COATED FUEL PARTICLES FOR HIGH TEMPERATURE GAS COOLED NUCLEAR REACTORS

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Motivation

ZrC either as a powder mixed with the fuel or as a thin layer on the fuel kernel has been proposed for use as an oxygen getter in the UO2 kernel TRISO fuel. The high oxidation potential of ZrC (above room temperature) is perceived to act as a reducing agent for the oxygen generated from the fission process of UO2. This avoids unfavourable oxidation reactions and reduces the formation of CO which are very detrimental more especially at high fuel burn-up levels. Another proposed use is the replacement of the SiC layer with a ZrC layer as barrier for containing fission products in the TRISO fuel particle. This is because ZrC has a low neutron capture cross section, an enhanced corrosion resistance to fission product attack and diffusion, and is stable even at high temperatures [17]. However, the properties of ZrC layers vary greatly depending on synthesis technique used and how well the deposition parameters are managed during its production. The detailed chemical vapour deposition reactor design for the deposition of ZrC layers will enable the production of ZrC layers that have properties which are desired for nuclear applications, especially as fission product barrier. Increased understanding of ZrC deposition parameters will assist in the production of ZrC layers with optimum quality. Although, the deposition process parameters may not entirely be transferable between different CVD designs, the understanding of the effects of these process parameters and CVD reactor geometry is crucial. This is because deposition parameters affect the properties of ZrC layers and hence make it hard to give specifications of ZrC layers to a particular application.

Aims

This work is aimed at detailing the development of a deposition process for the preparation and optimisation of ZrC layers from ZrCl4-Ar-CH4-H2 precursors using induction thermal CVD at atmospheric pressure. With objectives: 1. To fabricate and develop a laboratory scale CVD system so as to produce ZrC layers of various stoichiometry. 2. To investigate the microstructure and composition of ZrC coatings deposited as a function of the input variables. 3. To investigate the effect of the CVD deposition parameters on the properties of the deposited ZrC layers.

Approach

The vertical–wall RF inductively heated CVD system was developed in-house at The South African Nuclear Energy Cooperation (Necsa) SOC Ltd. This study was carried out experimentally on a laboratory scale. The effect of substrate temperature, methane/zirconium tetra chloride molar ratio, precursor partial pressures (methane, zirconium tetra chloride and hydrogen), substrate-gas inlet gap and deposition time were studied. The focus of this study was geared at fabricating the CVD reactor system and understanding the effect of deposition process on the properties of ZrC layers deposited at atmospheric pressure. The characterised properties were crystallite size, composition (stoichiometry), lattice strain, lattice parameter, dislocation density, surface morphology and preferred orientation. Layer thickness and growth rate were also investigated as a deposition process output at various deposition parameters.

Contributions

This study contributes significantly to the existing body of knowledge. As much as some of the techniques employed in this study are not new, many aspects of this study are unique. These include but are not limited to: 1. Design and fabrication of a vertical-wall thermal CVD system with special reference to ZrC layers. 2. Optimisation of the ZrC growth process by response surface methodology. 3. Investigation of the effect of substrate-gas inlet gap on the properties of the deposited ZrC layers. 4. Investigation of the effect of ZrCl4 partial pressure on the properties of the deposited ZrC layers. 5. The role of deposition time on the growth and evolution of microstructure and morphology of the ZrC layers.

Thesis layout

In this study ZrC layers were grown on a graphite substrate by CVD and characterised by scanning electron microscopy, X-ray diffraction, Raman spectroscopy and Energy dispersive X-ray spectroscopy. During the growth of ZrC layers, several growth parameters were varied in the CVD reactor. The most important of these are the temperature profile in the reactor, the gas flow parameters (gas speed, ratio of the different gases, partial pressures of the gases), deposition time and also the parameters of the reactor geometry. As each of these parameters is interdependent, experimentation, by changing these variables, is important for growing a stoichiometric ZrC layer with a polycrystalline microstructure.

