Graphite oxidation and structural strength of bottom support system in VHTR

Abstract

The air-ingress event by a large pipe break is an important accident considered in design of very high-temperature gas-cooled reactors (VHTR). Core collapse prediction is a main safety issue. Graphite oxidation model and structural failure model are technically required. In this study, one main target is to develop graphite oxidation model based on Arrhenius model for graphite oxidation reaction. Kinetic parameters of IG-430 were measured. Temperature was controlled at the range of 540 to 800 ºC to estimate activation energy. Oxygen concentration was controlled over the range of 0 to 0.34 mole fraction to find order of reaction. It turns out that activation energy Ea was 258.15 ± 1.5 kJ/mol and order of reaction was 0.37 ± 0.04. The other main target is to develop the structural failure model. Graphite support column is important for VHTR structural integrity. Graphite support columns are under the axial load. Critical strength of graphite column is related to slenderness ratio and bulk density. Through compression tests for fresh and oxidized graphite columns we show that compressive strength of IG-110 was 79.46 MPa. And, the buckling strength of IG-110 column was expressed by the empirical formula. $\sigma_0 = \sigma_{straight-line} -C\frac{L}{r}$ $\sigma_{straight-live} = 91.31 MPa, C=1.01$ The results of uniform and non-uniform oxidation tests show that the strength degradation of oxidized graphite column is expressed in the following non-dimensional form. $\frac{\sigma}{\sigma_0} = exp(-kd), k=0.111$ Also, from the results of the uniform oxidation test with a complicated-shape column, we found out that the above non-dimensional equation obtained from the uniform oxidation test is applicable to a uniform oxidation case with a complicated-shape column.