The Use of X-Ray Microtomography to Determine the Relationships Between the Microstructure and Mechanical Behaviour of Nuclear Graphite

X-ray microtomography (XMT) has been used to characterise the microstructure and density distribution in thermally-oxidised Pile Grade A (PGA) graphite in 3-D. The needle coke filler particles can be clearly distinguished from the binder phase. Quantitative measurements of density showed that thermal oxidation occurred predominantly in the binder phase. The spatial distribution of large-scale porosity was determined. The Young’s modulus and thermal diffusivity of specimens of virgin and nominally 15% and 30% weight loss graphite were measured. It was demonstrated that weight loss calculations do not necessarily reflect the degree of oxidation and material removal throughout the bulk of the specimen. Relationships between microstructure, bulk density, and the mechanical properties were determined. 0. INTRODUCTION Nuclear graphite undergoes microstructural changes on irradiation during service [1]. One of the main features of this rearrangement in Pile Grade A (PGA) graphite is the development of porosity [1]. This porosity adversely affects the mechanical behaviour of the graphite that has implications for the structural integrity of nuclear power plant. The objectives of this study are to use novel techniques (tomography, laser flash, and laser ultrasonics) to characterise the microstructure and microstructural changes on thermal oxidation, measure thermal diffusivity and Young’s modulus of these specimens, and correlate these microstructures with properties. 1. EXPERIMENTAL 1.1. Materials and Oxidation Process Thermal oxidation was performed on specimens of Pile Grade A (PGA) graphite. PGA is a mediumtextured, extruded graphite consisting of a pitch-based binder phase and needle coke filler particles. The extrusion process aligns the filler particles which, due to their high aspect ratio, results in anisotropy in the graphite’s mechanical and physical properties [2]. The tomography, thermal diffusivity, and Young’s modulus experiments were performed on cylindrical specimens of diameter 12 mm and length 6 mm. Specimens of two orientations were studied: WG, where the specimen cylinder axis is parallel to the extrusion direction; and AG where the specimen cylinder axis is perpendicular to the extrusion direction. In the thermal oxidation process, the samples were placed in a furnace at 900 oC in an atmosphere of CO and CO2 (5% CO in CO2). The gas flow rate was approximately 1 l min. The weight losses were calculated from mass measurements made using a 10 g Mettler balance. Under these conditions, the weight loss was approximately 3% per day up to weight losses of 35%. 1.2. Tomography The principle and practice of X-ray computed tomography has been described in detail elsewhere [3,4]. An X-Tek HMX 225 laboratory tomography system was used to collect the data, shown schematically in Figure 1. A series of 2-D radiographs were taken at equally-spaced angular orientations (separated by 0.32o) for 180o using x-rays generated from a copper target using an accelerating voltage of 30 kV THE USE OF X-RAY MICROTOMOGRAPHY TO DETERMINE RELATIONSHIPS BETWEEN MICROSTRUCTURE AND MECHANICAL BEHAVIOUR OF NUCLEAR GRAPHITE #E23 -2 -and beam current of 150 μA. The incident x-rays were filtered using a 0.25 mm aluminium sheet in front of the source. Volume reconstruction was performed using a 3-D filtered back projection algorithm [3]. Qualitative images of the specimens are shown in Figure 2. The coke particles can be clearly seen. The reduction in grey level of the binder phase corresponds to a reduction in its density. Figure 1: Schematic of tomographic set-up (not drawn to scale). Figure 2: Reconstructed images of specimens of nominal a) 0%, b) 17%, and c) 29.6% weight loss. 1.3. Quantitative Density Measurements by Tomography In order to convert the grey levels to quantitative measurements of density, a calibration experiment must be performed. The voxels (3D pixels) of the tomographic image represent a 3D map of the linear x-ray absorption coefficient, μ. For a given material and energy in the photoelectric domain, this factor, μ, is proportional to the density of the material, ρ [5]. For a single phase material such as graphite, the relationship between μ and ρ can be determined by a step wedge method on a fully dense graphite sample, in this study taken as Highly Oriented Pyrolitic Graphite (HOPG). The step wedge method consists of measuring the transmitted intensity of the beam through different thicknesses of material. The linear absorption coefficient is then simply determined as it is the gradient of the linear function ln(I0/I) = μ x thickness (1) where I0 is the incident x-ray intensity and I is the transmitted intensity. Perspex holder φ 13 mm Be Detector CCD camera φ 12 mm 6 mm 1 mm d = 10 mm DD = 556 mm PGA sample X-ray gun