Arbeitsbericht NAB 12-26

Nagra uses a compartment model for representing the distribution of radionuclides in the biosphere following release from a deep geological repository and for calculating potential effective radiation doses to persons. The compartment model is implemented as the Swiss Biosphere Assessment Code (SwiBAC) and adopts equilibrium assumptions for radionuclide distribution between solid phases, liquid phases and uptake by crops. Such assumptions are suitable for biosphere modelling of most radionuclides relevant to geological disposal. However, for C-14 a more specific modelling approach is appropriate.

This report documents the model that is used by Nagra to represent the distribution of C-14 in the biosphere. The conceptual and mathematical model for evaluating the transfer and accumulation of C-14 released to a local aquifer-soil-crop-atmosphere system is presented. The model draws on discussions and comparisons relating to C-14 modelling made within the international BIOPROTA forum. The data for the model are described and justified, along with details of numerical implementation and illustrative calculations.

The model assumes that C-14 is distributed in the same way as stable carbon and is hence based on masses and fluxes of stable carbon between various carbon pools. The mass balance demonstrates the dominance of the flux of stable carbon through the upper part of the plant canopy and the atmospheric layer above the plants. Organic matter represents the major carbon pool within the top soil in comparison with the amount in exchangeable inorganic form.

Illustrative calculations demonstrate a timescale of several hundred years to equilibrium, reflecting the assumed turnover rate of carbon within the local aquifer. Uptake of C-14 by plants from a layer within the canopy atmosphere that is conservatively represented as being diffusive-dominated is key in comparison to either root uptake from the top soil or uptake from a turbulent layer of the canopy atmosphere.

The C-14 model is used to calculate the following effective parameter values needed to represent the behaviour of C-14 within SwiBAC:

• effective solid / liquid distribution coefficients for the soil compartments and the local aquifer

• additional loss term from the top soil to reflect losses to the atmosphere and crops

• effective soil-to-plant concentration ratios for the different crops considered