Damage and fracture in neutron-irradiated graphite: In situ micro-mechancial testing

Polycrystalline artificial graphite has been used in nuclear reactors as a neutron moderator to sustain an efficient chain reaction and supports the core structure. It is not replaceable therefore limits the reactor life. During service, graphite core component subjects to high temperature neutron irradiation and corresponding changes in physical and mechanical properties. This paper concerns the evolution in mechanical properties such as elastic modulus and flexural strength with neutron irradiation damage. Using neutron irradiated highly ordered pyrolytic graphite materials as an example, Raman scattering has been adopted to characterise the irradiation damage and this is correlated to the mechanical properties measured by micro-mechanical testing. The modulus and flexural strength increased after neutron irradiation; twinning and kinking were found to occur in irradiated samples and the load-displacement curve has a linear-elastic, 'yielding' stage followed by plastic deformation prior to fracture. The results are compared with the data measured from an irradiated industrial nuclear graphite.