Technical Report NTB 14-09

Provisional Safety Analyses for SGT Stage 2Models, Codes and General Modelling Approach

In the framework of the provisional safety analyses for Stage 2 of the Sectoral Plan for Deep Geological Repositories (SGT), deterministic modelling of radionuclide release from the barrier system along the groundwater pathway during the post-closure period of a deep geological repository is carried out. The calculated radionuclide release rates are interpreted as annual effective dose for an individual and assessed against the regulatory protection criterion 1 of 0.1 mSv per year. These steps are referred to as dose calculations. Furthermore, from the results of the dose calculations so-called characteristic dose intervals are determined, which provide input to the safety-related comparison of the geological siting regions in SGT Stage 2. Finally, the results of the dose calculations are also used to illustrate and to evaluate the post-closure performance of the barrier systems under consideration.

The principal objective of this report is to describe comprehensively the technical aspects of the dose calculations. These aspects comprise:

  • the generic conceptual models of radionuclide release from the solid waste forms, of radionuclide transport through the system of engineered and geological barriers, of radionuclide transfer in the biosphere, as well as of the potential radiation exposure of the population,
  • the mathematical models for the explicitly considered release and transport processes, as well as for the radiation exposure pathways that are included,
  • the implementation of the mathematical models in numerical codes, including an overview of these codes and the most relevant verification steps,
  • the general modelling approach when using the codes, in particular the generic assumptions needed to model the near field and the geosphere, along with some numerical details,
  • a description of the work flow related to the execution of the calculations and of the software tools that are used to facilitate the modelling process, and
  • an overview of the quality assurance measures carried out.

Site-specific information used in the dose calculations is not documented in this report. Rather, it is compiled in Nagra (2014b) with links to other reports. The report Nagra (2014b) further describes the set of processes and parameters that are relevant to the provisional safety analyses, the definition of the calculation cases for the dose calculations and a discussion of the results. In this way, the report Nagra (2014b) and the present report together provide the required transparency and traceability with respect to the dose calculations for SGT Stage 2.

Other models and codes (e.g. groundwater models, mechanistic sorption models) that are used for the derivation of input parameters and for the justification of assumptions and simplifications used in this report are not described, but are dealt with in specific reference reports. The present report only occasionally refers to other reference reports; a more comprehensive presentation of the scientific basis for the dose calculations is given in Nagra (2014b).