Decontamination of nuclear graphite

Abstract

Graphite is used in gas-cooled reactors (e.g. AGR, MAGNOX, HTR) and Russian RMBK reactors as a moderator and reflector. About 250,000 Mg of irradiated graphite (i-graphite) has to be considered as radioactive waste in the next few centuries. Fission products and activation of impurities in the graphite contaminate this graphite during reactor operation. Key nuclides for waste management are Co-60 during decommissioning, if decommissioning is performed immediately after reactor shutdown, and the long living radionuclides 14C and 36Cl for long-term safety in the case of direct disposal. Most radioisotopes can, in principle, be removed by using the purification methods already applied during the manufacture of nuclear graphite. However, due to the same chemical behaviour as 12C, this does not seem to be applicable to 14C. Contaminated 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 [Millington, D.N., Sneyers, A., Mouliney, M.H., Abram, T., Brücher, H., 2006. Report on the International Regulation as regards HTR/VHTR Waste Management, Deliverable D-BF1.1 of the Raphael Project, EC Contract 516508, Confidential report]. Furthermore, the 14C activity of the graphite reflectors from the two German HTR reactors (AVR and THTR) would amount to more than 90% of the total 14C activity licensed for the underground disposal site Konrad in Germany for non-heat-generating radioactive waste [Brennecke, P., October 1993. Anforderungen an endzulagernde radioaktive Abfälle (Vorläufige Endlagerbedingungen, Stand: April 1990 in der Fassung vom Oktober 1993) – Schachtanlage Konrad –, BfS-ET-3/90-REV-2, Salzgitter, p. 51]. The burning of nuclear graphite would be an efficient method for volume reduction, but would not be accepted by the public as long as all the 14C were emitted into the atmosphere in the form of CO 2. The required separation of the 14C from the off-gas is difficult and not economic because this carbon isotope has the same chemical properties as the 12C from the graphite matrix. The solidification of the whole amount of CO 2 would cancel out the volume reduction advantage of burning. Thus, a process is required which benefits from the inhomogeneous distribution of the 14C in the graphite matrix leading to 14C-enriched and 14C-depleted off gas streams (Schmidt, P.C., 1979. Alternativen zur Verminderung der C-14-Emission bei der Wiederaufarbeitung von HTR-Brennelementen, JÜL-1567]. Tritium and other radioisotopes, including 36Cl and 129I, can be removed from graphite by thermal treatment. Even significant parts of the 14C inventory can be selectively extracted because most of the 14C may be adsorbed on the surface of the crystallites in the pore structure and not integrated into the crystal lattice. This has recently been demonstrated in principle by the HTR-N/N1 project. As an accompaniment to thermal treatments, steam reforming is an alternative method for decontaminating graphite from radionuclides. The decontamination rates are even higher in comparison to pure thermal treatment in an inert atmosphere, as was first evidenced by basic experiments in the HTR-N/N1 project.