Barriers prevent and retard the release of radionuclides from a repository.
High-level waste is confined by a multibarrier system consisting of different materials such as glass, metals and clays. Investigations of natural deposits and archaeological artefacts of these materials provide important information for evaluating their long-term behaviour.
Natural analogues for glass
Glass has an amorphous structure, which means that it components are arranged irregularly. When it breaks, uneven fragments with shell-like are produced. This is different to a crystal that generally breaks along even surfaces.
In nature, volcanic glasses (obsidian) can be found that consist of amorphous quartz and have been preserved in a chemically unaltered state for millions of years.
At Senzeilles in Belgium, tiny (0.05-1 mm) glass pearls produced by a meteorite impact 367 million years ago were found in a claystone. Because of their size, it could have been expected that they would have dissolved after such a long time. However, because they were enclosed in the clay, the glass pearls had not undergone any alteration.
Because of its favourable properties, glass forms the innermost engineered barrier in a geological repository for high-level waste. Radioactive elements are immobilised in a glass matrix which is very difficult to corrode or dissolve. The radioactive substances are thus safely contained for long time periods.
Natural analogues for metal
Steel is an iron alloy with a low content of carbon, which slows down the rusting process. On contact with oxygen-rich water, iron rusts on the surface, but the resulting rust layer provides protection for the underlying metal and delays the progress of the rusting process.
Archaeological findings of metal artefacts help scientists to estimate the lifetime of metal containers for high-level waste. Production of steel has been known for the last 2700 years, while iron has been in use for as long as 3500 years. Many iron artefacts are known particularly from Roman times.
Example of roman helmet
Rust has eaten away at this Roman helmet while it lay buried in the ground. However, after almost 2000 years, the rust has not completely eaten through the originally two to three millimetre thick helmet.
For comparison: Highly radioactive substances will first come into contact with water in the rock when the thick-walled (minimum of 15 cm) steel disposal container has corroded through. This is expected to take around 10,000 years. Only then can the radioactive substances be dissolved by water, by which time most of the radioactivity will have decayed away. The bentonite backfill in the tunnels and the surrounding host rock provide further containment.
Natural analogues for clay
Clay is capable of swelling and remaining watertight.
As a backfill material in the waste emplacement tunnels, the role of clay material is to keep infiltrating water away from the disposal containers and retard the transport of any released radionuclides. Bentonite clay fulfils both of these requirements. It can take up large amounts of water, which causes it to swell. It also has the ability to bind radionuclides on the long term and to retard their migration. For these reasons, the voids surrounding the waste containers are filled with swelling bentonite.
|Cracks in clayey soil. When clay dries out, it shrinks and forms cracks. When it rains, the water is absorbed, the clay swells and the cracks close. Image: Nagra|