Nuclear graphite strength degradation under varying oxidizing conditions

Abstract

The oxidized residual strength for two nuclear graphite grades, fine-grained IG-110 and medium-grained NBG-18, were determined over a mass loss range of 0% to 10% for three different oxidation temperatures. This study addresses the critical licensing issue of determining oxidized strength of graphite core components within the design rules of the American Society of Mechanical Engineers (ASME) Boiler Pressure and Vessel Code over a range of oxidation temperatures from 550 °C to 750 °C. Results generally demonstrate low-temperature oxidation to have a 30% greater strength reduction than similar mass losses at higher oxidation temperatures. These differences are attributed to whether the graphite oxidizes primarily within the kinetic-controlled regime or the diffusion-controlled regime which dictate the level of oxidation penetration depth into the interior microstructure. Finally, this study presents a new test method for determining mechanical compression strength from previously oxidized graphite material. This test procedure is based upon two approved American Society for Testing and Materials (ASTM) standards, ASTM D7542 and C695, and combines the requirements for both standards. It is hoped that this new test method can assist ASME BPVC users in determining oxidized residual strength since there are currently no approved standard for conducting such tests.