Release of metallic fission products from fully intact tri-structural isotropic (TRISO) fuel particles raises serious concern on the safety of high temperature gas-cooled reactors (HTGRs). In TRISO particles, SiC and/or ZrC coating is considered as the major barrier for the migration of the fission products. This thesis focuses on the migration mechanism study of Ag in SiC and Pd in ZrC.The mechanism of the migration of Ag in SiC is a long-lasting mystery. None of the currently existing models could satisfactorily explain the reported experimental facts. In this work, a new mechanism, termed as the “reaction-recrystallization” model, is proposed to explain the Ag migration behavior through SiC. Designed SiC/Ag diffusion couple experiments were carried out, and the results indicate that Ag migrates in SiC by the following three steps. First, Ag reacts with SiC to form an Ag-Si alloy (reaction). Second, carbon precipitates as a second phase and subsequently reacts with the Ag-Si alloy to form new β-SiC (recrystallization). Third, the Ag-Si alloy penetrates through the SiC layer by wetting its grain boundaries (migration). The validity of the proposed model was supported by thermodynamic calculations. (Chapter 3) The finding that SiC could be recrystallized in the presence of Ag inspires the idea of Ag-assisted crack healing in SiC. Cracks were intentionally generated by indenting the bulk SiC by a Vickers indenter. After vacuum annealing with Ag powder, the indent impressions were healed by newly-formed β-SiC grains with a recovery ratio of~ 60%. Median cracks were fully healed by both newly formed SiC and Ag-Si nodules. TEM observation reveals that the newly formed β-SiC layer is presented between the Ag-Si nodule and pristine SiC crack surface and smooths the tortuous crack surface. The above result is in potential to solve the problem of brittleness of SiC as a structural material. (Chapter 4)ZrC is considered as a candidate to replace SiC in TRISO fuel particles. The migration behavior of Pd in ZrC was investigated by designed Pd/ZrC diffusion couple experiments. It is found that ZrC reacts with Pd at temperatures higher than 600 °C to form Pd3Zr and amorphous carbon. The reaction kinetics parameters, i.e., the activation energy and the reaction order, along with the inter-diffusion coefficients of Zr and Pd, were calculated based on established models. These results provide preliminary explanation to the Pd migration in ZrC (Chapter 5).

In modern high temperature nuclear reactors, silicon carbide (SiC) is used as the main diffusion barrier for the fission products in coated fuel spheres called TRISO particles. In the TRISO particle, pyrolytic carbon and SiC layers retain most of the important fission products like xenon, krypton and cesium effectively at temperatures up to 1000 oC. Previous studies have shown that 400 oC to 600 oC implantation of heavy ions into single crystal 6H-SiC causes the SiC to remain crystalline with many point defects and dislocation loops (damage). The release of Xe at annealing temperatures above 1400 oC is governed by the normal volume diffusion without any hindrance of trapping effects. In this study two phenomena in single crystal 6H-SiC implanted by 360 keV Xenon ions were studied using Rutherford Backscattering Spectroscopy (RBS) and channeling. Radiation damage and its annealing behavior at annealing temperatures ranging from 1000 oC to 1500 oC, and the diffusion of xenon in 6H-SiC at these annealing temperatures were investigated. 360keV xenon ions were implanted into a single crystalline wafer (6H-SiC) at 600 oC with a fluence of 1 × 1016 cm-2. The sample was vacuum annealed in a computer control Webb 77 graphite furnace. Depth profiles were obtained by Rutherford backscattering spectrometry (RBS). The same set-up was used to investigate radiation damage of the 6H-SiC sample by channeling spectroscopy. Isochronal annealing was performed at temperatures ranging from 1000 to 1500 °C in steps of 100 oC for 5 hours. Channeling revealed that the 6H-SiC sample retained most of its crystal structure when xenon was implanted at 600 °C. Annealing of the radiation damage took place when the sample was heat treated at temperatures ranging from 1000 oC to 1500 oC. The damage peak almost disappears at 1500 oC but the virgin spectrum was not achieved. This happened because of dechanneling due to extended defects like dislocations remaining in the implanted region. RBS profiles showed that no diffusion of the Xe occurred when the sample was annealed at temperatures from 1000 oC to 1400 oC. A slight shift of the xenon peak position towards the surface after annealing at 1400 °C was observed for 600 oC implantation. After annealing at 1500o C, a shift toward the surface accompanied by a broadening of the Xe peak indicating that diffusion took place. This diffusion was not accompanied by a loss of xenon from the SiC surface. The shift towards the surface is due to thermal etching of the SiC at 1400-1500 °C. Modern high temperature gas-cooled reactors operate at temperatures above 600 oC in the range of 750 oC to 950 oC. Consequently, our results indicate that the volume diffusion of Xenon in SiC is not significant in SiC coated fuel particles. / Dissertation (MSc)--University of Pretoria, 2014. / gm2014 / Physics / unrestricted

