Non-destructive evaluation methods for degradation of IG-110 and IG-430 graphite

The lifetime extension of in-core graphite components is one of the key technologies for the VHTR. The residual stress in the graphite components caused by neutron irradiation at high temperatures affects their lifetime. Although oxidation damage in the components would not be significant in normal reactor operation, it should be checked as well. To evaluate the degradation of the graphite components directly by a non-destructive analysis, the applicability of the micro-indentation and ultrasonic wave methods were investigated. The fine-grained isotropic graphites of IG-110 and IG-430, the candidate grades for the VHTR, were used in this study. The following results were obtained. (1) The micro-indentation behavior was changed by applying the compressive strain on the graphite. It suggested that the residual stress would be measured directly. (2) The change of ultrasonic wave velocity with 1 MHz by the uniform oxidation could be evaluated by the wave–propagation analysis with wave–pore interaction model. (3) The trend of oxidation-induced strength degradation on IG-110 was expressed by using the proposed uniform oxidation model. The importance of the non-uniformity consideration was indicated.