Chinthaka Silva

X‐ray microcomputed tomography (μCT) was applied in characterizing the internal structures of a number of irradiated materials, including carbon‐carbon fibre composites, nuclear‐grade graphite and tristructural isotropic‐coated fuel particles. Local cracks in carbon‐carbon fibre composites associated with their synthesis process were observed with μCT without any destructive sample preparation. Pore analysis of graphite samples was performed quantitatively, and qualitative analysis of pore distribution was accomplished. It was also shown that high‐resolution μCT can be used to probe internal layer defects of tristructural isotropic‐coated fuel particles to elucidate the resulting high release of radioisotopes. Layer defects of sizes ranging from 1 to 5 μm and up could be isolated by tomography. As an added advantage, μCT could also be used to identify regions with high densities of radioisotopes to determine the proper plane and orientation of particle mounting for further analytical characterization, such as materialographic sectioning followed by optical and electron microscopy. In fully ceramic matrix fuel forms, despite the highly absorbing matrix, characterization of tristructural isotropic‐coated particles embedded in a silicon carbide matrix was accomplished using μCT and related advanced image analysis techniques.

Growth and microstructure of a protective or nonprotective SiO₂ scale and the subsequent volatilization of scale formed on high‐purity chemical vapor deposited (CVD) SiC and nuclear‐grade SiC/SiC composites have been studied during high‐temperature 100% steam exposure. The environmental parameters of interest were temperature from 1200°C to 1700°C, pressure of 0.1 to 2 MPa and flow velocities of 0.23 to 145 cm/s. Scale microstructure was characterized via electron microscopy and X‐ray diffractometry. The Arrhenius dependence of the parabolic oxidation and linear volatilization rate constants were determined. The linear volatilization rate exhibited a strong dependence on steam partial pressure with a weaker dependence on flow velocity. At high steam pressures, the oxide scale developed substantial porosity, which significantly accelerated material recession. The dominant oxide phase for the conditions studied was cristobalite. The oxidation behavior of SiC/SiC composite was strongly dependent on the state of the surface, specifically whether steam could find easy entry into the material via surface‐exposed interface layers. For the case where these as‐machined interfaces were surface coated with matrix CVD SiC, composite recession was found to be essentially that of high‐purity CVD SiC.

In the context of the demonstration of the UREX (uranium extraction) process, a separation of uranium and technetium using an anion exchange resin was performed on a simulant solution containing 98.95 g/L of²³⁸U and 130.2 mg/L of⁹⁹Tc. After sorption on the resin, TcO₄⁻was eluted with NH₄OH, the eluting stream was treated, and the technetium converted to Tc metal (yield=52.5%). The purity of the compound was analyzed: it contains less than 23.8 μg of²³⁸U per gram of⁹⁹Tc. Tc metal was characterized by X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS) spectroscopy; EXAFS analysis clearly confirms the hexagonal structure of the metal obtained after treatment. Uranium was converted to ammonium diuranate and to U₃O₈in a yield of 88.2%, analysis indicates that the compound contains less than 0.16 μg of⁹⁹Tc per gram of ammonium diuaranate.