Failure of graphite short bars with hemispherical seats

Abstract

A very high-temperature gas-cooled reactor (VHTR) has been developed by Japan Atomic Energy Research Institute and its core consists of so-called fuel blocks, removable reflector blocks and so on. These graphite blocks are supported by thin cylindrical graphite bars called support posts. These posts are in contact with the blocks through hemispherical seats to absorb the relative displacement of blocks by small inclination or rotation of the posts. The reliability of support post structure of this kind is still uncertain, but it is considered to be important because they are unreplaceable through a reactor lifetime. Failure mechanism of support post under compressave load was studied by using 1/6-scale models made of homogeneous graphite. The Weibull distribution was assumed as a probability density function of fracture strength of support post. The followings are the main conclusions. 1. (1) The compressive fracture strength decreases with increasing of slenderness ratio. 2. (2) The compressive fracture strength was independent of a parameter ( 1 r p − 1/r s ): the difference of reciprocal radii of curvature at the post-end and the post seat. 3. (3) The compressive fracture strength remains almost constant within 0–2° of the inclined angle of support post. 4. (4) The failure of support post mostly occurred near the post-end. The volume effect of graphite material and the apparent change of slenderness ratio by oxidation were also discussed.