In situ tests on backfilling and surrounding host-rock in the case of a radioactive waste disposal in a layered salt formation

In situ experiments carried out by ANDRA in a deep and homogeneous salt layer of a French mine (Amélie mine — Alsace Potash Mines S.A.) tested the behavior of the different components of a multibarrier system, designed for high-level waste disposal and particularly for heat-generating wastes. Canisters were placed in blind boreholes.

The experimental field is located at a depth of 520 m where two major tests have been carried out since 1987. The first test (1987–1989) tested the backfilling by heating different types of crushed salt (max. 130°C) emplaced in large-diameter boreholes. One hole remained empty as a control. The second experiment started in 1989 with the opening of a gallery and the boring of three different boreholes (7 m depth, 250 mm diameter) where the annulus between the cylindrical heating cell and the salt mass differs fundamentally in configuration: as tight as possible, empty, or backfilled with crushed salt. The first configuration was tested at the end of 1991.

In both test fields, a complete series of measurements has been carried out to record temperatures, pressures and deformations on the heating devices, as well as in the surrounding media. Backfill and host rock are monitored with sensors placed in different boreholes. High-resolution tiltmeters and accurate levelling complete the field of investigations. Thermo-mechanical behavior and the properties of the materials have been considered in detail using laboratory test results. Calculations showed how thermo-mechanical interactions behave and generally agreed in a satisfactory manner with the measurements of the test field.

Finally, after cooling and dismantling of the experiment, the microscopic change of the different crushed salts used for the backfilling is analysed.