Carbon-14 Sources, Speciation and Release from Radioactive Waste in the context of a Deep Geological Repository

pdf NTB 22-04 Carbon-14 Sources, Speciation and Release from Radioactive Waste in the context of a Deep Geological Repository(6.62 MB)

Carbon-14 (¹⁴C) is a key radionuclide for safety assessment that is released from high- and low- and intermediate-level waste due to the degradation of ¹⁴C-containing waste materials in deep geological repositories over time. ¹⁴C is contained in waste generated during the operation and decommissioning of nuclear power plants, from reprocessing and in waste from medicine, industry and research. The ¹⁴C released during the degradation of the various waste types is present in either inorganic or organic chemical form, in solution or gas phase. In the dissolved inorganic form, i.e., as H¹⁴CO₃- or ¹⁴CO₃^2-, 14C is strongly retained by the backfill materials (i.e., bentonite, cementitious materials) of the near-field and the host rock (Opalinus Clay) by isotopic exchange with carbonate of the calcite fractions and, therefore, decays within the repository. In the dissolved organic form, ¹⁴C is mainly present as ¹⁴C-bearing small molecules, such as ¹⁴Ccontaining carboxylates, which are only weakly retained by the backfill materials of the near-field and the host rock and can, therefore, contribute to the dose release via the groundwater pathway. In the gaseous form, ¹⁴C occurs mainly as ¹⁴C-bearing small hydrocarbons, in particular ¹⁴Cbearing methane, which migrate via the gas pathway. This report summarises the current state of knowledge on: (i) the sources of ¹⁴C in the various waste types relevant to the Swiss waste inventory; (ii) the chemical forms of 14C release through the degradation of ¹⁴C-containing waste materials, and (iii) the retention of the ¹⁴C-bearing species in the near -and far-fields of the deep geological repository. This report provides an overall appraisal of the behaviour of ¹⁴C in the context of a Swiss deep geological repository for both high- and low- and intermediate-level waste. It is the scientific basis for defining post-closure safety assessments in the framework of the general licence application.