Analysis of 14C-bearing compounds released by the corrosion of irradiated steel using accelerator mass spectrometry

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 ¹⁴C after their disposal in a DGR. The chemical form of ¹⁴C 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 (TI¹⁴C²) and organic ¹⁴C contents (TO¹⁴C) in the liquid and gas phases (TG¹⁴C), as well as individual ¹⁴C-bearing carbon compounds by compound-specific radiocarbon analysis (CSRA), were quantified using accelerator mass spectrometry (AMS). The AMS-based quantification allows the determination of ¹⁴C in the pico- to femtomolar concentration range. An initial increase in TO¹⁴C was observed, which could be attributed partially to the release of ¹⁴C-bearing oxygenated carbon compounds. In the long term, TO¹⁴C and the TI¹⁴C remain constant, while TG¹⁴C increases over time according to a corrosion rate of steel of 1 nm/yr. In solution, ¹⁴C-bearing carboxylic acids (CAs) contribute ~40% to TO¹⁴C, and they are the main ¹⁴C carriers along with ¹⁴C-bearing carbonate (¹⁴CO₃^2-). The remaining fraction of TO¹⁴C (~ 60%) is likely due to the presence of as yet non-identified polymeric or colloidal organic material. In the gas phase, ¹⁴CH₄ accounts for more than 80% of the TG¹⁴C, while only trace amounts of ¹⁴CO, and other small ¹⁴C-bearing hydrocarbons have been detected. In a DGR, the release of ¹⁴C 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.

Determination of ultra-low concentrations of gaseous 14C-bearing hydrocarbons produced during corrosion of irradiated steel using accelerator mass spectrometry

Compound-specific radiocarbon analysis (CSRA) was developed to identify and quantify gaseous 14C-bearing carbon compounds at the pico- to femtomolar concentration range and employed in a corrosion experiment with small specimens of irradiated steel. The approach is based on gas chromatographic separation of single 14C-bearing carbon compounds, their oxidation to 14CO2, sampling with a custom-made fraction collector and quantification by accelerator mass spectrometry (AMS). In addition to CSRA, a method allowing the quantification of the total 14C content of the gas phase was developed and tested. After validation of the two set-ups with standards, the gaseous 14C-bearing carbon compounds produced during alkaline anoxic corrosion of irradiated steel were quantified. Small hydrocarbons (HCs) like methane (14CH4) and ethane (14C2H6) were the only 14C-bearing compounds identified in the gas phase above the detection limit. 14CH4 was the main species (on average 5.4 × 10-14 mol L-1 gas) and contributed >90% to the total 14C content, whereas the concentration of 14C2H6 was much lower (7.9 × 10-16 mol L-1 gas). To our knowledge, this is the first study reporting CSRA of gaseous 14C-bearing HCs produced during anoxic corrosion of irradiated metallic radioactive waste at ultra-low concentrations.