OXIDATION EFFECTS ON THE STRENGTH AND FRACTURE TOUGHNESS OF THE SELECTED NUCLEAR GRAPHITE GRADES AT 600℃

摘要

The nuclear grade graphite is the proposed materials for the in-core components of the very high temperature gas-cooled reactor (VHTR) currently being developed. Understanding and evaluating the strength and fracture toughness of various graphite grades at operating conditions are critical in the design and operation of the reactor. In the current nuclear graphite database available for the design of VHTR, however, most of the strength and the fracture toughness data are of oxidized specimens tested at room temperature. Thus, limited data are available on the changes in the strength and fracture toughness at high temperature due to oxidation. Furthermore, no data are yet available to examine the possible decrease in the fracture toughness due to the increase in the strength with temperature after oxidation. In this study, we performed the bending tests for the selected nuclear graphite grades oxidized by 0 to 15 %. Specimens were oxidized and tested at 600℃ which is in the chemical oxidation regime and corresponds to helium coolant inlet temperature near the core support structure. The strengths and fracture toughness values were measured by 3-point bending test at the room temperature and at 600℃ in nitrogen environment for the oxidized specimens. Test results were discussed in view of the differences in the test temperature, oxidation, pore microstructure and the type of the coke.