An experimental study of oxidation of graphite structure and SiC layer for VHTR severe accident analysis

Abstract

The experimental study of oxidation behavior of the graphite structure and the SiC layer in TRISO was performed to estimate the safety relevant phenomena for Very High Temperature Reactors (VHTR) during air-ingress and steam-ingress postulated accidents. For the integrity of oxidized graphite during the air-ingress accidents we performed the experiments and the modeling on the strength degradation of oxidized graphite structures. We measured the compressive strength and the buckling strength of oxidized IG-110, IG-430, and NBG-10 graphite columns. We derived an empirical straight-line formula to estimate the buckling strength. The formula is expressed in terms of a single parameter, slenderness ratio over the intersection between compressive and buckling strength so that it may be applicable to any type-graphite column. The experimentally derived Knudsen relation with a corrected burn-off in the present study is applicable for predicting the strength degradation of the graphite column which is oxidized in Zone 1 and Zone 2. The strength degradation of the graphite column oxidized in Zone 3 can be evaluated by developing the empirical straight line formula. The combination of the empirical straight-line formula and the Knudsen relation with a corrected burn-off can enable us to predict the strength degradation of a graphite column with oxidization in Zones 1, 2 and 3. The strength of the oxidized graphite column is dependent on the initial strength and burn-off, even though the graphite columns have the different dimensions and the different failure modes. The strength degradation depends on two parameters, a corrected burn-off and a slenderness ratio. We developed and validated a unified model for the strength degradation of oxidized graphite structure. We introduced the concept of the f-value, which is defined as the fraction of the surface reaction to the total reaction. The fraction of the maximum allowable burn-off for the integrity of the support structu...