Table of Contents :

• • Summary
• CHAPTER INTRODUCTION
  • 1.1 Background
  • 1.2 Motivation
  • 1.3 Aims
  • 1.4 Approach
  • 1.5 Contributions
  • 1.6 Thesis layout
  • 1.7 References
• CHAPTER COATED FUEL PARTICLES FOR HIGH TEMPERATURE GAS COOLED NUCLEAR REACTORS
  • 2.1 The TRISO fuel particle
  • 2.2 SiC-TRISO coated fuel particle failure
  • 2.3 Diffusion mechanism of fission products
  • 2.4 References
• CHAPTER ZIRCONIUM CARBIDE
  • 3.1 Properties of zirconium carbide
  • 3.2 Cladding materials for nuclear reactor requirements
  • 3.3 Zirconium carbide phases
  • 3.4 Zirconium carbide synthesis
    • 3.4.1 Preparation of ZrC by solid phase reactions
    • 3.4.2 Preparation of zirconium carbide from solution-based precursors
    • 3.4.3 Preparation of ZrC from vapour phase based reactions
  • 3.8 References
• CHAPTER FILM DEPOSITION
  • 4.1 Overview of film growth
  • 4.2 Film growth modes
  • 4.3 Film deposition techniques
  • 4.4 Chemical vapour deposition (CVD)
    • 4.4.1 Thermal CVD
    • 4.4.2 Closed and open CVD systems
    • 4.4.3 Hot and cold wall CVD reactors
    • 4.4.4 High and Low pressure CVD
  • 4.5 The CVD process
    • 4.5.1 Precursor requirements for CVD process
  • 4.6 Induction heating process in CVD
    • 4.6.1 Electromagnetic formulation
  • 4.7 References
• CHAPTER ANALYTICAL AND CHARACTERISATION TECHNIQUES
  • 5.1 X-Ray Diffraction Technique
    • 5.1.1 Background
    • 5.1.2 Phase and composition identification
    • 5.1.3 Crystal lattice structure and Bragg diffraction
    • 5.1.4 Peak broadening of XRD patterns
    • 5.1.5 Microstrain and residual stress in films
    • 5.1.6 Texture analysis and preferred orientation
  • 5.2 Energy dispersive X-Ray spectroscopy
  • 5.2.1 Principle of analysis
  • 5.3 What happens inside the EDS detector
  • 5.4 Raman Spectroscopy
    • 5.4.1 Introduction to Raman
    • 5.4.2 Raman spectroscopy and Raman spectra
    • 5.4.3 Raman scattering
  • 5.5 Scanning electron microscopy
    • 5.5.1 Description of the technique
    • 5.5.2 Electron beam sample interaction and detection
    • 5.5.3 Sample requirements
  • 5.6 References
• CHAPTER EQUIPMENT FABRICATION AND CONSTRUCTION
  • 6.1 Design and fabrication of the CVD system
    • 6.1.1 Power supply system
    • 6.1.2 Power supply calibration
    • 6.1.3 The CVD reactor system
    • 6.1.4 The gas delivery system
    • 6.1.5 The ZrCl4 vaporisation system
    • 6.1.6 ZrCl4 mass transfer rate calibration
    • 6.1.7 The exhaust system
    • 6.1.8 Low Flammable limits of Hydrogen and Methane
    • 6.1.9 Leak detection
  • 6.2 Substrate Preparation
  • 6.3 References
• CHAPTER INVESTIGATING THE GROWTH OF ZrC LAYERS BY CHEMICAL VAPOUR DEPOSITION
  • 7.1 Optimisation of the synthesis of ZrC layers in an RF induction-heating CVD system using response surface methodology
    • 7.1.1 Introduction
    • 7.1.2 Thermodynamic feasibility of ZrC growth
    • 7.1.3 Design of experiments and response surface methodology
    • 7.1.4 Growth rate of zirconium carbide
    • 7.1.5 Crystallographic structure and phase composition
    • 7.1.6 Average crystallite size
    • 7.1.7 Optimisation of the experimental and model results
    • 7.1.8 Surface morphology
  • 7.2 Influence of the substrate gas-inlet gap on the growth rate, morphology and microstructure of ZrC layers
    • 7.2.1 Introduction
    • 7.2.2 Gas flow parameters in the reaction chamber
  • 7.3 The role of ZrCl4 on the growth characteristics of ZrC layers
  • 7.3.1 Introduction
    • 7.3.2 Gas flow behaviour and analytical description model of the gas flow process
    • 7.3.3 Effect of ZrCl4 partial pressure on ZrC growth rate
  • 7.4 Time dependence of microstructural and morphological evolution of ZrC layers
    • 7.4.1 Introduction
    • 7.4.2 Layer thickness and growth rate
  • 7.5 References
• CHAPTER CONCLUSIONS AND FUTURE WORK
  • 8.1 Conclusion
  • 8.2 Future work
• CHAPTER PUBLICATIONS AND CONFERENCES
  • 9.1 Publications
  • 9.2 Conferences

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