Microbial Treatment of Irradiated Graphite for Separation of Radioisotope ¹⁴C From Bulk Graphite ¹²C

The Pebble Bed Modular Reactor is being developed in South Africa. Important for PBMR implementation is a viable strategy for waste management. Irradiated graphite from fuel and structural components is too voluminous for practical treatment with traditional higher level waste methods and too radioactive to recycle. To clean the graphite of radionuclides, a two-step process is being pursued: (1) non-carbon radionuclides (activation products, fission products and actinides) are removed on an elemental basis by a chemical or microbial process. (2) ¹⁴C requires separation at an isotopic level, which would be impractical with established methods (gaseous diffusion or centrifuge). PBMR is investigating a method of isotope separation using biofractionation. Preliminary experiments indicate that microorganisms do separate radioactive ¹⁴C from stable ¹²C. An aqueous slurry of ¹⁴C-spiked, powdered graphite was “fed” to the microbes for 15–18 hours. The microbes initially contained only background levels of ¹⁴C, i.e. orders of magnitude less than the slurry. In post-experiment analyses, a sample of the microbes was found to contain approximately twice the amount of ¹⁴C present in the bulk slurry material. Experiments are underway to further quantify and verify these results, which indicate distinct microbial processing mechanisms for ¹⁴C and ¹²C. The most current results will be presented.