Abstract
Chronic oxidation by impurities in the VHTR coolant gas during normal operation and acute oxidation in off-normal conditions will produce microstructural damage and changes in pore configuration in nuclear graphite, which will cause a certain decay of mechanical properties including strength loss. A knowledge gap exists regarding the relationship between oxidation behavior of graphite and its mechanical properties, as identified by the documents of the workshop on nuclear graphite research organized by ORNL and sponsored by NRC in 2009.
This report contains experimental results aiming at understanding the effect of oxidation on porosity development in PCEA-grade nuclear graphite. A statistically significant number of specimens (25.4 mm length x 12.7 mm diameter) were oxidized in air at two temperatures (600 and 700 oC) and several levels of weight loss (5, 10, 15 %). The thickness, spatial distribution, and pore structure of the oxidized layer was characterized by optical microscopy coupled with automated image analysis (AIA). Mechanical properties, including compressive strength and elastic and shear moduli, were measured systematically. It was found that, at equal levels of weight loss, oxidation at 600 oC is more damaging on strength than oxidation at 700 oC, although the latter is faster. Linking information obtained from image analysis and mechanical tests, an original calculation model was developed which allows estimates of local properties (i.e. density and compressive strength) in the narrow oxidized layer developed by non-uniform oxidation at high temperatures.
