FEBEX: Full-scale engineered barriers experiment in crystalline host rock

• Research unit: EU RFP
• Project number: 95.0320

Key words (English)

Grimsel Test Site (GTS) Engineered Barrier System (EBS); In-situ test; Water content measurements; Time Domain Reflectometry (TDR); Bentonite; High Level Radioactive Waste (HLW)

Alternative project number (English)

EU project number: FI4W-CT95-0006

Research programs (English)

EU-programme: 4. Frame Research Programme - 5.2 Nuclear fission safety

Short description (English)

See abstract

Further information (English)

Full name of research-institution/enterprise:
Nationale Genossenschaft für die Lagerung radioaktive Abfälle NAGRA

Partners and International Organizations (English)

Enresa, (Ciemat, Aitemin,G3S, Andra, GRS)

Abstract (English)

The FEBEX-project consists of two larger scale tests and a series of supporting laboratory and modelling studies. The In-situ test is performed at the Grimsel Test Site, Nagra's Underground Research Laboratory, in the crystalline formation of the central Aare Massive. In 1995, Nagra and Enresa decided to implement the FEBEX in-situ test. One of the main objectives of the in-situ test is, beside the demonstration of the HLW-disposal concept in a 1:1 scale, to evaluate, improve and validate numerical models of coupled thermo-hydro-mechanical (THM) Models. In 1996, in a specially constructed 17m long tunnel section, two electrical heaters were emplaced. The backfill material consists of highly compacted bentonite blocks (in total 116 tonnes) instrumented with around 600 sensors. The electrical heaters, simulating the heat production of the HLW packages, were switched on in February 1997 with a target temperature of 100oC. Until now, the entire system is working without any problems, delivering the high quality dataset needed for the evaluation of coupled processes in the EBS system. Probably the most important parameter to be determined for the modelling of the THM-processes is the changing of water content of the bentonite. Thus, beside other activities in the project, Nagra decided to focus on the development and installation of the TDR system (Time Domain Reflectometry), monitoring the water content in the bentonite buffer and the surrounding granitic rock. 20 TDR probes were installed in the bentonite buffer in two sections and 4 probes in the granite. As the primary signal readings are temperature depended 24 temperature sensors have been also installed. The graphs illustrate the water content changes within one bentonite section for the time period from August 1996 to March 1999. The position of the sensors are indicated in the upper right of the diagrams. The corresponding temperatures vary between 45oC in the outer part and 78oC in the inner part and are almost constant since 1997.
The heating phase started on 27th February 1997 and will continue until year 2001. The data illustrate saturation in the outer part of the bentonite buffer shown in the left diagram and a de-saturation of the bentonite in the inner section near the heat source in the right hand diagram. However, significant differences between the horizontally and vertically aligned TDR probes are clearly evident and will be a focus for model testing.

References in databases (English)

Swiss Database: Euro-DB of the
State Secretariat for Education and Research
Hallwylstrasse 4
CH-3003 Berne, Switzerland
Tel. [phone]
Swiss Project-Number: 95.0320