The Radioactive Waste Management Directorate (RWMD) of the Nuclear Decommissioning Authority (NDA) is actively involved in the project, and commissioned Quintessa to undertake a modelling exercise to determine acceptable radionuclide release rates to the biosphere from disposed irradiated graphite. The intention of the study was not to assess particular features of an irradiated graphite waste disposal system, only to assess their required containment performance. As such, the cautious, scoping nature of the study is emphasised.
The study made generic and simplified assumptions for assessments of performance relating to treatment of waste graphite, engineered barrier systems (EBS), the geosphere and the biosphere. These assumptions were generally pessimistic, with factors which would improve the EBS and geosphere performance (e.g. sorption of radionuclides, path length to the biosphere) being considered in variant calculations. Consideration was given to groundwater and gaseous releases of radionuclides, with a focus upon C-14 and Cl-36, and also to human intrusion (which considered the probability of intrusion at different times after disposal). A range of biosphere dose assessment models for C-14 and Cl-36 was considered, including those currently used by NDA RWMD. Alternative dose assessment approaches, such as are currently under comparison within the international cooperation programme BIOPROTA, were also considered.