International audience In this paper, we present the study of thermal behaviour of 36Cl in nuclear graphite used in the St. Laurent A2 UNGG reactor (graphite moderated and CO2 cooled reactor). 37Cl, used to simulate 36Cl displaced from its original structural site by recoil, has been implanted into nuclear graphite samples (energy = 250 keV, fluence = 5 × 1013 at cm-2). The samples have been annealed in the 200-800 °C temperature range and analysed by SIMS. Structural modifications have been controlled by Raman microspectroscopy. This study shows that, in the considered temperature range and for a short annealing duration (4 h), chlorine is released almost athermally. At 500 °C, around 20% of the initial 37Cl content is released. At 800 °C, the release reaches a plateau and the loss of 37Cl is around 30%. Raman microspectroscopy shows that 37Cl implantation induces a structural disorder and that during annealing, the original structure is not completely recovered. © 2009 Elsevier B.V. All rights reserved.