High-temperature reactors make use of tri-structural coated fuel particles as basic fuel components. These TRISO particles consist of fissionable uranium dioxide fuel kernels, about 0.5 mm in diameter, with each kernel individually encased in four distinct coating layers, starting with a porous carbon buffer, then an inner pyrolytic carbon (IPyC) layer, followed by a layer of ceramic silicon carbide (SiC) and finally an outer pyrolytic carbon layer (OPyC). Collectively, the coating layers provide the primary barrier that prevents release of fission products generated during burn up in the UO2 fuel kernel. It is crucial to understand how the fission products contained within the fuel interact with the coating layers and how they are distributed within the fuel. The first step commonly performed to obtain the information on distribution is removal of the coating layers. The purpose of this study was to investigate the possible use of wet chemical etching techniques with the aim of removing the coating layers of ZrO2 coated fuel particles in a controlled way and to establish experimental parameters for controlled dissolution of irradiated fuel particles. Stepwise dissolution of coated fuel particle coating layers, containing zirconia kernels has been investigated by chemical etching experiments with acidic solutions of different mixtures. The heating methods used include heating by conventional methods, hot plates and a muffle furnace, a reflux-heating system and microwave-assisted digestion. The etching mixtures were prepared from a number of oxidizing acids and other dehydrating agents. The capability of each reagent to etch the layer completely and in a controlled manner was examined. On etching the first layer, the OPyC, the reflux heating method gave the best results in removing the layer, its advantage being that the reaction can be carried out at temperatures of about 130 ºC for a long time without the loss of the acid. The experimental results demonstrated that a mixture composed of equal amounts of concentrated nitric and sulfuric acid mixed with chromium trioxide dissolves the OPyC layer completely. The most favourable experimental conditions for removal of OPyC from a single coated fuel particle were identified and found to depend on the etching solution composition and etching temperature. Light microscopy yielded first-hand information on the surface features of the samples. It allowed fast comparison of etched and untreated sample features. The outer surface of particles prior to chemical etching of the outer pyrolytic carbon layer appeared black in colour with an even surface compared to the etched surfaces which appeared to have an uneven metallic grey, shiny texture. The scanning electron microscope (SEM) examination of the chemically treated outer carbon layer samples gave information on the microstructure and it demonstrated that the outer pyrolytic carbon layer could be readily removed using a solution of HNO3/H2SO4/CrO3, leaving the exposed SiC layer. Complete removal of the layer was confirmed by energy dispersive X-ray spectroscopic (EDS) analysis of the particle surface. For etching the second layer, the silicon carbide layer, microwave-assisted chemical etching was the only heating technique found to be useful. However, experimental results demonstrated that this method has limited ability to digest the sample completely. Also common chemical etchants were found to be ineffective for dissolving this layer. Only fluoride containing substances showed the potential to etch the layer. The results show that a mixture consisting of equal amounts of concentrated hydrofluoric and nitric acid under microwave heating at 200 ºC yielded partial removal of the coating and localized attack of the underlying coating layers. The SEM analyses at different intervals of etching showed: partial removal of the layer, attack of the underlying layers and, in some instances, that attack started at grain boundaries and progressed to the intra-granular features. The SEM results provide evidence that etching of the silicon carbide layer is strongly influenced by its microstructure. From these findings, it is concluded that etching of the silicon carbide under the investigated experimental conditions yields undesirable results and that it does not provide complete removal of the layer. This method has the potential to etch the layer to some extent but has limitations. Copyright / Dissertation (MSc)--University of Pretoria, 2013. / Chemical Engineering / unrestricted

