Density and Open/Closed Porosity Evolution of Low Temperature Oxidized Nuclear Graphite

Abstract

There are limited density and porosity data on nuclear graphite oxidised at temperatures less than 600 °C. In this work, three types of nuclear graphite (IG-110, NBG-17 and NBG-18) with different grain sizes and manufacturing processes were oxidized at 550 °C. The as-received IG-110, NBG-17 and NBG-18 graphite were characterized to create a baseline for studying the density and porosity evolution upon oxidation. Specimens with two different sizes were prepared to understand the specimen size effect on measured density and porosity. The geometric density of the as-received graphite blocks (20 (W)×25 (D)×30 (L) mm) was measured as per the C559-16 ASTM standard. In addition, open/closed porosities of the graphite blocks were measured with Helium Pycnometry. Density of disk graphite specimens (10 mm (Φ)×2 mm (D)) were characterized with physical dimension measurement and X-ray Computed Tomography (XCT) scaling. The results showed that the density calculated from XCT scaling with POCO graphite as a reference standard is consistent with geometric density. IG-110, as the only petroleum coke graphite, showed strong dependency of geometric density on specimen size. In addition, graphite oxidation phenomenon of IG-110, NBG-17 and NBG-18 at 550 °C showed significant differences from literature reported oxidation data. The results suggested that the surface porosity and open porosity ratio along with specimen size played a crucial role in oxidation kinetics at 550 °C.