The metal hydride used as hydrogen storage material has come into public notice. It is very important to understand the crystal structure and the permutation of hydrogen and the metal crystal structure of hydride for developing industrial hydrogen storage material. The tetrahedral formation of the atom intervals of titanium/zirconium alloys benefits the absorbing of hydrogen atoms. Especially under high temperature, the titanium/zirconium alloys have high affinity on hydrogen, and hydrogen atoms help to absorb within the titanium/zirconium alloys can spread and enter materials fast on the surface, this way will get uniform saturation solid solution. Therefore, we use hydrogen gas in the high-temperature gaseous phase to melt the titanium/zirconium alloys and observe the crystal structure and morphology of hydride precipitates with transmission electron microscopy. The research contents are as follows: (1) In Ti-H system, ageing of quenched specimens which contained the hydrides are found to have a face-centered tetragonal structure (£^-titanium hydride). Ageing of furnace-cooled specimens which contained the hydrides are found to have a face-centered cubic structure (£_-titanium hydride). (2) In Zr-H system, ageing of quenched specimens which contained the hydrides are found to have a face-centered tetragonal structure (£^-zirconium hydride). Ageing of furnace-cooled specimens which contained the grain-boundary hydrides are found to have a face-centered cubic structure (£_-zirconium hydride).

Molecular dynamics (MD) simulations were used in order to investigate structure and mechanical properties of zirconium and zirconium hydride. Calculation of temperature dependent failure of zirconium, diffusion of hydrogen in zirconium, properties of interfaces in zirconium and zirconium hydride and effect of hydrogen on crack nucleation and propagation were in good agreement with available experimental data. These are the first computer simulations where large-scale atomic/molecular massively parallel simulator (LAMMPS) code was used with the Embedded Atom Method (EAM) and Modified Embedded Atom Method (MEAM) to study structure and mechanical properties of zirconium hydrogen system (Zr-H) and zirconium hydride (ZrH2). Verification of methods was done in order to establish the best potential for zirconium and zirconium hydride. EAM and MEAM potentials successfully predicted lattice parameters, mechanical properties and variation of lattice parameters with temperature for α-Zr. MEAM potential was used to predict correctly the face centered structure for ZrH2 and also its mechanical properties. Temperature dependent stress-strain curves were calculated in order to predict yielding point for α-Zr. Results indicate early yielding and failure with increase of temperature in zirconium on application of tensile and compressive strains. Anisotropic stress variation with temperature in α-Zr was calculated. Hydrogen ingress through diffusion of hydrogen in zirconium is a mechanism responsible for formation of hydrides. Temperature-dependent hydrogen diffusion and activation energy for diffusion was calculated and the agreement with experiments was satisfactory. Anisotropy of diffusion of hydrogen is observed for Zr crystal. Hydrogen diffusion was also modeled under tensile and compressive strain and a possible formation of hydrides in the direction perpendicular to applied strain was observed. The effect of strain on orientation of hydride was investigated. Hydride {111} oriented crystal was strained along [1 1 ̅ 0] and [111] direction. Energy as a function of strain is calculated along both directions [111] and [1 1 ̅ 0] and it was found that energy of the system increase with increase in strain along [1 1 ̅ 0] and decrease with increase of strain along [111] direction. Calculated stress and strain curves indicate lower stresses along [111] direction and this causing the hydride to reorient in a direction perpendicular to applied strain. Structure of the interface (0 0 0 1) α-Zr // {1 1 1} δ-ZrH2 is modeled in order to investigate the crack initiation at this interface. Interfacial cracking of hydride under stress is observed. This observation is in good agreement with available experimental studies. Cracks are seen to nucleate earlier at higher temperature. Cracks and voids are common defects in zirconium fuel cladding. A crack is modeled along (0 0 0 1) plane of zirconium with hydrogen. In the presence of hydrogen cracks nucleate in zirconium causing fracture. This observation is in good agreement with previous experimental studies. Bonds surrounding atoms and stress concentration analysis were performed using OVITO and VMD software’s respectively. Weaker bonds and higher stress concentrations are observed in the presence of hydrogen in zirconium. The presented results clearly demonstrate that MD simulation can be used to predict structure and processes that are important for understanding failure in Zr based nuclear materials.

