New advances on the thermal behaviour of chlorine in nuclear graphite

This paper deals with the thermal behaviour of 36Cl in nuclear graphite used in UNGG French reactors (graphite moderated and CO2 cooled reactors). Implanted 37Cl simulates 36Cl displaced from its original structural site by recoil. The implanted nuclear graphite samples were annealed in the 200-1600 °C temperature range and the 6-50 h time range. Structural modifications were followed by Raman microspectrometry. 37Cl concentration depth profile evolution was determined by Secondary Ion Mass Spectrometry or by Rutherford Backscattering Spectrometry, depending on the implantation fluence. This study shows a correlation between the reordering of the graphite structure with annealing temperature and the chlorine release. It evidences also two distinct chlorine release steps with different kinetics (a rapid one followed by a much slower one) and suggests the presence of two different chlorine trapping sites. A low energy site at edge surface of crystallites (or coherent domains) and a high energy one located inside the crystallites (or coherent domains) for which temperatures higher than 1300 °C are required to allow chlorine removal.