Structural investigation of RBMK nuclear graphite modified by 12C+ ion implantation and thermal treatment

Abstract

Graphite is a major core component in the graphite-moderated RBMK-1500 nuclear reactor. Neutron irradiation induces radiation damage in the graphite matrix, which affects physical properties of material. In order to generate defects that could be similar to those induced by neutrons in the RBMK-1500 nuclear reactor, the raw RBMK graphite samples were implanted with 12C+ ions at the energy of 700 keV at varying fluences ranging from 1.2 × 1015 to 1.2 × 1016 ions/cm2. The ion beam interaction simulations were performed by using SRIM-2013 and GEANT4 codes. In order to study temperature effects on the evolution of the ion implanted graphite structure the thermal treatment was applied. The structural disorder level on the surface of RBMK graphite samples was investigated by using Raman spectroscopy before and after ion implantation as well as after thermal treatment. The decrease of the graphitic sp2-related content as well as formation of an amorphous structure were observed on the surface of the ion implanted samples, while thermal treatment resulted in significant reordering at the ion implantation fluence of 7.2 × 1015 ions/cm2 and lower. The understanding of structural dynamics of nuclear graphite under irradiation conditions provides insights into migration of impurities (e.g. 14N) as well as distribution of activation products (e.g. 14C, 36Cl) in the irradiated graphite matrix.