I Fracture and microstructure of

This review surveys the behaviour d graphite under loading conditions in which the stressing is on the whole tensile. The performance of a recently introduced model of graphite failure is reviewed in relation to the modeis from which it was developed, and against data obtained from a wide range of experiments on the failure and deformation properties of graphite In tension and bend. The new model represents, to some extent, an amalgamation of two earlier models, which in themselves are only capable of describing graphite behaviour io a limited extent. It is demonstrated to be very successful in describing a wide range of experimental failure behaviour of MT pitch coke and IM1-24 graphites in tensile and three-and four-point bend stress states. The introduction of non-conservative micrastructural processes allows strain hysteresis to be predicted. Furthermore, H is demonstrated that such processes may be associated with a component of acoustic emission which, together with a further component attributed to pore extension by cracking, account for the experimentally ObseNed frequency and energy d events from tensile stressed specimens. The possible extension d the new model, for instance to include the effects of fast neutron irradlailon and radiolytic oxidation experienced in a nuclear reactor environment or to allow its use in finite element stress analysis, is discussed. British commercial gas-cooled nuclear reactors make use of graphite both as a structural material and as a moderator to thermalize fast neutrons from the fission process. In both the Magnox and advanced gas cooled reactors (CAGRS) the core consists of a lattice of interlocking graphite bricks and keys. During the operation of such a reactor this structure is required to respond to thermally induced movements of the core support and restraint steelwork. In doing so the core lattice must retain the geometty of vertical channels machined into the bricks and passing through consecutive layers, so that the raking and lowering of fuel elements and control rods is not impeded. It is therefore necessaly that the graphite components have suficient strength to withstand the loads associated with the interactions between the graphite bricks which accompany the thermally induced movements, wen in their irradiated state. In ~ A G R S graphite is also used as a sleeve for the fuel elements which must remain intact to maintain the configuration of the fuel pins, and also to ensure there is no diversion of coolant gas which could lead to …