Zirconium alloys are susceptible to engineering problems associated with the uptake of hydrogen throughout their design lifetime in nuclear reactors. Understanding of hydrogen embrittlement associated with the precipitation of brittle hydride phases and a sub-critical crack growth mechanism known as Delayed Hydride Cracking (DHC) is required to provide the engineering justifications for safe reactor operation. The nature of bulk zirconium hydrides at low concentrations (< 100 wt. ppm) is subject to several contradictory descriptions in the literature associated with the stability and metastability of γ-phase zirconium hydride. Due to the differing volume expansions (12-17%) and crystallography between γ and δ hydride phases, it is suggested that the matrix yield strength may have an effect on the phase stability. The present work indicated that although yield strength can shift the phase stability, other factors such as microstructure and phase distribution can be as or more important. This suggests that small material differences are the reason for the literature discrepancies. DHC is characterised by the repeated precipitation, growth, fracture of brittle hydride phases and subsequent crack arrest in the ductile metal. DHC growth is associated primarily the ability of hydrogen to diffuse under a stress induced chemical potential towards a stress raiser. Knowledge of the factors controlling DHC are paramount in being able to appropriately describe DHC for engineering purposes. Most studies characterise DHC upon cooling to the test temperature. DHC upon heating has not been extensively studied and the mechanism by which it occurs is somewhat controversial in the literature. This work shows that previous thermo-mechanical processing of hydrided zirconium can have a significant effect on the dissolution behaviour of the bulk hydride upon heating. DHC tests with γ-quenched, furnace cooled-δ and reoriented bulk hydrides upon heating and DHC upon cooling suggest that the amount of hydrogen in solution is the primary factor controlling the occurrence of DHC and consistent with the postulation that the stress induced chemical potential is the driving force for DHC. / Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2012-01-24 06:14:14.152

As a part of the production of nitride nuclear fuel for use in fast nuclear reactors, zirconium is hydrided followed by nitriding and mixing with uranium nitride. This work concludes a study of unwanted metallic particles present in a powder that is supposed to be a zirconium hydride. Sponge zirconium was hydrided at different temperatures and different time intervals, and the resulting hydride was milled into a powder. The powders were analyzed using SEM and XRD after which the powders were pressed into pellets for light optical microscopic study. The primary goals were determination of the structure of the particles and thereafter elimination of them. It was seen that hydriding at 500 C results in less metal particles but more experiments need to be conducted to confirm this.

Zirconium alloys are widely used in the nuclear industry because of their high strength, good corrosion resistance and low neutron absorption cross-section. However, zirconium has strong affinity for hydrogen, which may lead to hydrogen concentration build-up over time during a corrosion reaction when exposed to water. Hydrogen stays in solution at higher temperature but precipitates as zirconium hydrides at ambient temperatures. The formation of zirconium hydrides is considered to be a major cause of embrittlement, in particular as a key step in the mechanism of delayed hydride cracking. Despite the fact that zirconium hydrides have been studied for several decades, the basic nature and mechanisms of hydride formation, transformation and exact structure are not yet fully understood. In order to find the answer to some of these problems, the precipitation and dissolution of hydrides in commercial grade Zr powder were monitored in real time with high resolution synchrotron and neutron radiations, and the whole pattern crystal structure analysis, using Rietveld and Pawley refinements, were performed. For the first time all commonly reported zirconium hydride phases and complete reversible transformation between two different Zr-hydride phases were recorded with a single setup and their phase transformation type have been analyzed. In addition, the preparation route of controversial γ-zirconium hydride (ZrH), its crystal structure and formation mechanisms are also discussed. / <p>Note: The papers are not included in the fulltext online.</p><p>Paper II and III in thesis as manuscript, paper II with title "The phase transformation between the δ and ε Zr hydrides"</p>

An electrolytic cell was designed, built, and tested with several proof-of-concept experiments in which Zircaloy material was charged with hydrogen in order to generate zirconium hydride formations. The Electrolytic Charging with Hydrogen and a Thermal Gradient (ECH-TG) system has the ability to generate static 20°C to 120°C temperatures for a H2SO4 and H2O bath for isothermal experiment conditions. This system was designed to accommodate a molten salt bath in future experiments to achieve higher isothermal temperatures. Additionally, the design accommodates a cartridge heater, which when placed on the inside of the sample tube, can be set at temperatures up to 350 °C and create a thermal gradient across the sample. Finally, a custom LABVIEW VI, L2.vi, was developed to control components and record data during experimentation. This program, along with web cameras and the commercial StirPC software package, enables remote operation for extended periods of time with only minor maintenance during an experiment. While proving the concept for this design, 19 experiments where performed, which form the basis for a future parametric study. Initial results indicate formations of zirconium hydrides which formed rim structures between 8.690 +/- 0.982 μm and 12.365 +/- 0.635 μm thick. These electrolytically produced rims were compared with hydrides formed under a previous vapor diffusion experiment via Scanning Electron Microscope (SEM) imaging and Energy dispersive X-ray Spectroscopy (EDS) analysis. While the existing vapor diffusion method formed gradients of zirconium hydride, it failed to produce the gradient in the correct direction and also failed to create a hydride rim. The successful use of the ECH-TG system to create said rim, and some of the methods used to direct that rim to the OD of the tube can be used for future work with the vapor diffusion method in order to create zirconium hydrides of the correct geometry. The procedures and apparatus created for this project represent a reliable method for creating zirconium hydride rim structures.

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.