Uranium-10wt% zirconium (U-10Zr) alloy nuclear fuels have been used for decades and new variations are under consideration ranging from U-5Zr to U-50Zr. As a precursor to understanding the fission gas behavior in U-Zr alloys using ion implantation, a basic study on the U-Zr metallurgy was completed using EPMA, DSC, XRD, Optical microscopy, and TEM with a focus on solid state phase transformations in alloys containing 2, 5, 10, 20, 30, and 50wt% zirconium. Alloys were cast by crucible melting using high temperature furnace under argon atmosphere in yttrium oxide crucibles and various thermal profiles were used to study phase transformations in these alloys. Using TEM, XRD, and DSC data, it was ascertained that athermal-ω, along with martensitic α1, formed in all alloys quenched from γ phase. XRD could detect the presence of athermal-ω only in U-20, 30 and 50wt%Zr alloys. BSE images for as-cast alloys of 2, 5, 10, 20, and 30wt%Zr had lamellar microstructure with lamellae rich in zirconium. All alloy samples clearly showed a heating transformation pertaining to δ → γ in DSC data while XRD could only confirm the presence of δ phase in U-20, 30, and 50wt%Zr alloys. An explanation is offered for the absence of δ phase peaks in uranium-rich alloys based on its formation mechanism. Alloy samples of U-2, 5, and 10wt%Zr were step-cooled from γ phase by annealing in the (α + δ) phase field before cooling to room temperature revealed broad peaks for δ phase indicating incomplete collapse of {111}γ planes. Both as cast and γ- quenched alloys were annealed at 600degreeC, in the (α + δ) phase field for 1, 3, 7, and 30 days. Microstructures of the samples in both cases contained uranium-rich matrix and zirconium-rich precipitates and WDS analyses were consistent with their being α-U and δ phase respectively. However, XRD data for annealed alloys never showed peaks for δ phase even though it’s area fraction was within the detection limits. Moreover, the peaks which were present in U-20wt%Zr vanished after annealing for 7 days. Based on the data obtained, it is suggested that it is more appropriate to consider the presence of metastable diffusional-ω instead of a stable δ in the as-cast alloys and that it is not stable at 600degreeC.

The purpose of this project has been to expand the η8 binding mode of the permethylpentalene ligand into uranium, zirconium and hafnium chemistry. All three of these elements have shown intriguing, high-hapticity carbocyclic chemistry and, because of their relatively large size, are excellent candidates for the development of organometallic permethylpentalene chemistry. Chapter one of this thesis will review previous work on η n carbocyclic ring chemistry of these elements, where n = 6 - 8. This introduction will include the unsaturated rings systems where all the ,carbons are bonded to the metal centre, specifically η6 arene systems, η 7 cyclohept.atriene systems, and η 8 cyclooctatetraene and pentalene systems. Species of lower hapticity (e .g. the η 6 binding mode of cycloheptatriene) will not be covered but reviews, where available, will be referenced. Chapter two documents the successful synthesis and characterisation of η 8 permethylpentalene uranium (IV) species. Initially, the uranocene equivalent, UPn*2 was synthesised and characterised structurally, magnetically and electrochemically. From here, a half-sandwich synthon [U Pn*CI4][Li(TMEDA)h was synthesised which was used for further salt metathesis chemistry in order to make a number of mixed sandwich complexes. Chapter three is an account of the synthesis and characterisation of zirconium and hafnium η 8 permethylpentalene species. Initial work focused on the synthesis of a suitable synthon analogous to that used for the previously synthesised titanium species. However, this route was unsuccessful and an alternative species was formed, [MPn*(μ-Cl)3/2]2(μCl)2[Li(THF)x(Et2O)y]. This species could be made on a multi-gram scale and proved to be a sui table synthon for further synthesis. Salt metathesis reactions were undertaken and a number of new species were synthesised and characterised including mixed-sandwich, alkyl, aryl and allyl species. Chapter four reports the results of polymerisation testing that was undertaken for selected synthesised compounds. All compounds catalysed the formation of poly(ethylene), with the group 4 mixed sandwich species being particularly active catalysts. Two of the zirconium species, ZrPn*CpCI and ZrPn*Cp2 were therefore used for further optimisation experiments which were somewhat limited due to the high activity of the compounds. These were useful in gaining insight into conditions that should be investigated on a larger reaction scale. Chapter five gives the full experimental details for all the syntheses described in chapters two and three as well as details of instrumentation used for characterisation, and also gives the respective loadings of catalyst and co-catalyst employed in the polymerisation testing reported in chapter four. Chapter six presents the full characterisation data obtained for the compounds synthesised and the electronic appendix attached as a CD at the back of the thesis contains the crystal data .cif files and the DFT output files (.out). ,

The metallic binary-alloy fuel Uranium-Zirconium is important for use in the new generation of advanced fast reactors. Uranium-Zirconium goes through a phase transition at higher temperatures to a (gamma) Body Centered Cubic (BCC) phase. The BCC high temperature phase is particularly important since it corresponds to the temperature range in which the fast reactors will operate. A semi-empirical Modified Embedded Atom Method (MEAM) potential is presented for Uranium-Zirconium. This is the first interatomic potential created for the U-Zr system. The bulk physical properties of the Uranium-Zirconium binary alloy were reproduced using Molecular Dynamics (MD) and Monte Carlo (MC) simulations with the MEAM potential. The simulation of bulk metallic alloy separation and ordering phenomena on the atomic scale using iterative MD and MC simulations with interatomic potentials has never been done before. These simulations will help the fundamental understanding of complex phenomena in the metallic fuels. This is a large step in making a computationally acceptable fuel performance code, able to replicate and predict fuel behavior.

