Natural background radiation
Natural background radiation
Naturally-occurring radioactive materials are an inherent part of our living environment. The ground below our feet and the concrete or brick walls around us emit radiation. We are exposed to radiation from space everywhere; the exposure is higher aboard an aeroplane than on the ground. This radiation is called natural background radiation.
Natural background radiation exposes us to a dose of slightly under one millisievert, that is, approximately 25 per cent of our annual radiation dose. Radiation from space accounts for approximately 0.33 millisieverts (mSv), and the ground and building materials account for approximately 0.5 mSv. There is not much we can do about the dose we receive from background radiation.
External radiation from the ground and buildings
We receive external radiation from the gamma radiation emitted by radioactive substances in the ground and building materials. These substances include uranium, thorium and potassium. We spend most of our time at home, work and stores, that is, indoors. The radiation dose we receive indoors is five times higher than the dose received outdoors. Stone-based building materials, such as concrete and stone slabs, have the highest concentrations of radioactive materials. In a flat, we receive radiation from the walls, floors and ceiling; in a wooden detached house only from the floor. Outdoors the radiation comes from soil.
Thus, the dose received from external radiation mainly comes from building materials indoors and amounts to 0.5 mSv/year per Finn on average. The dose varies from 0.17 to 1 mSv/v depending on the municipality. The external radiation level is highest in the rapakivi granite district of South-East Finland.
Cosmic radiation comes from space. The cosmic radiation exposure of Finns is approximately 0.3 mSv per year. Variation in different parts of Finland is nearly nonexistent. If there was a village on top of the Haltiatunturi fell, the highest point in Finland, the inhabitants’ cosmic radiation dose would be 1.5 times the dose of people living at the sea level in Helsinki. Mexico City is located over two kilometres above sea level. The cosmic radiation dose of the inhabitants is approximately 0.8 mSv per year.
The maximum dose received by aircrew from cosmic radiation is approximately 5 mSv per year. There is not much you can do to protect yourself from cosmic radiation.
Cosmic radiation comes from outer space and the Sun. Particles from outer space are mainly protons and alpha particles with a very high energy content. Particles from the Sun have a lower energy level. On the ground and in the lower atmosphere, these particles cannot be detected directly. What is detected is secondary radiation caused by them.
Uranium concentrations in Finnish bedrock and waters
With regard to the presence of uranium, the bedrock in Finland can be divided into two main types: deep-seated rock types (mostly granites) and foliated rock and gneiss types. Uranium concentration is often indicated in part per million (ppm). The average uranium content in Finnish bedrock is estimated at 4 ppm. The Nuclear Energy Decree specifies uranium as a mineral in which the average content of uranium is at least 1,000 ppm.
Uranium concentrations in bedrock vary greatly. The map shows that granitic rock types commonly have higher than average concentrations (4–50 ppm), particularly in the rapakivi areas of South-East Finland. Typical uranium concentrations in foliated rock and gneiss zones are not higher than in granites, even though practically all of Finland’s uranium deposits are located in them. In the deposits surveyed so far, the average uranium concentration has been 300–1,200 ppm and the total amount of uranium approximately 1,000 tonnes.
In the black schist of Talvivaara, the uranium concentration is 15–20 ppm, which is exceptionally high with regard to the concentration variations in the Finnish bedrock. The concentration is much too low for actual uranium mining business, but recovery of uranium is profitable in connection with other mining and enriching operations.