Effects of Pore Discovery Rate on the Ultrasonic Properties of Nuclear Graphites Undergoing Oxidation

- The ultrasonic behaviors of nuclear graphites are heavily influenced by defect structures in these materials. Defects include bulk porosity comprised of pores ranging in size from the nano- to the mesoscale, microcracking and intracrystallite nanocracking (Mrozowski cracks). These structures are affected by oxidation processes which generally increase the overall porosity, but simple models that relate the elastic moduli to porosity do not accurately capture the variation in the moduli that accompany oxidation processes. During oxidation, the porosity increases as chemical reactions occur between exposed surfaces of graphite and a gas phase oxidizing agent. The exposed surfaces include open porosity in the graphite and the reactions on these surfaces take place at basal plane and edge plane sites. As the oxidation proceeds, sufficient material can be removed to open pores that were previously closed allowing them to join the open-pore population with the result that new surfaces are subject to oxidation. The rate at which closed pores become open is known as the pore discovery rate. Differences in reactivity between edge and basal plane sites reduces the connectivity of graphite crystallites and alters the transfer of stresses through the microstructure. Even though the overall oxidation rate governs the generation of porosity, microstructural connectivity in open pores evolves as oxidation progresses and the discovery process causes the open pore population to increase. These processes produce a mean connectivity that depends on site oxidation rates and on the pore discovery rate. This work considers the effects of site reactivities along with the pore discovery rate to assess the impact of the porosity generation process on the ultrasonic behavior of graphite.