An Approach to Modelling the Impact of 14 C Release from Reactor Graphite in a Geological Disposal Facility 2

7 Carbon-14 (C-14) is a key radionuclide in the assessment of a geological 8 disposal facility (GDF) for radioactive waste. In the UK a significant proportion of 9 the national C-14 inventory is associated with reactor core graphite generated by 10 the decommissioning of the UK’s Magnox and AGR reactors. 11 There are a number of uncertainties associated with the fate and transport of 12 C-14 in a post-closure disposal environment that need to be considered when 13 calculating the radiological impacts of C-14 containing wastes. Some of these 14 uncertainties are associated with the distribution of C-14 containing gaseous 15 species such as CH4 and CO2 between the groundwater and gaseous release 16 pathways. As part of the C14-BIG programme, a modelling framework has been 17 developed to investigate these uncertainties. This framework consists of a 18 biogeochemical near-field evolution model, incorporating a graphite carbon-14 19 release model, which interfaces with a geosphere/biosphere model. The model 20 highlights the potential impact of the microbial reduction of CO2 to CH4, 21 through the oxidation of H2, on C-14 transport. The modelling results could be 22 used to inform the possible segregation of reactor graphite from other gas 23 generating wastes. 24