Oxidation effects on the fracture toughness of the nuclear graphite at high temperature

Abstract

The nuclear grade graphite is the potential candidate materials for the in-core components of the very high temperature gas-cooled reactor (VHTR). Therefore, understanding and evaluation of the strength and fracture toughness for various grades of graphite at operating conditions are essential for design and operation of the reactor. Currently, most of the nuclear graphite data available for the design of VHTR are measured at room temperature. Thus, only limited data are available on the high temperature and the oxidized conditions. 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 3-point bending tests for the selected nuclear graphite oxidized by 0 to 15 %. Specimens were pre-oxidized and tested at $600^\circ C$ which is in the chemical oxidation regime and corresponds to helium coolant inlet temperature near the core support structure. The fracture toughness values were measured by 3-point bending test at the room temperature and at $600^\circ C$ at $700^\circ C$ in nitrogen environment for the oxidized specimens. Test results were discussed in view of the differences in grain size, pore size, and microstructure. We investigated on effects of temperature and oxidation on fracture toughness. Comparing the same manufacturer, the difference in grain size affects the increase rate of fracture toughness with temperature. Increase rate of fracture IG-430 is higher than IG-110, and NBG-17 is higher than NBG-18 at $600^\circ C$. As the proximity of pore makes primary crack to jump one pore to next pore as oxidation progresses, normalized fracture toughness is exponentially decreased by weakening effect. We considered the effects of oxidation and temperature. Since the fracture toughness is increased by thermal expansion of the grain at high temperature, increasing effect of fracture toughness shows up at a little oxida...