Deutsch (10.2 MB) - Nagra

-V- NAGRA NTB 94-09
SUMMARY
As part of its overall responsibility for finding a safe means of disposal for all
categories of radioactive waste, Nagra has investigated a range of potential host rocks
for a repository for low- and intermediate-level waste. Comprehensive investigation
programmes were carried out at four sites - Piz Pian Grand (GR), Oberbauenstock
(NW), Bois de la Glaive (VD) and Wellenberg (NW). The Valanginian Marls at
Wellenberg have a number of properties which make them suitable as a host
formation, namely their sorption properties, their plastic behaviour under mechanical
loading and, in particular, their low water permeability.
On 17th June 1987, Nagra submitted an application (NSG 18) for permission to carry
out an extensive exploration programme at Wellenberg, with a view to assessing the
suitability of the site for construction of a repository for low- and intermediate-level
waste. The decision of the Federal Council of 31 st August 1988 granted permission to
carry out investigations which would not disturb the potential disposal zone. As well as
investigations from the surface, i.e. seismic surveys and deep boreholes, this included
construction of an exploratory drift. For licensing reasons, construction of the drift had
to be put on hold but the surface-based investigations went ahead.
Initial activities in Phase I of the programme included continuation of mapping studies
and hydrogeological surveying. Geophysical investigations (reflection and refraction
seismics and geoelectrics) were then carried out between 18th September and 27th
November 1989. In the Engelberg Valley, between Dorfli and Mattli, six piezometer
boreholes were drilled and equipped between 21 st November 1989 and 12th January
1990.
The first five exploratory boreholes formed the focal point of the Phase I investigations.
The programme began, on 20th November 1989, with construction of the drillsites at
Schwandrain (SB4) and Wilershochi (SB3). Actual drilling started on 15th May 1990 at
SB4 and was completed on 5th March 1993 with the Allmend borehole (SB2). The
latter site is located in the future repository portal zone and was partly recultivated.
Apart from ongoing long-term monitoring programmes, the installation of a packer
system in SB6 in summer 1993 completed the work in this first investigation phase.
Building on the results from Phase I, a second stage in the Wellenberg site
characterisation programme was initiated in summer 1994. Phase II included a further
surface seismics campaign, several piezometer boreholes and shallow cored
boreholes. Borehole SB4a consisted of both a vertical and an inclined (45 degrees)
hole and sealing of the inclined hole brought this stage of investigations to an end in
November 1995.
The first chapter of this report discusses regional geology, the aims of the investigation
programme and the timetable which was followed. Licensing and environmental issues
are discussed in chapter 2, which deals both with general aspects of construction and
operation and with the specific measures taken for noise protection. Chapter 3
describes the drilling techniques used and chapters 4 to 10 each discuss one borehole
in detail. Chapter 11 addresses supervisory bodies and documentation of the
programme.
The total number of metres drilled from the six sites was around 7869, of which 96%
were drilled using the wire line coring technique. Core recovery was 97%. The aim of