Technischer Bericht NTB 93-28

Untersuchungen zur Standorteignung im Hinblick auf die Endlagerung schwach- und mittelaktiver AbfälleGeologische Grundlagen und Datensatz zur Beurteilung der Langzeitsicherheit des Endlagers für schwach- und mittelaktive Abfälle am Standort Wellenberg (Gemeinde Wolfenschiessen, NW)

This report forms part of the supporting documentation for the low- and intermediate-level waste repository site selection procedure. The aim of the report is to present the site-specific geological data, and the geosphere database derived therefrom, which were used as a basis for evaluating the long-term safety of a repository at Wellenberg. These data also form a key component of other reports appearing simultaneously with the present one, first on the intercomparison of the four potential sites (Bois de la Glaive, Oberbauenstock, Piz Pian Grand and Wellenberg; NTB 93-02) and, second, on the safety assessment of the Wellenberg site itself (NTB 93-26).

The level of detail of the present report is determined by the requirements of the other two reports mentioned, which would include presenting, discussing and justifying the geosphere dataset used in the performance assessment model calculations. The dataset was defined in sufficient depth at the beginning of 1993, prior to the still ongoing, detailed evaluation of the extensive experimental data acquired during the investigations. It became apparent that, apart from deriving the geosphere dataset and establishing its geological basis, the report should include a very detailed picture of the geology and hydrogeology at the Wellenberg site. The additional report, which will be prepared to accompany the application for a general repository licence in mid-1994, will build on the present report by including the results of the in-depth data interpretation and a geological synthesis based on the results of the exploration campaign which was completed at Wellenberg in spring 1993.

The introductory chapter discusses procedures and goals. The second chapter provides an overview of the geographical and geological situation at Wellenberg, followed by a discussion of previous geological investigations carried out in the siting area and the region. The chapter closes with a discussion of the pre-existing information on the site which formed the basis for planning the field investigation programme.

Chapter 3 then discusses the planning and progress of the field programme, and the current status of investigations is presented. Beside investigations which do not require a licence, such as mapping and spring surveys, the main components of the programme were seismic surveys and the five exploratory boreholes which were drilled to depths between 430 m and 1870 m and in which detailed geological, geophysical and hydrogeological investigations were performed. These investigations were carried out between autumn 1989 and spring 1993.

The fourth chapter presents the geological situation at the Wellenberg site and describes the concepts and models formulated on the basis of this information. These models were used to derive a dataset for performance assessment relating to site-selection and application for a general licence. The host rock and other geological formations and units which were either drilled through or mapped are described, together with the structural geology of the region. Water flow paths and water-bearing systems in the host rock are then characterised and conceptualised for the purpose of hydrodynamic and transport modelling. The hydrochemistry and isotope hydrology of both near-surface and deep groundwaters are described. All aspects of the hydrogeology of the siting area are then presented and models and conclusions are derived from these data. Chapter 4 closes with a section on neotectonics and an overview of a study on long-term scenarios.

Chapter 5 then derives the performance assessment and engineering datasets, based on the investigations, concepts and modelling exercises described in chapter 4. For performance assessment purposes, the dataset includes parameter values and data on large-scale groundwater flow systems, groundwater flow and flow path characteristics in the vicinity of the repository, characterisation and conceptualisation of water flow paths and reference waters and the mineralogy of water-bearing systems. The engineering dataset contains data on the host rock boundaries and the resulting project variants, as well as a description of the geotechnical characteristics of the formations which will be important in terms of underground construction.

In summary, it can be said that, to date, the investigation results from Wellenberg have confirmed predictions in all relevant respects and, in some cases, have even exceeded expectations (e.g. in relation to the available volume of host rock). The investigations represent an important step forward in the geological characterisation of the Wellenberg repository site and the results will form the basis of the general licence application which will be submitted in mid-1994. The application to the Federal Government for a permit to construct a nuclear facility and, later, for an operating permit for emplacement of the waste will of course require further investigations to be carried out. These will begin by continuing with the federal permit which already exists (for an exploratory drift outwith the disposal zone and related measurements). Further investigations will also take place when accessing the actual disposal zone during the construction phase.