MATERIALS PERSPECTIVE Fracture behaviour of radiolytically oxidised reactor core graphites: a view A. Hodgkins* 1 , T. J. Marrow 2 , M. R. Wootton 3 , R. Moskovic 3 and P. E. J. Flewitt 3,4 This paper provides a view on the fracture behaviour of polygranular graphites, used to moderate gas cooled nuclear reactors. Graphite is often cited as a classic example of a brittle material because failure, in tension, is associated with small strains. However, attempts to characterise the fracture behaviour of graphite by linear elastic fracture mechanics methods have been largely unsuccessful. Observations of graphite fracture show that elastic strain energy may be dissipated by the formation of distributed microcracks, and their formation may be responsible for non- linearity in the rising load–displacement curve. Progressive softening behaviour may also be observed in some specimens after the peak load. This type of load–displacement behaviour is a characteristic of quasi-brittle materials. Radiolytic oxidation increases the proportion of porosity within reactor core graphite so that the microstructure becomes increasingly skeletal. Consideration is given to the fracture of radiolytically oxidised graphite to support an argument for quasi-brittle behaviour. Keywords: Reactor core, Graphite, Quasi-brittle, Fracture, Process zone, Porosity Introduction Graphite moderator bricks, for UK gas cooled reactors, were manufactured in batches, from a mixture of dry calcined coke and liquid pitch binder. The mixed and graded raw materials were fed into either a mould or extrusion die. Subsequently, the billets were baked at a temperature of ,800uC to remove volatile substances. Forced pitch impregnation was used to control the volume of porosity in the microstructure. Finally, the billets were re-baked and then graphitised at a tempera- ture of ,2800uC. 1,2 The microstructure of graphites, produced by these methods, consists of relatively large graphitised filler particles (i.e. 0?10–1?0 mm scale) dispersed in a matrix of finer (below y10 mm) graphite crystallites defined as ‘flour’ (fine calcined filler particles) and graphitised pitch binder. The porosity, arising from gas evolution and accommodation cracking due to thermal strains, results from both baking and graphiti- sation. 1 Porosity is observed in both the large filler particles and the matrix, and complex interconnected pathways are formed throughout the microstructure. Porosity remains, after production, at about 20% com- prising both open and closed pores. The closed pores are associated with the filler particles and result from shrinkage during the calcination stage of manufacturing, whereas open pores are contained mainly within the binder regions and arise from gas evolution. 1 The UK Magnox reactors and the UK advanced gas reactors (AGR) used graphites supplied by British Acheson Electrodes Limited and Anglo Great Lakes. Pile grade A (PGA) graphite components, used to moderate the Magnox reactors, were formed by extru- sion. Graphite moderator bricks for the AGR pro- gramme were moulded from Gilsocarbon IM1-24. Both grades of graphite exhibit variations in the properties and the microstructure due to manufacturing. The variations are observed between different bricks and within the same brick 2 but were limited by the quality procedures adopted. The crystalline form of manufac- tured graphite is orthorhombic and consists of layers of hexagonal carbon atom rings that form the basal planes. Successive basal planes are arranged in an alternating ABABAB sequence. The atoms in each layer are located directly above and below the centre of the hexagonal rings in adjacent layers. The bonds between atoms in the same layer are covalent, 2,3 and the weak bonds between the basal planes provide a low energy pathway for cleavage fracture. 3,4 The filler particles in PGA graphites become aligned during extrusion so that mechanical and fracture properties are different in the axial and trans- verse directions. Such orientation differences are not observed in the Gilsocarbon material due to the micro- structure and shape of the filler particles. Fast neutron irradiation leads to energy deposition that gives rise to radiolytic oxidation in CO 2 gas cooled reactors. The graphite microstructure and properties change over the service life of the reactors due to two processes: first, neutron irradiation introduces defects into the crystal lattice that increase the strength as a 1 Serco TAS, Faraday Street, Birchwood Park, Warrington, WA3 66A, UK 2 Materials Performance Centre, School of Materials, The University of Manchester, Manchester, M13 9PL, UK 3 Magnox North Ltd, Oldbury Naite, Oldbury-on-Severn, Bristol, BS35 1RQ, UK 4 Departmet of Physics, HH Wills Laboratory, University of Bristol, Bristol, BS8 1TL, UK *Corresponding author, email Andrew.hodgkins@Sercoassurance.com ß 2010 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 25 June 2009; accepted 11 August 2009 DOI 10.1179/026708309X12526555493477 Materials Science and Technology 2010 VOL 26 NO 8 899