Development of evaluation method with X-ray tomography for material property of IG-430 graphite for VHTR/HTGR

Graphite materials are used for the in-core components of High Temperature Gas-cooled Reactor (HTGR) which is a graphite-moderated and helium gas-cooled reactor. The HTGR is particularly attractive due to capability of producing high temperature helium gas, and its passive and inherent safety features. The Very High Temperature Reactor (VHTR) is one of the most promising candidates for the Generation-IV nuclear reactor systems. IG-110 graphite having high strength and resistance to oxidation is used in the HTTR of JAEA. IG-110 is a major candidate for the in-core graphite components of VHTR, too. From the standpoint of the safety at air ingress accident, it is important for graphite materials to have adequate resistance against oxidation damage. IG-430 graphite having higher strength and resistance to oxidation than IG-110 is an advanced candidate for the VHTR. Recently, X-ray tomography method is expected to apply the evaluation of neutron irradiation effects by measuring the irradiation-induced change of geometry of graphite grains and pores. This method is also applicable to evaluate the oxidation damage on graphite from the oxidation-induced change of grain/pore microstructures. In this study, in order to develop evaluation method for material properties and to evaluate the irradiation-induced property changes under higher neutron doses for IG-430, the oxidation and densification effects on elastic modulus of IG-430 were investigated. Moreover, the correlation of the microstructure based on the X-ray tomography images and the material properties was discussed. It was shown that the elastic modulus of the densified graphite depends on only the open pores and it is possible to evaluate the material properties of graphite by using X-ray tomography method. However, it is necessary to take into account of the change in the number and shape of closed pores in the grain to simulate the elastic modulus of the highly oxidized and irradiated materials by the homogenization analysis