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.
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. (Image: Nagra)
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.