2013:20 Terrestrial Biosphere Modelling of C-14 Research

The need to address radiological impacts from 14C released to the biosphere has been recognized for some time. However, because of its role in biological processes and its ecological cycling, the standard methods employed to model long-term radionuclide transport and accumulation in the biosphere cannot be used satisfactorily for 14C. In 2011, the Swedish Radiation Safety Authority (SSM) commissioned a study to develop a 14C model of the soil-plant-atmosphere system that would provide an independently modelling capability in order to support the oncoming review of dose assessments associated with a license application for extension of a low- and intermediate-level radioactive waste repository and releases of radionuclides during normal operation of Nuclear Power Plants (NPPs). This study is comprised of a review of contemporary models, the development of a new conceptual model, SSPAM14C, and the application of SSPAM14C to a set of experimental data, relating to the atmospheric exposure of cabbages.

The purpose of this study is to evaluate, both qualitatively and quantitatively, the modelling of 14C in the biosphere.  In particular, this study is focused upon the soil-plant-atmosphere system. Consideration is given to the following aspects of 14C in the biosphere:

  • Releases from the geosphere into soils (cf. the scenarios for waste disposal), and also releases from aboveground sources (relevant for routine release from NPPs). 
  • An understanding and description of 14C uptake processes in the soil – roots – canopy atmosphere and external atmosphere system.
  • Spatio-temporal scales, including inter-annual processes and the necessary degree of complexity for long-term assessments.
  • Development of a conceptual/mathematical model incorporating the key features derived from the review of a comprehensive model for 14C transport and uptake in the soil – roots – canopy atmosphere and external atmosphere system including validation against experimental data.

In this study a review of existing 14C soil-plant-atmosphere models has been used to formulate a new model, SSPAM14C. This model contains relatively detailed sub-models for the soil, plant and atmosphere systems making it, in principle, suited to modelling both long term and acute releases to the soil-atmosphere environment.  The application of this model to a set of experimental data relating to a short term atmospheric release of 14C-bearing gas has highlighted a number of areas requiring further development, as discussed above, both in terms of the model parameterisation, and also missing experimental data to further the empirical understanding of such systems and for model validation. Equivalent data are not yet available for assessing the performance of the model for the soil sub-system.