Inconspicuous and smaller than a walnut: this little rock comes from a place in the Black Forest where uranium ore was mined in granite rock until the early 1990s. If you hold a Geiger counter to the rock, it will start to make clicking sounds: it will have measured a dose around 100 times above that of natural, or “normal”, radioactivity.
What makes this feel somewhat sinister is that our senses – eyes, ears, nose and mouth – cannot detect ionising radiation. It is produced, for example, when radioactive atoms decay. No wonder then that radioactivity can trigger fear. At the same time, it has been so well researched worldwide that it is safe when proper precautions are taken. In Switzerland, the dose limits prescribed in the Radiation Protection Act are based on a cautious general assumption.
This assumption is based on the so-called linear no-threshold model according to which the risk of cancer increases with the radiation dose. According to this model, even the smallest dose can potentially cause cancer. To date, however, international expert committees have not recognised any studies scientifically proving a relationship between low radiation doses and health.
Nonetheless, radiation protection in Switzerland is based on this conservative and therefore cautious assumption. Any remaining uncertainties are covered in this way – the dose limits are accordingly strict.
Small but loaded with energy
How can radioactive substances harm you? The atomic nuclei of certain chemical elements are unstable and decay, during which some of the particles emit ionising radiation. These particles are tiny but travel at very high speeds. As a result, they contain a lot of energy, allowing them to penetrate body cells, where they can cause damage.
Sievert is the unit used to measure the exposure to ionising radiation that can impact health. In this context, it is irrelevant whether the radiation source is natural or artificial. On average, every person living in Switzerland is exposed to a dose of around 6 millisieverts (mSv) per year, i.e. 6,000 microsieverts.
Radiation from the ground beneath our feet, from outer space and from our own bodies contributes around 0.35 mSv, respectively. Muscles, for example, contain the element potassium, of which a very small proportion is naturally radioactive. Medical treatments such as X-rays add an average of 1.5 mSv.
However, by far the greatest proportion comes from radon: around 3.3 mSv per year. This radioactive gas occurs naturally and is produced when uranium, of which small quantities are present in the underground rock, decays. Radon rises to the earth’s surface, where it does not pose a threat to health as it dilutes in the open air.
However, if radon enters enclosed spaces via leaking underground foundations, it can accumulate in the air we breathe. This means that when radon decays, the resulting ionising radiation can reach the lung tissue. As a rule, the more radon is in the air and the longer it is inhaled, the greater the calculated cancer risk.
Even the Opalinus Clay radiates
In Switzerland, radioactive waste must be disposed of in a deep geological repository. The repository protects humans, the environment and, above all, the groundwater from radioactive substances in the long term.
The centrepiece of the deep geological repository is the Opalinus Clay. The tight rock safely encloses the radioactive waste. Among other components, the clay rock consists of weathered granite, which contains traces of natural uranium. For this reason, the Opalinus Clay itself is slightly radioactive.
In line with legal stipulations, the maximum additional radiation exposure from a Swiss deep geological repository cannot exceed 0.1 mSv. However, Nagra’s safety analyses show the additional radiation dose will be far below this legal limit – despite basing the analyses on conservative, i.e. particularly cautious, assumptions.
In comparison, natural radioactivity is around 0.1 microsieverts per hour in the Swiss Plateau. Humans are constantly exposed to this. After 10 hours, they will have received a dose of 1 microsievert. About half of this originates in space and is called cosmic radiation. The other half, terrestrial radiation, comes from underground rock – for example, granite that contains uranium.
Depending on where you live, the level of cosmic or terrestrial radiation can vary. People living in Davos, for example, are exposed to higher levels of cosmic radiation than those living on the Swiss Plateau. This is because the protective layer of the atmosphere is thinner at high altitudes.
Locarno in Ticino, for example, is built on uranium-bearing gneiss. As a result, the radiation there is around 50 per cent higher than in Zürich. Extrapolated to one year, this results in an increase in radiation exposure of around 0.5 millisievert, which is five times higher than the legal requirement for the deep geological repository.
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