Technical Report NTB 93-22

Kristallin-ISafety Assessment Report

This report presents a comprehensive description of the post -closure radiological safety assessment of a repository for vitrified high-level radioactive waste (HLW), sited in the crystalline basement of Northern Switzerland. The assessment considers a repository concept similar to that described in Project Gewähr 1985, but also takes account of a recent synthesis of information from geological investigations. The safety assessment, geological synthesis and an exploration study together form the Kristallin-I project; the last two studies are reported elsewhere. The aims of the Kristallin-I safety assessment are: to re-evaluate the crystalline basement of Northern Switzerland as a host rock for a HLW repository; to improve understanding of the performance of the engineered and geological barriers; to identify key geological characteristics and establish desirable ranges for corresponding parameters; to develop and test a more complete methodology and sets of models and computational tools.

The Kristallin-I safety assessment employs a hierarchy of deterministic calculations to investigate uncertainty in the geological environment and in the performance of the repository. In this, three types of uncertainty are distinguished:

  • uncertainty in the selection and combination of relevant features, events and processes (FEPs); this is explored by performing calculations for a Reference Scenario and number of alternative scenarios;
  • uncertainty in the way in which important FEPs are modelled; this is explored by performing calculations for a set of Reference Model Assumptions and a number of alternative model assumptions within the Reference Scenario;
  • uncertainty in the rate and extent of important FEPs; this is explored by means of variations in the values assigned to model input parameters; parameter variations are performed around a Reference Case, which is based on the Reference Scenario and Reference Model Assumptions.

At each stage, conservatisms are introduced. In modelling the Reference Scenario, some FEPs that could be beneficial to safety are not represented. Where alternative models are identified, the model leading to highest consequences is adopted in the Reference Model Assumptions. Data are also selected conservatively in the Reference Case.

The current interpretation of the geological situation indicates that conditions in the crystalline basement can provide a suitable environment for a safe repository for vitrified HLW. The peak annual individual dose calculated for the Reference Case is more than two orders of magnitude below the limit of 0.1 mSv y-1 established in Swiss regulatory guidelines and occurs more than 200’000 years after repository closure. All of the scenario, model and parameter variations calculated also lead to doses well below the regulatory guideline.

With the conservative model of geosphere transport appropriate to the current level of uncertainty in the hydrogeological regime and in the characteristics of water-conducting features in the crystalline basement, the engineered barriers provide the principal constraint on radionuclide release and transport. Indeed, the engineered barriers alone, together with the expected dilution in the near-surface environment, are sufficient to ensure peak doses below the regulatory limit. In this case, the main role of the geological barriers is to provide an environment favouring the longevity and adequate performance of the engineered barriers. Such an environment gives mechanical protection, favourable geochemical conditions and sufficiently low groundwater flows. In future, a more detailed characterisation of the host rock, and of the water-conducting features therein, may allow a less conservative approach to geosphere-transport modelling. It may then be concluded that the host rock forms a very efficient additional safety barrier.