<p>  </p> <p>Metallic uranium-zirconium (U-Zr) nuclear fuel is a primary candidate for future fast reactors. The U-Zr system has been studied for decades with thousands of fuel pins being irradiated, yet the phase boundaries and lattice evolution with respect to temperature and composition remain poorly quantified. Historic engineering scale testing has resulted in empirical models for fuel evolution and subsequent fuel performance. However, these historic tests are on a convoluted system, consisting of dynamic temperatures, evolving thermal gradients, varying irradiation damage and damage rates, evolving compositions via fission and redistribution of primary constituents, and morphological evolution. This system proves exceedingly difficult to describe mechanistically due to the coexistence of various intertwined thermodynamic driving forces (e.g., temperature, composition, fluence, and fission rate which all vary concurrently). The driving forces influence the manifestation of the primary life-limiting phenomena present within the U-Zr system, specifically fuel-cladding mechanical interaction, fuel-cladding chemical interaction, fuel swelling, and fuel constituent redistribution. Although the phenomena present in the U-Zr system are known and qualitatively described, they are lacking in fundamental descriptions due to the historic inability to deconvolve the effects of temperature, composition, and fission rate. This study evaluates the current understanding of U-Zr fuel swelling and constituent redistribution in a uniquely quantified manner using Phenomena Identification and Ranking Tables. </p> <p><br></p> <p>In response to these findings, a novel separate effects irradiation test vehicle, housing uniquely fabricated U-Zr alloys, was proposed, developed, and successfully fabricated to provide the community with a means to decouple temperature, composition, initial microstructure, and fission rate from one-another. Initial out-of-pile characterization was conducted with scanning electron microscopy, transmission electron microscopy, and neutron diffraction with in-situ heating on various U-Zr alloys (U- 6, 10, 20, and 30 wt.% Zr). This work quantifies the initial microstructure throughout the fabrication process and the thermal response of the material. Results include the phase morphology, phase boundaries, absolute lattice parameters, and lattice specific coefficients of thermal expansion. The phase boundaries identified in this study were then used to develop a new U-Zr phase diagram. The isolation of thermal and compositional dependencies furthers the understanding of the fuel system and can be used to increase fuel longevity.</p>

<p>  </p> <p>The next generation of nuclear reactor designs promise to provide clean, safe, and efficient energy to address our current climate crisis. But with these new technologies, nuclear fuel materials must be carefully designed and understood to meet these demands. Candidate oxide and metallic nuclear fuel materials being considered for use in these new reactor technologies, despite their potential, still have significant remaining materials challenges in understanding their long-term performance and integrity under extreme reactor conditions. As such these candidate fuels require extensive materials characterization to understand their long-term performance under reactor conditions. The objective of this study is to evaluate the microstructural evolution of candidate fuels U-50wt%Zr and ThO2 under the following contexts: 1) Investigation of phase stability in candidate metallic fuel U-50wt%Zr under thermal and irradiation treatment; 2) Investigate localized thermal properties of candidate oxide fuel ThO2 under irradiation through a novel correlative microscopy approach. </p> <p>The influence of thermal and irradiation treatment on phase stability in δ-U50wt%Zr was investigated through conventional APT-TEM methodology. U-Zr is a candidate metallic fuel for advanced fast reactor applications. However, there is still work remaining to better understand how these materials evolve under extreme reactor conditions, especially for the δU-50wt%Zr composition. Metallic fuels are susceptible to significant chemical redistribution under extreme conditions resulting in potential degradation of fuel properties and performance. In these experiments, U-50wt%Zr was subjected to thermal annealing and proton irradiation respectively. These treatments produced very different modulated structures in U-50wt%Zr, and the implications of such on phase stability in U-50wt%Zr will be discussed.</p> <p>Additionally, long-term nuclear reactor operation hinges upon efficient thermal transport in nuclear fuels. There is a critical need to understand localized thermal transport in these materials to enable intelligent design of high-performance fuels. A novel correlative atom probe tomography (APT)-transmission electron microscopy (TEM) approach was developed to investigate the influence of irradiation defects on localized thermal diffusivity in ThO2 upon proton irradiation, and implications of such results will be discussed. </p>

In this work a manufacturing method for UN, ZrN and (U,Zr)N pellets was established at the nuclear fuel laboratory at KTH Stockholm/Sweden, which consists of the production of nitride powders and their sintering into pellets by spark plasma sintering. The nitride powders were produced by the hydriding-nitriding route using pure metal as starting material. This synthesis was performed in a stream of the particular reaction gas. A synthesis control and monitoring system was developed, which can follow the reactions in real time by measuring the gas flow difference before and after the reaction chamber. With the help of this system the hydriding and nitriding reactions of uranium and zirconium were studied in detail. Fine nitride powders were obtained; however, the production of zirconium nitride involved one milling step of the brittle zirconium hydride. Additionally uranium and zirconium alloys with different zirconium contents were produced and synthesized to nitride powders. It was found that also the alloys could be reduced to fine powder, but only by cyclic hydriding-dehydriding. Pellets were sintered out of uranium nitrides, zirconium nitrides, mixed nitrides and alloy nitrides. These experiments showed that relative densities of more than 90% can easily be achieved for all those powders. Pellets sintered from mechanically mixed nitride powders were found to still consist of two separate nitride phases, while nitride produced from alloy was demonstrated to be a monophasic solid solution both as powder and as sintered pellets.