The Pebble Bed Modular Reactor (PBMR) is a high temperature graphite-moderated nuclear reactor that uses helium as a coolant. The triple coated (TRISO) particles contain enriched uranium oxide fuel which is coated with layers of various forms of pyrolytic carbon and silicon carbide. The TRISO particles are further embedded in the matrix of spherical graphite pebbles. The graphite matrix is a composite moulded from a compound containing natural flake graphite (64 wt.%), synthetic graphite (16 wt.%) and a phenolic resin binder (20 wt.%) heated to 1800 °C in inert atmosphere. The graphitic composite provides structural integrity, encasement and act as a moderator material. In this work, low density model graphite composites similar to those used in nuclear applications as encasement material in fuel pebbles were made by uniaxial cold compression moulding. The graphitic composites contained various ratios of natural flake graphite and synthetic graphite at fixed phenolic novolac resin binder content of 20 wt.% (green state). The fabrication process employed entails mixing the graphite powders, followed by addition of methanol phenolic resin solution to the graphite powder mix, drying, grinding, milling and sieving; and finally compression moulding in a stainless steel die at 13 MPa using a hydraulic press. The green moulded disc specimens were then carbonized at 900 °C in nitrogen atmosphere to remove volatiles followed by annealing at 1800 °C in helium atmosphere. The annealing step diminishes structural defects and result in densification of the composites. The microstructure of fabricated graphitic composites was characterized using various techniques. Particle Size Distributions determined using Laser diffraction showed that the inclusion of the binder leads to agglomeration. The composite powders had larger mean particle sizes than the raw graphite powders showing the binding effect of the novolac phenolic resin. X-ray diffraction studies showed that the graphitic composites had a hexagonal crystal structure after annealing. Raman spectroscopy revealed the presence of the structurally disordered phase derived from the resin carbon (indicated by the pronounced D-band in the Raman spectra). XRD and Raman observations were consistent with literature and gave results supporting existing knowledge base. Optical microscopy revealed a flake-like microstructure for composites containing natural graphite and needle-coke like particles for composites containing mainly synthetic graphite. Optical microscopy confirmed that the effect of the manufacturing route employed here was to align the particles in the direction perpendicular to the compression moulding direction. As a result, the graphitic composites exhibited anisotropic property behavior. The bulk density of the composites increased with the increase in the natural graphite content due to compactability of natural flakes in the manufacturing route. Thermogravimetric analysis studies on the composites showed that they were stable in air to 650 °C. Composites containing mainly synthetic graphite were thermally more stable in air compared to their natural graphite counterparts. The linear coefficients of thermal expansion of the composites were measured using thermomechanical analysis (20-600 °C). In the moulding direction, the average CTE (αP) values were in the range (5-9) × 10-6 K-1 and increased with increment in the natural graphite content in the composite. In the direction perpendicular to moulding direction, the average CTE (αN) values were in the range (1.7-2.1) × 10-6 K-1 showing that the expansion was similar or constant in this direction. Therefore an anisotropic expansion ratio, i.e. αP:αN, of about 3 was observed in the composites. This anisotropy is attributable to the alignment of the filler particles in the manufacturing route. The thermal conductivity of the annealed composites were measured in the pressing direction from 100 to 1000 °C and the values ranged from 19 to 30 W m-1 K-1. Anisotropy was also observed as far as strength was concerned. A composite containing 64:16:20 wt.% ratio had the best mechanical properties, high thermal conductivity and slightly high expansion coefficient. This work demonstrates the complimentary properties of the graphite fillers in the composites. It also reports for the first time, data on the effect of variation of the filler graphites on microstructure and properties of model low density compression moulded graphitic composites. / Thesis (PhD)--University of Pretoria, 2013. / gm2014 / Chemistry / unrestricted

This thesis documents research done in the development and the exploration of feasibility for a high-throughput method to measure local thermal properties. The present capabilities in the measurement of local thermophysical properties such as thermal conductivity, thermal diffusivity, and Kapitza resistance are very inefficient and impractical to fully understand and characterize heat transport through certain materials and features. This work follows up on past work in local thermal property measurement via the spatial domain thermoreflectance (SDTR) method, and explores the possibility of parallelizing the process. The parallelized SDTR (P-SDTR) method involves using laser projector sources to periodically heat and measure the changes of reflectivity of a sample surface at multiple locations simultaneously. These measurements are made possible by the development of a lock-in camera that can measure the characteristics of modulated light using lock-in amplification at several spots across an area with an advanced camera sensor. This method allows for the measurement of local thermal properties across features such as grain boundaries, or directional properties in anisotropic materials. An experimental setup is developed to determine at which heating and probing parameters a thermoreflectance signal can be measured. A finite element model is also made to simulate the P-SDTR process, and validate that the assumptions made in SDTR can be made in P-SDTR measurements. It is shown that at an appropriate separation of heating/measurement locations, the solutions from the simulation approach that of a single measurement spot. An initial device design is proposed and tested. Future work in the development of the P-SDTR device is also laid out.

High temperature gas cooled nuclear reactors often make use of Tristructural Isotropic (TRISO) coated fuel particles. In these particles, a layer of silicon carbide plays the key role of providing mechanical strength and acting as a diffusion barrier so preventing the release of fission products. TRISO particles are produced by a chemical vapor deposition (CVD) process in a spouted bed coater. Operating conditions of chemical vapor deposition processes are known to influence the properties of the deposited material. In the case of silicon carbide deposited by pyrolysis of methyltrichlorosilane (MTS) in a hydrogen atmosphere, process parameters that may influence the properties of the silicon carbide deposited include deposition temperature, MTS concentration and hydrogen flow rates. In this study the coating process was investigated using a laboratory scale spouted bed CVD coater. In all the test work conducted, carbon coated zirconia particles were used as a starting material. Only silicon carbide was deposited during these trials. Process parameters investigated were temperature, MTS concentration and hydrogen flow rate. The range investigated was 1250 °C to 1550 °C for temperature, 0.5 % to 2.5 % for MTS concentration and 10.0 l.minute-1 to 15.0 l.minute-1 for hydrogen flow. This covered the range that is typically used for small-scale production coaters. Two different gas inlet configurations, a conventional water cooled inlet and an inlet without any cooling, were used in the investigation. Properties of the coating process, such as the deposition rate and coating efficiency, as well as material properties were measured. Material properties investigated included: density, crush strength, micro-hardness, fracture toughness, nano-hardness, Young’s modulus, elemental composition, phase composition and microstructure. It was found that, of the variables investigated, temperature had the strongest effect while hydrogen flow rate had the least effect on material properties. There was considerable variability in all measured parameters; this introduced considerable uncertainty into the predicted effects of process conditions on material properties. / Thesis (PhD)--University of Pretoria, 2012. / Materials Science and Metallurgical Engineering / unrestricted