Abstract
In the first part of this document we report on measurements of oxidation rates in air and development of porosity in the oxidized layer of nuclear graphite grade IG-110. Oxidation rate measurements were performed using the equipment and the procedure recommended by ASTM D 7542-09. These measurements used standard shape and size specimens (25.4 mm diameter x 25.4 mm long cylinders). In parallel we performed additional measurements on smaller size specimens (12.7 mm x 25.4 mm long cylinders), in order to determine whether it would be advantageous to modify the recommendation from ASTM D 7542 regarding the standard shape and size of oxidation specimens. The results were not positive, and therefore the proposal to modify the ASTM D 7542 was not further supported. After oxidation, the development of porosity in the oxidation layer was monitored by optical microscopy with automated image analysis. This was done for all specimens oxidized at four temperatures (600, 650, 700, 750 oC) up to 15 % weight loss.
Second part of this report compares the oxidation behavior of three graphite grades with very different structure: PCEA and NBG-18 (considered as possible candidates for NGNP) versus the IG-110 graphite considered by many a standard graphite for VHTR. The results show distinct differences in the oxidation behavior, which were assigned to known differences in the microstructure of these nuclear grade graphite. While IG-110 is fine-grained with high surface area and fine pores, the other two grades have larger grain sizes, larger pores, and smaller surface area. These differences induce variations with respect with both (i) the oxidation rates in identical conditions and the associated kinetic parameters (apparent activation energy, frequency factor) and (ii) the penetration depth of the oxidant and porosity development in the oxidized layer. The relationship between graphite microstructure and its oxidation resistance demonstrated in this report underline one more time the importance of performing comprehensive oxidation characterization studies of the new grades of nuclear graphite considered as candidates for VHTR.
