Stages of the Project
Overview of the steps of the preliminary Gorleben safety analysis (vorläufige Sicherheitsanalyse Gorleben -VSG). The preliminary Gorleben safety analysis (VSG) comprises of four consecutive stages.
1. Establish fundamental principles
The first step is to work on the basic principles. This step is then further divided in to the following three sections:
The first part consists of a geo-scientific description of the site and a prognosis of its geological development in the next million years. The basis, therefore, is to establish all the data that from the beginning of exploration through to the interruption through the moratorium in the year 2000. This is not only about the compilation of this data, but also, for example, about the appraisal of their quality.
The second part deals with the characterization of waste, that might be stored at Gorleben. This data is necessary, amongst other things, to establish the size of a possible repository mine.
Finally, in the third section, the so-called Safety and Verification Concept will be developed. It serves the purpose of specifying the BMU's ‘Safety Requirements for the Disposal of Heat-Generating Radioactive Waste’ that are essential for the VSG in connection with the following work stages. One of the basic requirements is described as the permanent safe isolation of the radionuclides in an ‘isolating rock zone’ which describes the area around the repository mine in the salt rock. This area lies in the deep interior of the salt dome.
2. Repository concept
The safety analysis presupposes not only the exact knowledge of the geological conditions and their possible development but also a description of the repository mine as well as of the concept of waste emplacement and of how the mine will be sealed. For example, the kind of storage (e.g. drift emplacement or emplacement in boreholes) and of container (e.g. Pollux) must be described. In the second stage of VSG, concrete repository concepts will be developed that are optimised with regard to operational safety, long-term safety and retrieval of the waste.
3. System Analysis
The key part of the system analysis is the long-term safety analysis. This analysis looks at whether radionuclides are contained in the isolating rock zone through the combined function of geological barriers (i.e. rock salt) and geotechnical barriers (for example: sealing of shafts or drifts).
To be able to analyse a period of over a million years, it is necessary to identify the possible evolution of a repository system. This has to be based e.g. on prognoses of the geological evolution at the site and of any changes in the salt rock that occur during the construction and operation of a repository. From this scenarios are derived that are used for the following analyses. By way of example the repercussions of any future ice ages on the repository will be investigated.
Possible influences on the efficiency of the barriers are determined for the scenarios with the help of special geomechanical and geohydraulic simulation programs. In particular, the analysis looks at whether the protective function of the isolating rock zone remains preserved.
The permanent safe containment of the radionuclides in this rock zone is only then given if regardless of the impacts assumed in the scenarios either no radionuclides reach into the surrounding salt from the edge of this area (this is referred to as complete containment) or the radionuclides released can be classified as minor according to the assessment criteria of the BMU Safety Requirements.
4. Synthesis of the Results
In the last stage of the process, the VSG findings of the previous investigations are summarised and assessed regarding the reliability of the information. Subsequently, the following results are emphasized in line with the objectives of the VSG: On the one hand, a preliminary assessment of the suitability of the site is provided on the basis of the current knowledge; on the other hand, the research needs that might still be there are identified and suggestions for further exploration are made. During the project period, the results of the VSG are regularly reviewed by an external panel of German experts.