The composition of radioactive waste is known: it consists of different types of radioactive isotopes. Based on this, it is possible to calculate the decay rate of the radioactivity for each waste type. The radioactive decay of the waste reduces its toxicity. After a certain time, the substances will have decayed to their natural levels although the decay rate varies.
How long will the waste have to be contained?
After approximately 30,000 years, low-and intermediate-level waste reaches the radiotoxicity of natural granitic rock. The radioactivity of spent uranium fuel reaches natural levels after around 200,000 years.
The environment must be protected from radioactive waste during this period. Disposal in geologically stable rock formations ensures the safe containment of the waste for a sufficiently long time period – this fact is recognised worldwide. For this reason, the waste has to be disposed of in a so-called deep geological repository.
How to avoid excessive radiation exposure
Radioactive substances emit ionising radiation that can break up chemical compounds. The effect of radioactive substances on human health is greater when they are ingested than when they impact the body externally. Ingested radioactive substances decay inside the body and can alter the surrounding cell tissue. Excessive radiation exposure (i.e. a high radiation dose) can therefore directly damage the organs and lead to long-term effects such as cancer.
Safe containment using barriers
In a repository, the waste is enclosed in a
- disposal containers holding the packaged waste
- (bentonite) backfill of the emplacement drifts
- the surrounding rock
These barriers prevent unacceptable amounts of radioactive substances from being transported by water to the earth’s surface. A repository is therefore ideally constructed in low-permeability rock. In Switzerland, the so-called Opalinus Clay serves as the host rock. The Opalinus Clay is largely impermeable to water, can self-seal fissures and has the capacity to bind radioactive substances. For these reasons, this clay rock is the most important geological barrier of the deep geological repository.
Strict protection objectives in Switzerland
A deep geological repository has to ensure the permanent protection of humans and the environment, i.e. post-closure safety. To this end, the authorities have specified protection objectives and protection criteria. At no time may the release of radionuclides from a sealed repository result in an excessive radiation exposure to humans. Safety requirements specify a protection criterion of 0.1 millisieverts annually. This is equivalent to less than one fiftieth of the total annual radiation exposure in Switzerland.
Using safety analyses, Nagra’s scientists have been able to transparently demonstrate that the multi-barrier system can meet these protection objectives. The safety barriers effectively retain almost all radionuclides over a long time period. Even when based on very pessimistic assumptions, all calculated dose values were well below the protection objective of 0.1 millisieverts per year.
How rapidly does the waste from nuclear power plants decay?
After a period of 200,000 years, the radioactivity of spent fuel assemblies from nuclear power plants will have decayed to the natural radiotoxicity level, i.e. the original level of the uranium ore when it was mined.
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