Radioactive waste contains radioactive forms of a range of different elements.

Its composition is known, which allows the decrease in radioactivity with time to be calculated for different waste types. It is true for all waste types that their toxicity is decreased by radioactive decay until it reaches natural levels. However, the time required for this varies widely for the different waste types.

After around 30,000 years, low- and intermediate-level waste has a radiotoxicity equivalent to that of granite. After around 200,000 years, the radioactivity of spent uranium fuel reaches that of the naturally occurring uranium that was originally mined to produce it.

Radioactive waste has to be isolated from our living environment during these long time periods. It is recognised worldwide that disposal in geologically stable rock formations can ensure safety over the long time spans involved. Radiation from the waste is shielded by the waste containers, the tunnel backfill, the repository installations and the surrounding rock. The safety barriers also prevent radioactive substances from being dissolved in water and transported to the earth's surface, where they could enter the food-chains. A geological repository is therefore constructed in low permeability rock.

A geological repository has to ensure the long-term protection of man and the environment. The safety authorities have specified objectives that quantify the required level of protection. At no time shall the release of radionuclides from a sealed repository give rise to individual doses that exceed 0.1 millisieverts per year. Using safety analyses, Nagra has been able to show that these protection objectives can be met thanks to the isolation function of the planned multiple safety barrier system in the repository. All calculated dose values lie well below the protection objective of 0.1 millisieverts per year.

Radioactive decay

Image: Nagra

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