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 radioactivewaste (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 formof 14C releasedfrom
the irradiatedmetallicwastes determines the pathway ofmigration fromthe 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 themain 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 organicmaterial.
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 bemainly in gaseous form andmigrate
via the gas pathway from the repository near field to the surrounding host rock and eventually to the environment.