Understanding of thermodynamic behaviour of neutron-irradiation graphite from CIRUS reflector

Graphite is used as a moderator and reflector material in nuclear fission reactors. Graphite also serves as a non-replaceable structural component in reactor cores, which must remain stable over the lifetime of the reactor while operating at elevated temperatures. The irradiation of graphite in a nuclear reactor results in a complex population of defects. Heating of the irradiated graphite at high temperatures results in annihilation of the defects with release of an unusually large energy, called the Wigner energy. We have performed neutron diffraction experiments on highly irradiated graphite samples by neutrons over 50 years in the CIRUS research reactor at Trombay and ab-initio simulations. We have identified various 2-, 3- and 4-coordinated topological structures in defected graphite, and provided microscopic mechanism of defect annihilation on heating and release of the Wigner energy. The annihilation process involves cascading cooperative movement of atoms in multiple steps involving an intermediate structure. Our experimental observation of magnetism in defected graphitic materials is of fundamental importance as these materials find wide applications. Our work provides new insights in understanding of the defect topologies, observation of magnetism and defect annihilation in graphite, which is of considerable importance to wider areas of graphitic materials including graphene and carbon nanotubes