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 after their disposal in a DGR. The chemical form of C 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 (TIC) and organic C contents (TOC) in the liquid and gas phases (TGC), as well as individual C-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 in the pico- to femtomolar concentration range. An initial increase in TOC was observed, which could be attributed partially to the release of C-bearing oxygenated carbon compounds. In the long term, TOC and the TIC remain constant, while TGC increases over time according to a corrosion rate of steel of 1 nm/yr. In solution, C-bearing carboxylic acids (CAs) contribute 40% to TOC, and they are the main C carriers along with C-bearing carbonate (CO). The remaining fraction of TOC ( 60%) is likely due to the presence of as yet non-identified polymeric or colloidal organic material. In the gas phase, CH accounts for more than 80% of the TGC, while only trace amounts of CO, and other small C-bearing hydrocarbons have been detected. In a DGR, the release of C 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.