The few radionuclides that leave the CRZ are eventually transported via aquifers into a local aquifer of the surface environment, which is part of the biosphere. Note that, in the quantitative assessments in Chapter 8, this transport, which is expected to further delay, disperse and dilute the radionuclides, is not taken into account.

Consistent with ENSI Guideline G03 (ENSI 2023), radiological consequences of radionuclide releases to the biosphere are evaluated by assessing potential doses to humans. The purpose of biosphere modelling described in NAB 24-06 (Nagra 2024n) is to provide the means to carry out this assessment and to demonstrate that the disposal system meets the quantitative protection criteria, thus protecting humans and the environment. Specifically, biosphere modelling is used to provide sets of radionuclide-specific biosphere dose conversion factors (BDCFs), that are applied to calculated release rates to the biosphere in order to obtain effective dose rates that can be compared with regulatory criteria.

The biosphere model (i) represents the behaviour of the released radionuclides in the human habitat, and (ii) determines the potential radiation exposure for a representative individual from the most affected population group based on major exposure pathways.

The modelling aims to be realistic in terms of representing radionuclide migration in the biosphere, but pessimistic in evaluating potential doses. This includes the assumption that the aforementioned population group is making maximum use of local resources through a self-sufficient lifestyle. To help avoid undue pessimism within this assumption, the assumed behaviour of the population group aims to be realistic and credible, based on present-day behaviour relevant to the assumed local and climatic conditions.