Temperature effects on the behavior of carbon 14 in nuclear graphite

The dismantling of the 1st French generation UNGG (Uranium Naturel Graphite Gas) nuclear reactors operated by the French utility, EDF (Electricité de France) will generate around 17,000 tons of irradiated graphite wastes that have to be disposed of. 14C is one of the main radioactive dose contributors over 10,000 years. For the management of this waste, it is mandatory to get an accurate estimation of 14C. The general aim of our work is therefore to simulate the behavior of 14C in nuclear graphite and to elucidate the coupled and decoupled effects of temperature, irradiation and radiolytic corrosion that mainly influence 14C behavior in graphite during reactor operation. This paper focuses on the behavior of 13C implanted into nuclear graphite and used to simulate the presence of 14C displaced from its original structural site through recoil during neutron irradiation. It aims at evaluating both the temperature and the disorder level of the implanted graphite structure effects on 13C migration using two complementary techniques, NRA and SIMS, to evaluate the 13C distribution at the millimeter and micrometer lateral scales respectively. Raman micro-spectroscopy is used to check the graphite structure evolution. The results show that 13C is not released up to 1600 °C whatever the initial structural disorder level of the implanted graphite. This might be due to the fact that 13C might be trapped into interstitial clusters. The extrapolation of our results to the behavior of 14C shows that reactor temperatures (200-500 °C) did not induce any 14C release. Moreover, as long as there is no gasification of the graphite matrix, high temperatures tend to stabilize 14C into the remaining graphite structure. This fact has to be considered in case of high temperature purification of 14C from irradiated graphite.