Thermal conductivity is a critical fuel performance property not only for current UO2 oxide fuel based light water reactors but also important for next-generation fast reactors that use U-Zr based metallic fuels. In this work, the thermal transport properties of both UO2 based oxide fuels and U-Zr based metallic fuels have been studied. At first, molecular dynamics (MD) simulations were conducted to study the effect of dispersed Xe fission gas atoms on the UO2 thermal conductivity. Numerous studies have demonstrated that xenon (Xe) fission gas plays a major role on fuel thermal conductivity degradation. Even a very low Xe concentration can cause significant thermal conductivity reduction. In this work, the effect of dispersed Xe gas atoms on UO2 thermal conductivity were studied using three different interatomic potentials. It is found that although these potentials result in significant discrepancies in the absolute thermal conductivity values, their normalized values are very similar at a wide range of temperatures and Xe concentrations. By integrating this unified effect into the experimentally measured thermal conductivities, a new analytical model is developed to predict the realistic thermal conductivities of UO2 at different dispersed Xe concentrations and temperatures. Using this new model, the critical Xe concentration that offsets the grain boundary Kapitza resistance effect on the thermal conductivity in a high burnup structure is studied. Next, the mechanisms on how Xe gas bubbles affect the UO2 thermal conductivity have been studied using MD. At a fixed total porosity, the effective thermal conductivity of the bubble-containing UO2 increases with Xe cluster size, then reaches a nearly saturated value at a cluster radius of 0.6 nm, demonstrating that dispersed Xe atoms result in a lower thermal conductivity than clustering them into bubbles. In comparison with empty voids of the same size, Xe-filled bubbles lead to a lower thermal conductivity when the number ratio of Xe atoms to uranium vacancies (Xe:VU ratio) in bubbles is high. Detailed atomic-level analysis shows that the pressure-induced distortion of atoms at bubble surface causes additional phonon scattering and thus further reduces the thermal conductivity. For metallic fuels, temperature gradient and irradiation induced constituent redistribution in U-Zr based fuels cause the variation in fuel composition and the formation of different phases that have different physical properties such as thermal conductivity. In this work, a semi-empirical model is developed to predict the thermal conductivities of U-Zr alloys for the complete composition range and a wide range of temperatures. The model considers the effects of (a) scattering by defects, (b) electron-phonon scattering, and (c) electron-electron scattering. The electronic thermal resistivity models for the two pure components are empirically determined by fitting to the experimental data. A new mixing rule is proposed to predict the average thermal conductivity in U-Zr alloys based on their nominal composition. The thermal conductivity predictions by the new model show good agreement with many available experimental data. In comparison with previous models, the new model has further improvement, in particular for high-U alloys that are relevant to reactor fuel compositions and at the low-temperature regime for the high-Zr alloys. The average thermal conductivity model for the binary U-Zr fuel is also coupled with finite element-based mesoscale modeling technique to calculate the effective thermal conductivities of the U-Zr heterogeneous microstructures. For a U-10wt.%Zr (U-10Zr) fuel at temperatures below the ɑ phase transition temperature, the dominant microstructures are lamellar δ-UZr2 and ɑ-U. Using the mesoscale modeling, the phase boundary thermal resistance R (Kapitza resistance) between δ-UZr2 and ɑ-U has been determined at different temperatures, which shows a T-3 dependence in the temperature range between 300K and 800K. Besides, the Kapitza resistance exhibits a strong dependence on the aspect ratio of the δ-UZr2 phase in the alloying system. An analytical model is therefore developed to correlate the temperature effect and the aspect ratio effect on the Kapitza resistance. Combining the mesoscale modeling with the newly developed Kapitza resistance model, the effective thermal conductivities of many arbitrary δ-UZr2 + ɑ-U heterogeneous systems can be estimated. / Doctor of Philosophy / Thermal transport in nuclear fuels is critical for both energy conversion efficiency and nuclear energy safety. Therefore, understanding the thermal transport properties such as thermal conductivity of nuclear fuels is not only important for current UO2 oxide fuel-based light water reactors but also critical for next-generation fast reactors that use U-Zr based metallic fuels. The thermal transport