Characterisation of the Deformation and Fracture of Nuclear Graphite using Neutron Diffraction

It has been demonstrated that the initiation and growth of localised, heterogeneously-distributed process zones is associated with the non-linear stress-strain response of graphites used to moderate UK gas-cooled civil nuclear reactors. These graphites, such as Gilsocarbon graphite, have heterogeneous complex polygranular microstructures which contain pores and flaws arising from their fabrication. The macroscopic properties of such nuclear graphites are dictated by their microstructure. Due to the presence of pores and aggregates, the lattice strain is not expected to change 1:1 with the externally bulk strain applied to macro-scale specimen. Deformation of the material containing pores and flaws causes localisation of strains and, hence, initiation of inelastic damage. The length-scale at which the localised damage develops during loading can be characterised by the lattice strain in a bulk volume of material. Therefore, in situ neutron diffraction on a Gilsocarbon graphite bend geometry test specimen has been undertaken at the ENGIN-X, ISIS facility. It was found that lattice strain changes linearly with applied bulk strain but with reduced magnitude. The results are discussed with respect to the evolution of characteristic process zones, as deformation is increased, and the associated microcracking.