Carbon-14 release and speciation during corrosion of irradiated steel under radioactive waste disposal conditions

Carbon-14 is a key radionuclide in the safety assessment of deep geological repositories (DGR) for low-and intermediate-level radioactive waste (L/ILW). Irradiated metallic wastes generated during the decommissioning of nuclear power plants are an important source of C-14 after their disposal in a DGR. The chemical form of C-14 released from the irradiated metallic wastes determines the pathway of migration from the DGR into the environment. In a long-term corrosion experiment with irradiated steel simulating the hyper-alkaline, anoxic conditions of a cement-based DGR, total inorganic ((TIC2)-C-14) and organic C-14 contents ((TOC)-C-14) in the liquid and gas phases (TG(14)C), as well as individual C-14-bearing carbon compounds by compound-specific radiocarbon analysis (CSRA), were quantified using accelerator mass spectrometry (AMS). The AMS-based quantification allows the determination of C-14 in the pico-to femtomolar concentration range. An initial increase in (TOC)-C-14 was observed, which could be attributed partially to the release of C-14-bearing oxygenated carbon compounds. In the long term, (TOC)-C-14 and the (TIC)-C-14 remain constant, while TG(14)C increases over time according to a corrosion rate of steel of 1 nm/yr. In solution, C-14-bearing carboxylic acids (CAs) contribute similar to 40% to (TOC)-C-14, and they are the main C-14 carriers along with C-14-bearing carbonate ((CO32-)-C-14). The remaining fraction of (TOC)-C-14 (similar to 60%) is likely due to the presence of as yet non-identified polymeric or colloidal organic material. In the gas phase, (CH4)-C-14 accounts for more than 80% of the TG(14)C, while only trace amounts of (CO)-C-14, and other small C-14-bearing hydrocarbons have been detected. In a DGR, the release of C-14 will be mainly in gaseous form and migrate via the gas pathway from the repository near field to the surrounding host rock and eventually to the environment.