Leaching of UNGG graphite Study of $^{36}$Cl behaviour
Résumé
Graphite was used as a moderator in the French-designed first generation nuclear reactors (9 UNGG reactors operated by CEA and EDF). Their decommissioning is planned in the coming years and it will generate in France about 23,000 tons of irradiated graphite representing around 100,000 m$^3$ of conditioned waste in cementitious packages that need to be disposed. The disposal behaviour of the radionuclides is one of the main issues when considering graphite waste long-term management scenarios. In order to design a dedicated disposal facility for irradiated graphite, it is crucial to increase the knowledge on this issue. During operation, some graphite impurities were neutron activated, giving rise to radionuclides such as $^{36}$Cl. $^{36}$Cl is a long-lived radionuclide (half life 3.105 y) although minor in term of activity level in irradiated graphite but considered in impact studies as the first radionuclide to be released from the repository containment. This communication compiles data collected during leaching experiments in France on $^{36}$Cl release from irradiated graphite. A first labile fraction of $^{36}$Cl is found to be controlled by diffusion transport through graphite matrix and, as a result, is very fast compared to disposal timescale and $^{36}$Cl half-life. A second fraction is identified with a lower kinetic release. These results are of the utmost importance for choosing irradiated graphite disposal solutions in France.
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