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

Abstract

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.