mechanisms in the two fuel types are fundamentally different: the predominant heat carriers in UO2 are phonons while they are electrons in U-Zr. This work studies the thermal transport properties for both types of nuclear fuels. At first, molecular dynamics (MD) simulations were conducted to study the effect of dispersed xenon (Xe) fission gas atoms on the UO2 thermal conductivity, because Xe is the major fission gas product and even a small concentration of Xe can cause significant fuel thermal conductivity reduction. In this work, three different interatomic potentials were used to study the Xe effect. It is found that although these potentials result in significant discrepancies in the absolute thermal conductivity values, the normalized values are very similar at a wide range of temperatures and Xe concentrations. By integrating this unified effect into the experimentally measured thermal conductivities, a new analytical model is developed to predict the thermal conductivities of UO2 at different Xe concentrations and temperatures. Then this new model is used to study how dispersed Xe influences the effective thermal conductivity of heterogeneous UO2 microstructures with different grain sizes. Next, we focused on how the presence of Xe bubbles degrades the effective UO2 thermal conductivity using MD. The effects of both Xe gas bubble size and pressure were examined. Our results show that dispersed Xe gas atoms or small Xe clusters result in a lower thermal conductivity than clustering them into larger bubbles if the total porosity is fixed. In comparison with empty voids of the same sizes, a Xe-filled bubble leads to a lower thermal conductivity when the bubbles pressure is high, because the distorted bubble surface can cause additional phonon scattering effect. Besides the UO2 based oxide fuels, U-Zr based metallic fuels are promising fuel forms for next-generation fast reactors due to their high thermal conductivity. In this work, a semi-empirical model with a single set of parameters is developed to predict the average thermal conductivities of U-Zr alloys for the complete composition range and a wide range of temperatures. The thermal conductivities predicted by the new model have good agreement with many available experimental data, even if some experimental data are not included in the model fitting. The above thermal conductivity model for the binary U-Zr alloy has been coupled with finite element-based mesoscale modeling to calculate the effective thermal conductivities of U-Zr heterogeneous microstructures containing ɑ-U and δ-UZr2 lamellar phases. Using the mesoscale modeling, the phase boundary thermal resistance R (Kapitza resistance) between δ-UZr2 and ɑ-U has been determined for a wide range of temperatures as well as the aspect ratio of the lamellar δ-UZr2 phase. An analytical model is therefore developed to correlate the effects of temperature and aspect ratio on the Kapitza resistance. Combining the mesoscale modeling with the newly developed Kapitza resistance model, the effective thermal conductivities of many U-Zr heterogeneous systems can be accurately estimated.

Metallic fuels have an important role to play in "fast breeder" Gen-IV type nuclear reactors, and U-Pu-Zr is one of the prototypical systems. Because of the variability in fuel chemistry during burn-up, it is important to understand the effect of minor actinides and fission products on phase stability. Within this framework, we present a study on phase equilibria in the binary Np-Zr alloy system on which little work has been published. To resolve the contradictory reports on the ordering/ clustering trends of the bcc phase, a thermodynamic study is performed using the CALPHAD method. The calculated Np-Zr phase diagram is consistent with two sets of data: formation enthalpies of the bcc phase that are calculated with ab initio KKR-ASA-CPA electronic-structure method and lattice stabilities of solution phases obtained from first-principles technique. Another important feature in the Np-Zr alloy system is the non-stoichiometric delta-NpZr2 phase that forms in a hexagonal C32 structure similar to the delta-phase in the U-Zr system and the w-phase in pure Zr. An increase in the homogeneity range of the delta-phase when going from Pu to Np and to U is attributed to a lowering of its heat of formation that is caused by an increase in d-band occupation. Two different possibilities for the stability of the delta- and w- phases have been proposed in the present work. Additionally, calculated changes in enthalpy versus temperature are plotted for two alloy compositions of the Np-Zr system to guide future experimental work in resolving important issues in this system. Finally, an ab initio study, implemented with the L(S)DA U formalism, is performed for pure Np that reveals a transition from a non-magnetic to a magnetic state at a critical U parameter.