ATOMISTIC AND EXPERIMENTAL DETERMINATION OF THE STRUCTURAL AND THERMOPHYSICAL PROPERTIES OF THE ACCIDENT TOLERANT FUEL MATERIALS

The tragic nuclear accident at the Fukushima-Daiichi power station in Japan brought in to our attention the risk associated with the current design of reactors based on uranium dioxide (UO2) fuel and zirconium cladding. As an offshoot, the research towards accident tolerant nuclear fuel (ATF) that can withstand the loss of coolant for a long time while improving thermal efficiency has gained momentum. Most desirable thermophysical properties expected of an ATF is high thermal conductivity, the lack of which leads to the poor dissipation and rapid meltdown at the core of the fuel pellet during the loss of coolant. Several approaches are being considered by researchers across the world to improve the thermal conductivity of nuclear fuels. Apart from the state of art of uranium-based fuels, there is a renewed interest in thorium-based fuels (especially thorium dioxide (ThO2) and thorium nitride (ThN)) in the quest of ATF. This thesis focuses on evolutionary fuel concepts based on thoria fuels. Unlike UO2, the information regarding the thermophysical properties of ThO2 fuels, and the additive materials under the normal operating conditions and the extreme accident conditions are not well known. Therefore, in this thesis, the computational techniques such as density functional theory (DFT) and classical molecular dynamics (MD) are used to determine the thermophysical properties of the thoria fuel, surrogate of thoria CeO2 and additive materials such as SiC and BeO. One of the significant limitations in the front end of the thoria fuel cycle has the difficulty of fabricating dense pellets by conventional sintering techniques. Hence the processing of thoria fuels by the spark plasma sintering (SPS) was proposed, and the effect of the sintering parameters on the density, microstructure and the thermal conductivity of ThO2 fuel was established. Finally, using SPS, a novel composite fuel of ThO2-SiC has been fabricated with the enhanced thermal conductivity

First-principles determination of the phonon-point defect scattering and thermal transport due to fission products in ThO2

This work presents the first principles calculations of the lattice thermal conductivity degradation due to point defects in thorium dioxide using an alternative solution of the Pierels-Boltzmann transport equation. We have used the non-perturbative Green's function methodology to compute the phonon point defect scattering rates that consider the local distortion around the point defect, including the mass difference changes, interatomic force constants and structural relaxation near the point defects. The point defects considered in the work include the vacancy of thorium and oxygen, substitution of helium, krypton, zirconium, iodine, xenon, in the thorium site, and the three different configuration of the Schottky defects. The results of the phonon-defect scattering rate reveals that among the considered intrinsic defects, the thorium vacancy and helium substitution in the thorium site scatter the phonon most due to substantial changes in the force constant and structural distortions. The scattering of phonons due to the substitutional defects unveils that the zirconium atom scatters phonons the least, followed by xenon, iodine, krypton, and helium. This is contrary to the intuition that the scattering strength follows HeTh > KrTh > ZrTh > ITh > XeTh based on the mass difference. This striking difference in the zirconium phonon scattering is due to the local chemical environment changes. Zirconium is an electropositive element with valency similar to thorium and, therefore, can bond with the oxygen atoms, thus creating less force constant variance compared to iodine, an electronegative element, noble gas helium, xenon, and krypton. These results can serve as the benchmark for the analytical models and help the engineering-scale modeling effort for nuclear design.Comment: 10 page

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

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