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

Abstract

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.