Carbon-14 has been identified as one of the more significant radionuclides in solid radioactive wastes in a repository, due to the potential radiological impact arising if 14C were to be released and enter the biosphere. However, the assessment of radiation doses is complicated by the major role of carbon in biological processes, and this has tended to lead to the adoption of a cautious assessment approach.
An international comparison of five models used to predict uptake of 14C to agricultural crops has been undertaken, within the BIOPROTA framework. Processes investigated include conversion of 14C-labelled CH4 into CO2 in soils, carbon accumulation in and release from soil carbon pools, gaseous emanation to, and dispersion from, the plant canopy atmosphere and, incorporation into plants by photosynthesis.
For a unit rate of entry of 14C to soil, modelled activity concentrations in cereal crops differ by three to five orders of magnitude. This reflects, in part, differing assumptions for mixing and dispersion of air above the soil surface and within the crop canopy layer. For a unit activity concentration of 14C in air, the modelled uptake to cereal crops converges significantly. Following an assumed irrigation of crops with groundwater containing unit activity of 14C, the predicted uptake to crops varied by two to four orders of magnitude, again largely dominated by assumptions regarding the canopy atmosphere. In all cases, there is some convergence in model predictions as field size increases.
A continuing programme of field research is being undertaken in parallel with the assessment work.