A unified model for strength degradation of oxidized IG-430 graphite column in VHTR

Air-ingress events caused by large pipe breaks are important accidents considered in the safety analysis of Very High-Temperature Reactors (VHTRs). A main safety concern is the possibility of core-collapse following the failure of the graphite support column, which can be oxidized by ingressed air. The strength of the oxidized cylindrical graphite specimens under compressive load was tested using IG-430 graphite, a candidate material for the support structure of VHTR concept. The temperature conditions were 600 °C where the reaction rate is limited by the kinetics (Zone 1) and 800 °C where the reaction rate is affected by both kinetics and mass transfer rate (Zone 2). The graphite specimens were oxidized up to ∼32% of total burn-off. In this study we suggest a new parameter for the unified strength degradation model. The derived Knudsen relation with corrected burn-off in this study is applicable to predict the strength degradation of a graphite column that oxidized in Zone 1 and Zone 2. The exponent k in the Knudsen relation was empirically obtained as 0.079. The obtained relation is applicable to a maximum corrected burn-off of 22%. The Zone 3 is surface oxidation. The strength degradation of graphite column oxidized in Zone 3 can be evaluated by developing the empirical straight line formula. This combination of the empirical straight line formula and the Knudsen relation in terms of corrected burn-off enables us to predict the strength degradation of a graphite column oxidized in Zones 1, 2 and 3.