Technischer Bericht NTB 88-18

Untersuchungen zur Standorteignung im Hinblick auf die Endlagerung schwach- und mittelaktiver Abfälle:Berichterstattung über die Untersuchungen der Phase I am potentiellen Standort Oberbauenstock (Gemeinde Bauen, UR)

On 30th September 1985, Nagra's applications for investigation permits for the potential sites Bois de la Glaive, Oberbauenstock and Piz Pian Grand were partially granted by the Federal Government. The partial approval allowed the performance of scientific investigations to assess potential disposal zones at the three sites, but withheld approval in granting a licence for construction of exploratory drifts. Permission for granting a construction license was made conditional on: the evaluation of the results from the licensed scientific investigations; and preparation of supporting reports, which would be submitted with the application for a license for construction at each of the three sites. This report presents the results of scientific investigations conducted under the restricted permits, and will accompany the application to the Federal Government for permission to construct an exploratory drift at Oberbauenstock. Similar reports and applications will be submitted simultaneously for the sites Bois de la Glaive and Piz Pian Grand.

By way of introduction, the report presents the initial research guidelines created by the granted licences, and the influence this had on the associated investigation objectives and performance. Following this discussion, an overview of available data and a description of investigations conducted for the site are presented.

The second chapter presents results of the geological investigation program. Information concerning the structural geology of the area was obtained by regional studies, local surface mapping and geophysical measurements in boreholes. The results of these new studies were integrated into an updated tectonic model, which provides an improved conception of geological conditions at the site.

Contained also within the chapter is a summary of: the lithologic and stratigraphic data obtained from boreholes; a description of mineralogical investigations and related results; and physical parameter determinations obtained from geophysical surveys. A physical description of the hydrologic flow-systems encountered is then presented on the basis of the geological, lithological, and mineralogical data obtained.

A description of the hydrogeological investigations conducted is presented, which discusses results obtained from spring measurements and hydraulic parameters determined from borehole tests. Of particular interest is the discussion concerning: the abnormally low hydraulic potentials encountered; the origin of the natural gas present in the host rock and its possible effect on abnormal hydraulic potential distribution; and, an analysis of the possible influence of gas on the results of the hydraulic tests. An overview is also provided of the hydrochemical investigations performed on: spring samples, water samples from boreholes, and reference, baseline measurements obtained for precipitation.

The third chapter is dedicated to studies which, based on the results of the previously described investigations, assess the feasibility of a potential repository from a technical and safety point of view. The chapter briefly refers to earlier studies conducted for Project Gewähr (Guarantee) 1985, and presents a description and results for the new three-dimensional modelling which encompasses an area four times larger than previously covered. These results are used as a basis for recommending an improved orientation (from a safety viewpoint) for the tunnel axis. A second study is presented that embodies a provisional engineering concept for a repository at Oberbauenstock, i.e. in terms of the governmental decision of 30th September 1985. This is a precondition for removing the restrictions on construction of an exploratory drift. The study presents: the most important basic data and boundary conditions affecting the repository concept; describes the various components of the disposal facility; and, details its operational procedure. In a third study, the long-term safety of a repository for low- and intermediate-level waste is evaluated based on results form previous site investigations. After listing relevant release scenarios, presenting modified assumptions for the geosphere model and discussing significant aspects of near-field and biosphere modelling, the findings are discussed and pertinent conclusions drawn.

The fourth and final chapter summarises the results of the investigation program and draws relevant conclusions, the most important of which are:

  • Investigations conducted to date have not revealed any reasons for ruling out Oberbauenstock as a potential repository site.
  • Additional investigations which will advance the present state of knowledge are feasible only in conjunction with construction of an exploratory drift.