EFFECT OF DIFFERENTIAL OXIDATION OF GRAPHITE CRYSTALLITES ON THE ELASTIC MODULI OF NUCLEAR GRAPHITES

The physical properties of nuclear graphites are heavily influenced by a range of defects that arise as a result of the manufacturing processes used to produce these materials. Among these defects, porosity plays an important role in determining properties of bulk graphite including the elastic responses, strength and toughness. It is well known that oxidation processes affect porosity and impact these properties as a result of the modification of the graphite pore structure. During oxidation, porosity increases as a result of chemical reactions between graphite and gas phase species. These reactions occur at rates governed by the availability of reaction sites including those on graphite crystallite basal planes and edge planes. Beyond reaction site availability, differences in reactivity among various portions of the graphite microstructure influence morphological and physical changes to the graphite as porosity evolves. In particular, differences in reactivity between edge and basal plane sites reduce the connectivity of graphite crystallites sharing edge connections more rapidly than the junctions between other portions of the microstructure. This work examines the effect of site reactivities using first-order rate equations to describe differential changes to the graphite microstructure. These changes are related to the elastic connectivity of graphite crystallites and are used to assess the changes in the moduli as porosity increases.