The geological barrier comprises the host rock and all overlying rock formations up to the earth's surface.
The host rock is the body of rock immediately surrounding the repository. In Europe, crystalline rock, clays and salt are being considered as host rocks for a high-level waste repository.
Crystalline rocks such as granite are formed from magma deep in the earth's crust.
During cooling, shrinkage cracks and voids are formed in which, under favourable conditions, beautiful crystals may form. Fault zones are generated when the rock body can no longer withstand the stresses to which it is subjected.
Landscape at the Grimsel pass with crystalline rocks
|The rock can then break suddenly or begins to deform at points of weakness. Water can often flow relatively easily and quickly along such faults and fractures. Between these unevenly distributed fault zones are large areas of rock that are only slightly disturbed and, because of their high stability, these come into consideration for hosting a deep repository. Crystalline rocks that would potentially be suitable in Northern Switzerland are partly located under thick sediment layers, making them difficult to explore.|
A crystal cavern at the Grimsel test site. Image: Comet
Clay formations are characterised by an excellent isolation and sealing capacity, as well as the ability to immobilise water and dissolved substances over geological timescales.
Clays are soft and plastic at the earth's surface but, at greater depths, they form hard rock.
Clays have a «braking effect» on migrating toxic substances, immobilising them and retarding their transport.
At depth, the Opalinus Clay, which is around 175 million years old, still contains ten to twenty grams of dissolved salts per litre of porewater from the original seawater. Because this seawater has been contained in the rock for many millions of years, scientists assume that the properties of the rock will remain virtually unchanged during the next several 100,000 years. It is therefore suitable as a host rock for a geological repository.
Example ammonite, «Leioceras opalinum»
|This extremely well preserved ammonite of the type «Leioceras opalinum» was found in the Opalinus Clay in the Benken borehole. It was protected and preserved by the clay from outside influences. The name of the fossil is derived from the opalescent sheen of the shell that has remained intact for around 180 million years. Image: Comet|
Example salt deposit in Northern Switzerland
|In Rheinfelden (east of Basel), a salt layer around 50 metres thick can be found at a depth of a hundred metres. It lies beneath water-bearing strata. Although rock salt is highly soluble in water, this salt layer, which is around 240 million years old, has still not been dissolved away by water - even though it is located in an area that experiences earthquakes. Responsible for its preservation are the fifty-metre thick clayey-sulphate layers in which it is embedded. These prevent the infiltration of water. This shows that clay formations have very good isolation properties. Image: Comet|