Biosphere modelling for C-14: Description of the Nagra model

pdf NAB 23-25 Biosphere modelling for C-14: Description of the Nagra model(1.40 MB)

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.

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 data for the model are described and justified, along with details of numerical implementation and illustrative calculations. The model draws on discussions and comparisons relating to C-14 modelling made within the international BIOPROTA forum and modelling and associated research undertaken by other radioactive waste management organisations.

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 demon-strates the dominance of exchanges between soil solution, soil gas and the exchanges of stable carbon between the soil, plant canopy and above-canopy atmosphere. Organic matter represents the major carbon pool within the topsoil 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. The degree of atmospheric mixing within the plant canopy atmosphere means that the small amount of plant carbon derived from root uptake of dissolved inorganic carbon dominates as a pathway for C-14 uptake by plants for a source term to the soil.

Nagra’s biosphere model for 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, the variably saturated zone and the local aquifer;
• the loss term from the topsoil to reflect degassing losses to the atmosphere and losses through harvested crops; and
• effective soil-to-plant concentration ratios for the different crops considered.