High temperature gas cooled reactors

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The shortage of electricity, constant decrease in fossil fuel supply and increase in energy demands around the world require alternative methods of electricity generation. One of these alternative methods is the use of high temperature gas cooled reactors (HTGRs) for generating electricity. In a nuclear reactor, a slow neutron (n) reacts with uranium (U235) to form U 236 which is unstable. The unstable U236 fissions to two fission products, three neutrons and release a lot of energy. Some of the neutrons formed will further reacts with the other U 235 causing them to fission (forming more FPs, 3n and releasing more energy). Basically one fission will trigger others, which triggers more until there is a chain reaction.
Rods are inserted amongst the tubes which are holding the uranium fuel to control the nuclear reaction. The control rods, inserted or withdrawn may slow down or accelerate the reaction [1]. Water separates fuel tubes in the reactor. The water is turned into steam by the heat produced via fission. The steam drives a turbine, which spins a generator to create the electricity. The HTGRs, such as the pebble bed modular reactor (PBMR), uses coated fuel particles shown the schematic diagram in figure.1-1. Uranium oxide or uranium carbide kernels are used as nuclear fuels. The fuel kernel is a small spherical ball covered with four layers and is called the tristructural isotropic (TRISO) particle.
The four layers are: low-density pyrolytic carbon i.e. the buffer layer, inner high-density pyrolytic carbon (IPyC), silicon carbide (SiC) and outer high-density pyrolytic carbon (OPyC), as illustrated in figure 1-1. The fuel kernel is 0.5 mm in diameter, the buffer layer is 95 μm thick, IPyC and OPyC are each 40 μm thick and the SiC is 35μm thick [2]. The low-density pyrolytic carbon buffer acts as voids for the gaseous fission products (FPs) and carbon monoxide produced and protect the IPyC from damage by reducing the fission recoils and accommodate the deformation of the fuel kernel during burn-up. The IPyC layer acts as a diffusion barrier for gaseous FPs. The laminar structures of the carbon sphere in both the buffer and IPyC layers also have stress and gas storage function [3]. The SiC layer is the main barrier for FPs release and the OPyC protects the SiC mechanically. Most of the HTGR operates at a temperature of about 950 °C [4][5]. Under this normal operation conditions of the reactor, SiC is a reliable diffusion barrier for most of the FPs. The release of some key FPs (silver (Ag), europium (Eu) and strontium (Sr)) and the reaction of SiC with other FPs during reactor operating [6][7], raised some concerns on the integrity of SiC as a coating layer [8][9] and the ability of SiC to act as the main diffusion barrier. Reports have shown that the interaction of palladium (Pd) with SiC enhances the migration of silver [10]–[13]. The interaction between high yield FP such as zirconium (Zr) and SiC has also been reported [14][15]. Due to these problems a solution to stop the migration of FPs, and reaction of FPs with the SiC has to be investigated. Tungsten is suggested as a solution to assist SiC as a diffusion barrier for the TRISO particle.

Silicon Carbide

SiC is a binary compound with the same number of Si and C atoms. The structural unit of SiC is considered to be covalently bonded, with an ionic contribution of about 12%, due to the difference in valency of Si (positively charged) and C (negatively charged), making the Si and C bonds to be nearly pure covalent. The basic structural unit of SiC is a tetrahedron, and is either SiC4 or CSi4 as shown in figure 1-2. SiC has chemical and physical properties like: extreme hardness, high thermal conductivity, small neutron capture cross-sections, high temperature stability, radiation resistance, etc. [16]–[18]. It is a wide-bandgap semiconductor 2.4-3.4 eV [19] that has a good potential for electronic and optoelectronic applications in advance devices such as LEDs, lasers, microwave power, etc. [20].
It has a Mohs hardness of 9.5 making it one of the hardest naturally occurring material known. The bond length between Si and C atom is 1.89 Å which results in excellent hardness and high bond strength [21]. SiC is a good abrasive with high corrosion resistance and it has high thermal conductivity allowing for high operation temperatures. It also has good dimensional stability (its ability to be able to maintain or keep its shape over a long period of time, and also under specific conditions) under neutron radiation. The sublimation temperature of SiC is around 2800 °C and decomposes at temperatures above 1600 °C [22]–[24]. SiC comes in a different number of crystal structures, called polytypes. All the polytypes of SiC chemically consist of 50% of C atoms and 50% of Si covalently bonded. SiC has more than 200 polytypes which have been identified [19][25]. All the polytypes have different electrical properties [26]. The most common polytypes of SiC are the cubic 3C-SiC, hexagonal 4H-SiC, 6H-SiC, and rhombohedral 15R-SiC (shown in figure 1-3). The difference in polytypes is characterized by the stacking sequence of the bi-atom layers of the SiC structure.

