Natural radioactivity in the environment

Radioactivity occurs when the nuclei of unstable atoms (radionuclides) attempt to become stable and, in the process, emit energy in the form of ionising radiation. Each radionuclide has a specific type of radiation emitted by its nucleus, and half-life. Radioactivity can be natural or anthropogenic (artificial).

Earth's radioactivity

The Earth itself is a source of terrestrial radiation. Radioactive materials (uranium's and thorium's series, and by potassium's K-40 isotope) exist naturally in soil and rock. .

The decay series of uranium-238 (U-238), starting with the long-lived U-238 (half-life of 4.5 billion years), has 14 radionuclides – 8 of them are alpha emitters and 6 are beta emitters. The most important radionuclides are Ra-226, Rn-222 and Pb-210. The radon-222 isotope (Rn-222) is present in the air and gives off the biggest radiation dose humans are exposed to (half of the yearly effective dose).

Thorium's series with the long-lived Th-232 (half-life of 14.1 billion years) has 12 radionuclides – 7 of them are alpha emitters and 5 are beta emitters (Bi-212 emits alpha and beta particles). Because of terrestrial external radiation, the biggest share of the radiation dose is left behind by the radionuclides of thorium's decay series.

The potassium radionuclide K-40 (half-life of 1.28 billion years) emits gamma and beta radiation. Because of the natural sources, K-40 represents about 10% of the yearly dose humans are exposed to.

In addition, long-lived radionuclides with half-lives of 109–1011 years found in nature are also Rb-87, La-138, Sm-147 and Lu-176.

Cosmic radiation

Cosmic radiation is an ionizing radiation producet when primary photons and alpha particles from outside the solar system interact with components of the earth´s atmosphere. A second source of cosmic radiation is the release of charged particles from the sun, which become significant during periods of solar flare (sun storm). Galactic cosmic rays consist of protons (90%), Helium nuclei (9%), and electrons (1%). When these rays strike the Earth's atmosphere, they cause secondary nuclear reactions.

In the process, secondary neutrons are formed which cause further nuclear reactions. The ionising component of cosmic rays is by far the most prevalent on the Earth's surface, measuring around 32 nSv/h (at sea level). The neutron component of cosmic radiation represents only about 6–8 nSv/h. In multi-storey buildings, dose rate caused by cosmic radiation is from 2× to 10× lower than outdoors.

Two thirds of the radiation field for secondary radiation decays into muons (≈20 nSv/h), one third into electrons and positrons (≈10 nSv/h), while a small percentage (>2 nSv/h) decays into photons.

With the exposure to cosmic rays, humans receive about 10% of the yearly effective dose of radiation. Under the influence of cosmic rays, radionuclides such as H-3, Be-7, C-14, Na-22 and others are perpetually formed in the upper layers of the atmosphere.

Anthropogenic (artificial) radioactivity in the environment

Artificial radioactivity created in a controlled environment is geographically limited and localised, mostly to closed areas, e.g. reactor buildings, radioactive waste storage areas, laboratories, etc.

Global dispersion of radioactivity

In past cases of radioactivity in uncontrolled environments, such as nuclear airbursts or large nuclear accidents, most of the radionuclides that were formed during the chain reaction were released into the environment and, because of the high temperatures, also into the upper levels of atmospheric layering. Here, strong air currents carried them all over the globe and they contaminated the ground levels of the atmosphere and the Earth's surface. During the first days and weeks, the scale of pollution was large because of the presence of many short-lived radionuclides, whereas, in the following months and years, longer-lived radionuclides, which were formed as nuclear fission products, became more prevalent.

Nuclear airburst tests

In the period 1945–1980, global superpowers such as the US, Soviet Union, Great Britain, France, and China performed 520 nuclear airburst tests (UNSCEAR 1993). Long-lived radionuclides Cs-137 and Sr-90 are still present in the soil today due to this, while C-14 is partially still present in plants, and H-3, to a lesser extent, in precipitation and surface waters.

Nuclear accidents

Until now, five large nuclear accidents with significant consequences for the wider environment and population have occurred. Three of those occurred in nuclear facilities (Three Mile Island, US, 1979; Chernobyl, Soviet Union, 1986; and Fukushima, Japan, 2011), and two in military facilities (Kyshtym, 1957, and Windscale, Great Britain, 1957).

Significant radioactive contamination reached Slovenia only after the nuclear reactor disaster at the Chernobyl nuclear power plant in April 1986. Then, more than one third of the radioactive material was dispersed throughout Europe outside of the former Soviet Union. In Slovenia, the radioactive contamination with I-131, and especially with Cs-137, was several times higher than that caused by the all previous nuclear airbursts together.
Artificial radionuclides (H-3, C-14, Kr-85, I-129) are globally dispersed, albeit in very small amounts, which are being perpetually released from fuel reprocessing plants (in Europe from France and Great Britain).

Locally elevated levels of radioactivity caused by nuclear and radiation facilities

Artificial radioactivity released into the environment by nuclear and radiation facilities is usually found in the local environment and their immediate surroundings. Nuclear and radiation facilities in Slovenia:

• Krško Nuclear Power Station (NEK),
• research reactor in Brinje (TRIGA),
• interim radioactive waste storage facility in Brinje (CSRAO),
• former uranium mine at Žirovski vrh (RUŽV),
• some hospitals also release artificial radionuclides into the environment.

The nuclear power station continuously releases activation and fission products, including noble gases, radioactive aerosols, and iodine, tritium and C-14 isotopes. During its operation, the research reactor releases the Ar-41 noble gas. While it was still operational, the former uranium mine and concentration plant released a lot of natural radioactive nuclide activity from the uranium decay chain, especially Rn-222, U-238, Ra-226 and Pb-210. Radiopharmaceuticals (I-131, Tc-99m) are being released periodically each week by hospitals with departments of nuclear medicine. Radioactive contamination in the environment that is caused by nuclear and radiation facilities is low and, in most cases, hard to measure.