In this paper, it is clearly shown that the distribution of the initial porosity is a critical factor in the prediction of damage evolution and initiation of failure in a cast AM60B magnesium notch Bridgeman tensile specimen. Using X-ray computed tomography, the actual initial porosity distribution was obtained, and this distribution was input into a finite element code as an initial condition. The predicted damage evolution from this simulation was compared to the damage evolution of the experimental specimen as well as other simulated porosity distributions. This study shows that the simulation of the actual porosity distribution predicted well the damage evolution observed in the experiment. It is also shown that the initial distribution of porosity plays a vital role in the predicted elongation to failure of a notched specimen. The actual distribution was shown to fail at a significantly lower strain than random or uniformly distributed damage.

The effects of metastable tetragonal zirconia on the properties of Ordinary Portland Cement were observed during which the effect of crystallite size pH on the preparation solution, precursor salt, and the presence of co-precipitates, Fe(OH)3, SnO2 and SiO2 on the crystallization temperature, enthalpy and crystal structure, immediately following the crystallization exothermic burst phenomenon in ZrO2 were measured. Thermal analysis and x-ray methods were used to determine crystallite sizes and structures immediately following the exothermic burst. Comparisons were made for zirconias prepared from oxychloride, chloride and nitrate solutions. The existence of tetrameric hydroxidecontaining ions in oxychloride precursor is used to rationalise low values of crystallization enthalpy. The position of the crystallization temperature, Tmax was not dependent on crystallite size alone but also on the pH at which the gel was made, the surface pH after washing, and the presence of diluent oxides. Enthalpy v r1/2 and Tmax v (diluent vol)1/3 relationships indicate that surface coverage effects dominate a surface nucleated phenomenon. The data established for ZrO2 systems was used to develop tetragonal-ZrO2-SnO2 powders capable of improving the mechanical properties of Ordinary Portland Cement discs. The ZrO2-OPC discs were prepared by powder mixing, water hydration and uniaxial pressing. Vicat needle tests showed that tetragonal-ZrO2 increases the initial setting rate. Microscopy indicated that porosity distribution changes near to ZrO2 particles. Zirconia has also been introduced into OPC discs by vacuum infiltration methods developed for solutions and colloidal suspensions. Comparisons between OPC discs and the OPCtetragonal ZrO2 composites have been made on the basis of diametral compression strength, Young’s modulus, hardness and toughness (K1c), as estimated by the cracked indentation method. Bell-shaped curves are found for the way the mechanical properties are changed as a function of Zirconia content.

The effects of metastable tetragonal zirconia on the properties of Ordinary Portland Cement were observed during which the effect of crystallite size pH on the preparation solution, precursor salt, and the presence of co-precipitates, Fe(OH)3, SnO2 and SiO2 on the crystallization temperature, enthalpy and crystal structure, immediately following the crystallization exothermic burst phenomenon in ZrO2 were measured. Thermal analysis and x-ray methods were used to determine crystallite sizes and structures immediately following the exothermic burst. Comparisons were made for zirconias prepared from oxychloride, chloride and nitrate solutions. The existence of tetrameric hydroxidecontaining ions in oxychloride precursor is used to rationalise low values of crystallization enthalpy. The position of the crystallization temperature, Tmax was not dependent on crystallite size alone but also on the pH at which the gel was made, the surface pH after washing, and the presence of diluent oxides. Enthalpy v r1/2 and Tmax v (diluent vol)1/3 relationships indicate that surface coverage effects dominate a surface nucleated phenomenon. The data established for ZrO2 systems was used to develop tetragonal-ZrO2-SnO2 powders capable of improving the mechanical properties of Ordinary Portland Cement discs. The ZrO2-OPC discs were prepared by powder mixing, water hydration and uniaxial pressing. Vicat needle tests showed that tetragonal-ZrO2 increases the initial setting rate. Microscopy indicated that porosity distribution changes near to ZrO2 particles. Zirconia has also been introduced into OPC discs by vacuum infiltration methods developed for solutions and colloidal suspensions. Comparisons between OPC discs and the OPCtetragonal ZrO2 composites have been made on the basis of diametral compression strength, Young’s modulus, hardness and toughness (K1c), as estimated by the cracked indentation method. Bell-shaped curves are found for the way the mechanical properties are changed as a function of Zirconia content.

A procedure to evaluate the leaching properties of radionuclides from irradiated graphite waste has been developed by combining ANSI 16.1 (USA) and NEN 7345 (Netherlands) standardised diffusion leaching techniques. The ANSI 16.1 standard has been followed to the acquire the leachates and to determine the leach rate/ diffusion coefficient and NEN 7345 standard technique has been used to determine the diffusion mechanism of radionuclides. The investigation employs simulated Drigg groundwater as a leachant using semi-dynamic technique for the production of leachate specimens. From gamma spectroscopy analysis the principal radionuclides present in terms of activity were 60Co, 137Cs, 134Cs, 155Eu, 133Ba and 46Sc. The dominant radionuclides are 60Co, 134Cs and 133Ba which together account for about 91 % of the total activity. The 91 % can be broken down into 73.4 % 60Co, 9.1 % 134Cs and 8.1 % 133Ba. Analysis of total beta and total beta without tritium activity release from Magnox graphite was measured using liquid scintillating counting. Preliminary results show that there is an initial high release of activity and decreases when the leaching period increases. This may be due to the depletion of contaminants which were absorbed by the internal pore networks and the surface. During the leaching test approximately 275.33 ± 18.20 Bq of 3H and 106.26 ± 7.01 Bq of 14C was released into the leachant within 91 days. Irradiation induced damages to the nuclear graphite crystal structure have been shown to cause disruption of the bonding across the basal planes. Moreover, the closures of Mrozowski cracks have been observed in nuclear graphite, the bulk property are governed by the porosity, in particular, at the nanometre scale. Therefore, knowledge of the crystallite structure and porosity distribution is very important; as it will assist in understand the affects of irradiated damage and location and the mechanism of the leaching of radionuclides. The work reported herein contributed several key findings to the international work on graphite leaching to offer guidance leading toward obtaining leaching data in the future: (a) the effective diffusion coefficient for 14C from graphite waste has been determined. The diffusion process for 14C has two stages resulting two different values of diffusion coefficient, i.e., for the fast and slow components; (b) the controlling leaching mechanism for 3H radionuclide from graphite is shown to be surface wash–off; and for that of 14C radionuclide the initial controlling leaching mechanism is surface wash-off following by diffusion which is the major transport mechanism ; (c) The weight loss originates from the open pore structure which has been opened up by radiolytic oxidation; at the higher weight losses much of the closed porosity in the graphite has been opened. The investigation indicates that weigh loss has a major influence on the leaching of elements from the irradiated graphite; and (d) the analysis of the pores in nuclear graphite can be categorised into three types. These three types of pores are: (1) small pores narrow which are slit-shaped pores in the binder phase or matrix, (2) gas evolution pores or gas entrapment pores within the binder phase or matrix and (3) lenticular pores which are large cracks within the filler particles. It is shown in this thesis that by using tomography to study the morphology of the different pores coupled with the distribution of impurities an understanding of the role of porosity in leaching is possible.