Carbon-14 is a key radionuclide in the assessment of the safety of a geological disposal facility for radioactive waste because of the calculated assessment of the radiological consequences of gaseous carbon-14 bearing species [i]. It may be that such calculations are based on overly conservative assumptions and that better understanding could lead to considerably reduced assessment of the radiological consequences from these wastes. Alternatively, it may be possible to mitigate the impact of these wastes through alternative treatment, packaging or design options. The Radioactive Waste Management Directorate of the UK’s Nuclear Decommissioning Authority (NDA RWMD) has established an integrated project team in which the partners are working together to develop a holistic approach to carbon-14 management in the disposal system [ii].
For a waste stream containing carbon-14 to be an issue:
• There must be a significant inventory of carbon-14 in the waste stream; AND
• That waste stream has to generate carbon-14 bearing gas; AND
• A bulk gas phase has to entrain the carbon-14 bearing gas: AND
• These gases must migrate through the engineered barriers in significant quantities; AND
• These gases must migrate through the overlying geological environment (either as a distinct gas phase or as dissolved gas); AND
• These gases must interact with materials in the biosphere (i.e. plants) in a manner that leads to significant doses and risks to exposed groups or potentially exposed groups.
The project team has developed and used this “AND” approach to structure and prioritise the technical work and break the problem down in a manageable way. We have also used it to develop our approach to considering alternative treatment, packaging and design options. For example, it may be possible to pre-treat some wastes to remove some of the inventory or to segregate other wastes so that they are removed from any bulk gas phase which might facilitate migration through the geosphere.
Initially, the project team has undertaken a six month programme of work to examine the current understanding of these aspects and has captured this in the Phase 1 report [ii], in a modelling basis spreadsheet and in scoping assessments, which help us better understand the potential significance of carbon-14. Using the current modelling basis, but ignoring any potential benefits from the geosphere in retarding or preventing gas from reaching the surface, the calculated release of carbon-14 is dominated by: corrosion of irradiated reactive metals (in the operational and early post-closure time frame); corrosion of irradiated stainless steel and leaching of irradiated graphite (in the longer term). The Phase 1 work has shown that there is considerable scope for reducing the calculated radiological consequence for these wastes and a roadmap has been developed for a second Phase of work.