Natural Sources

The largest source of ionizing and non-ionizing radiation in the solar system is the Sun. The Sun, in essence, is a giant nuclear fusion reactor that gives off massive amounts of ionizing radiation every second. Ionizing radiation includes alpha particles, beta particles, x-rays and gamma rays. Fortunately the Earth’s atmosphere and magnetic field protects us from most of the ionizing radiation given off by the Sun. The Earth also receives small amounts of ionizing radiation from outer space in the form of cosmic rays from distant stars, supernovas and black holes.

The amount of natural background radiation a person receives depends on where they live. An area with some of the highest levels of natural background radiation in the world is found in the hot spring areas of Ramsar, Iran.  Natural background radiation levels reach as much as 100 times the world average. This is due primarily to high concentrations of radium-226 (a product of uranium-238 decay) and its decay products in the rocks, soil, and water. The local population of Ramsar has not suffered any ill effects despite living in an area with higher than the average natural background radiation for many generations.

Natural Sources of Radiation
Source
Millisieverts per Year
Inhalation (radon)
1.2
Terrestrial
0.48
Cosmic
0.4
Ingestion
0.3

About 70% of the ionizing radiation your body receives comes from a natural source, the Earth; which was formed from material left over shortly after the Sun formed. Some of that material was radioactive. Some of these radioactivie isotopes (e.g. uranium-238, potassium-40, thorium-232) have long-enough half-lives to be found on Earth today, along with their shorter-lived decay products (e.g. radium-226, polonium-210, radon-222).  Other natural radioisotopes, like carbon-14 and tritium (hydrogen-3), are formed by the interaction of cosmic radiation with earth’s atmosphere.  All of these natural radioactive isotopes are found in rocks, soil, water and air. Because all living things consume water, air, and nutrients from the soil or other living things, all are exposed to and contain small amounts radioactive isotopes. This type of radiation is known as natural background radiation.

Radon

A large percentage of the natural background radiation found in our environment is in the form of radon-222, an odourless, colourless gas which is part of the decay chain of uranium-238 found in rock and soil. Because radon is a gas, it can escape into the atmosphere where it decays into a number of radioactive daughter isotopes such as polonium-218 and polonium-214, both of which emit alpha particles.

Alpha particles cannot penetrate the skin but can damage internal tissues if taken inside the body. Thus, if radon gas is inhaled over long periods of time in large enough quantities lung tissue can be damaged and a person’s risk of lung cancer is increased. Because radon seeps up from the ground as uranium-238 decays, the wind usually dissipates or spreads the radon gas making the level of radon outside very low. Where the build-up of radon can be a concern is in the basements of homes and buildings as well as mines. Radon gas can enter buildings through cracks in the floor, drains, around basement windows, cracks in foundation walls and around pipes. If the rate of air exchange in these areas is poor, a build up radon gas can result. Radon may also be found in well water.

Mines usually have large air exchangers to give miners a fresh supply of air and to remove any gas build-up. But what can you do at home to help limit exposure to radon?

  • If a person feels there may be a risk, first have a test done. Test kits are available from the provincial and federal governments for a fee
  • Seal any cracks in the basement walls and floor
  • Seal any openings around pipes and cables entering the basement
  • Improve the ventilation in the basement using fans, air exchangers or periodically opening basement windows
Radon
Radon gas enters buildings through cracks in floors, drains, basement windows, cracks in foundation walls and pipes.

It should be noted that the levels of radon vary from community to community. Most municipalities have information on the radon levels that can be expected in their municipality.

In the late 1970s and early 1980s, a survey conducted by Health Canada measured radon levels in 18 cities across Canada. The mean radon levels (measured in Bq/m3) and the percentage of homes measuring greater than 200 Bq/m3 were recorded. Winnipeg and Regina had the highest mean measurements, while Vancouver had the lowest. Winnipeg also had the highest percentage of homes with levels over 200 Bq/m3. As part of that study, Health Canada found that there was no correlation between radon levels in the home and lung cancer and that less than 0.1% of Canadian homes had high enough radon levels to warrant corrective action.

Fossil Reactor

In 1972, a strange discovery was made by French physicist Francis Perrin while examining samples from a deposit of uranium at Oklo in Gabon, Africa. He discovered that the sample had a lower percentage of uranium-235 than would normally be expected. This was very curious because the radioactive decay rate of uranium-235 is precisely known.

Oklo
1.5 billion years ago natural conditions created a nuclear fission reactor at the uranium deposit in Oklo, Gabon.

More samples and measurements were taken and Francis Perrin came to a startling conclusion. A little over 1.5 billion years ago, conditions existed that created a natural, nuclear fission reactor at the Oklo uranium deposit. The possibility of natural reactors had been predicted by chemist Paul Kazuo Kuroda of the University of Arkansas in 1956 but, until then, no one had ever found one.

The natural nuclear reaction was made possible by oxygen rich water running through cracks in the rocks rich in uranium ore, which dissolved the uranium and then deposited it in concentrations rich enough to start the fission reaction. The water also acted as the moderator needed for the fission to take place. When water was present, the reactor would start and the deposit in the ground would reach hundreds of degrees Celsius. When this happened, the water boiled away, stopping the reaction because the moderating action of the water was gone. This on-again, off-again cycle happened every two and a half hours for over 100,000 years consuming over six tons of uranium-235. Since 1972, several of these natural reactors have been found in the Oklo uranium ore deposit.

Oklo
Geological situation that lead to the creation of a natural nuclear fission reactor in Oklo, Gabon. 1. Nuclear reactor zones; 2. Sandstone; 3. Uranium ore layer; 4. Granite.

Because the fission by-products of these natural reactors are essentially the same as those found in used fuel from nuclear power plants to generate electricity, scientists study these natural reactors to learn how the nuclear fuel used today will impact the environment in the future. What they have found is surprising. After almost two billion years, the radioactive by-products, including plutonium, have moved less than two metres from where they originally formed within the deposit. The Oklo site has provided valuable data and information on how geology can be used to safely store Canada’s used nuclear fuel deep underground.

The ore formation at the Oklo site appears to be somewhat similar to that in the Athabasca basin deposits in Saskatchewan. In that case, the ground water chemistry transported uranium from the bedrock into the sandstone where it was deposited.

Sources: 

Environmental Health Program, Health Canada.
American Nuclear Society, http://www.ans.org/pi/np/oklo/.