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TABLE OF CONTENTS :

Chapter 1 : Introduction
- 1.1 High temperature gas cooled reactors
- 1.2 Silicon Carbide
- 1.3 Tungsten
- 1.4 W coating on SiC (Metal-ceramic composites)
- 1.5 Research Motivation
- 1.6 Research Objective
- 1.7 Thesis outline
- 1.8 References
Chapter 2 : Diffusion
- 2.1 Diffusion Mechanism
  - 2.1.1 Vacancy Mechanism
  - 2.1.2 Interstitial and Interstitialcy Mechanism
  - 2.1.3 Grain boundary Diffusion and Dislocation
- 2.2 References
Chapter 3 : Thermodynamics
- 3.1 Mass Action Law
- 3.2 Gibbs Free Energy
- 3.3 Phase Diagrams
  - 3.3.1 Tungsten-Carbon Binary phase diagram
  - 3.3.2 Tungsten-Silicon binary phase diagram
  - 3.3.3 Ternary phase diagram for W-Si-C
- 3.4 References
Chapter 4 : Metal-SiC reaction
- 4.1 Metal-SiC Reactions
  - 4.1.1 Non-reactive metal-SiC system
  - 4.1.2 Reactive Metal-SiC systems
- 4.2 References
Chapter 5 : Previous studies on W-SiC
- 5.1 References
Chapter 6 : Experimental Procedure
- 6.1 Samples Preparation
  - 6.1.1 Sputter Deposition
  - 6.1.2 Annealing
- 6.2 Analysis Techniques
  - 6.2.1 Rutherford Backscattering Spectrometry (RBS)
  - 6.2.2. Scanning Electron Microscopy (SEM)
  - 6.2.3 X-ray Diffraction (XRD)
  - 6.2.4 Atomic Force Microscopy (AFM)
- 6.3 References
Chapter 7 : Results and Discussion
- 7.1 As-deposited samples
  - 7.1.1 Rutherford Backscattering Spectrometry
  - 7.1.2 Grazing Incident X-ray Diffraction
  - 7.1.3 Scanning Electron Microscopy and Atomic Force Microscopy
- 7.2 Vacuum Annealing
  - 7.2.1 Rutherford Backscattering Spectrometry
  - 7.2.2 Grazing Incident X-ray Diffraction
  - 7.2.3 Scanning Electron Microscopy
  - 7.2.4 Atomic Force Microscopy
- 7.3 Hydrogen (H2) Ambient Annealing
  - 7.3.1 Rutherford Backscattering Spectrometry
  - 7.3.2 Grazing Incident X-ray Diffraction
  - 7.3.3 Scanning Electron Microscopy
  - 7.3.4 Atomic Force Microscopy
- 7.4 Argon (Ar) Ambient Annealing
  - 7.4.1 Rutherford Backscattering Spectrometry
  - 7.4.2 Grazing Incident X-ray Diffraction
  - 7.4.3 Scanning Electron Microscopy
- 7.4. 4 Atomic Force Microscopy
- 7.5 Discussion
- References
Chapter 8 : Conclusions and Future work
- 8.1 Conclusions
- 8.2 Future studies
- Appendix