operations.https://www.scientific.net/JERAhj2023Mechanical and Aeronautical Engineerin

Data-driven analysis and prediction of stable phases for high-entropy alloy design

Abstract High-entropy alloys (HEAs) represent a promising class of materials with exceptional structural and functional properties. However, their design and optimization pose challenges due to the large composition-phase space coupled with the complex and diverse nature of the phase formation dynamics. In this study, a data-driven approach that utilizes machine learning (ML) techniques to predict HEA phases and their composition-dependent phases is proposed. By employing a comprehensive dataset comprising 5692 experimental records encompassing 50 elements and 11 phase categories, we compare the performance of various ML models. Our analysis identifies the most influential features for accurate phase prediction. Furthermore, the class imbalance is addressed by employing data augmentation methods, raising the number of records to 1500 in each category, and ensuring a balanced representation of phase categories. The results show that XGBoost and Random Forest consistently outperform the other models, achieving 86% accuracy in predicting all phases. Additionally, this work provides an extensive analysis of HEA phase formers, showing the contributions of elements and features to the presence of specific phases. We also examine the impact of including different phases on ML model accuracy and feature significance. Notably, the findings underscore the need for ML model selection based on specific applications and desired predictions, as feature importance varies across models and phases. This study significantly advances the understanding of HEA phase formation, enabling targeted alloy design and fostering progress in the field of materials science

Accident tolerant composite nuclear fuels

Investigated accident tolerant nuclear fuels are fuels with enhanced thermal conductivity, which can withstand the loss of coolant for a longer time by allowing faster dissipation of heat, thus lowering the centerline temperature and preventing the melting of the fuel. Traditional nuclear fuels have a very low thermal conductivity and can be significantly enhanced if transformed into a composite with a very high thermal conductivity components. In this study, we analyze the thermal properties of various composites of mixed oxides and thoria fuels to improve thermal conductivity for the next generation safer nuclear reactors

Accident tolerant composite nuclear fuels

Investigated accident tolerant nuclear fuels are fuels with enhanced thermal conductivity, which can withstand the loss of coolant for a longer time by allowing faster dissipation of heat, thus lowering the centerline temperature and preventing the melting of the fuel. Traditional nuclear fuels have a very low thermal conductivity and can be significantly enhanced if transformed into a composite with a very high thermal conductivity components. In this study, we analyze the thermal properties of various composites of mixed oxides and thoria fuels to improve thermal conductivity for the next generation safer nuclear reactors