Decontamination of nuclear graphite by thermal methods

Contaminated nuclear graphite cannot be stored in low level surface disposal facilities such as Centre de L'Aube, in France, due to the long half life of 14C. Furthermore, the 14C activity of the graphite reflectors from the two German HTR reactors (AVR and THTR) only would already cover more than 90% of the 14C activity licensed for the underground disposal facility Konrad in Germany for non heat-generating radioactive waste. Therefore, alternative waste management strategies must be developed for nuclear graphite. The total burning of nuclear graphite with free release of 14C is problematic due to public acceptance. The solidification of the whole amount of CO2 would negate the volume reduction benefit of burning. Therefore, a separation of the 14C from the off-gas is required. However, this carbon isotope has the same chemical properties as the 12C from the graphite matrix. Physical separation methods like pressure swing sorption, cryogenic distillation or ultracentrifugation are not economic for the large amounts of CO2. Pyrolysis in an inert atmosphere or steam reforming seems to be a possibility to separate high amounts of 14C. First experiments under an inert atmosphere showed a high selective removal of 14C from graphite. Higher 14C decontamination factors were obtained in a steam atmosphere, but selectivity decreased. Next step will be a combination of these processes to optimize the 14C release rates and high selectivity for an industrial process development. (author)