Frequently Asked Questions: Tritium
Q1. What is tritium?
Q2. Where does tritium come from?
Q3. How is tritium used?
Q4. What are the levels of tritium typically found in the Canadian environment?
Q5. What radiation dose do Canadians typically receive from tritium?
Q6. What are the potential health effects of exposure to tritium?
Q7. In what form is tritium released from nuclear facilities?
Q8. What happens to tritium in the environment?
Q9. What happens when tritium enters the body?
Q10. How is tritium regulated?
Q11. What are the tritium dose limits?
A1. Tritium is a rare isotope (form) of hydrogen, the only radioactive form of this widespread natural element. In regular hydrogen, the atomic nucleus contains only one particle, a proton, whereas the tritium nucleus has three particles, a proton and two neutrons.
Tritium starts to decay as soon as it is formed, by emitting electrons (beta radiation from the nucleus). The half-life of tritium is 12.33 years: it takes just over 12 years for the number of disintegrations (radioactivity) to be reduced by half, another 12 years to be reduced to one-quarter of the original amount, and so on, until it has nearly all changed to helium (a stable, non-radioactive element).
A2. Tritium is produced naturally from interactions of cosmic rays with gases in the upper atmosphere.
It is also produced in nuclear reactors in several ways:
- from fission of uranium in reactor fuel
- from neutron irradiation of heavy water (which, unlike regular water, is created from oxygen and deuterium – another naturally-occurring hydrogen isotope). Heavy water is used in some nuclear reactors such as the Canadian CANDU reactor
A3. Tritium is used in many ways:
- in sealed light sources, like emergency exit signs and airport runway lights
- in medical and academic research
- in some countries, as fuel for thermonuclear weapons
- as fuel for some experimental nuclear fusion machines being developed to harness fusion energy for electrical power
A4. The natural concentration of tritiated water in air is most often below analytical limits of detection with occasional values found of up to about 1 Becquerel per cubic metre (Bq/m3). There is considerable seasonal and latitudinal variation in measurements depending on humidity levels and proximity to large bodies of water. Other forms of tritium, such as tritium gas and tritiated methane, are also present in small amounts in the air.
Tritium produced from nuclear weapons tests in the 1950s and 1960s was dispersed into the global atmosphere and reached 120 Bq/L in precipitation in Ottawa in the mid-1960s. Concentrations since then have steadily declined and are now about 2 to 3 Bq/L across Canada.
A5. In Canada, members of the public receive annual doses of radiation from tritium ranging from 0.0001 to 0.013 millisieverts (mSv) (although very few members of the public receive doses in the upper end of this range).
Near nuclear facilities, where tritium levels are slightly higher, the average annual dose to adults is about 0.0015 mSv.
The worldwide average natural background radiation dose from all sources for a human being is about 2.4 mSv per year, according to United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR).
A6. Tritium exposure can pose a health risk if it is ingested through drinking water or food, or inhaled or absorbed through the skin in large quantities.
The Canadian public is not at risk from tritium intakes at current levels. There is no evidence of adverse health effects, based on biological experiments, observations of humans following accidental intakes of tritium, or routine surveillance of radiation workers at these levels.
A7. Nuclear facilities may emit tritium in a variety of chemical forms. Canadian nuclear reactors emit tritium mostly in the form of tritiated water (only a very small fraction of the water molecules in the environment actually contain tritium). Some tritium may be emitted as tritiated hydrogen or in organic forms such as methane or pump oil.
A8. In the environment, tritium may change from one chemical form to another, e.g. tritium can bind to carbon in organic compounds. For example, tritiated hydrogen may convert to tritiated water, which can become part of the organic molecules in plants and animals; these organic molecules may then eventually end up in soils.
Tritium naturally occurs in the air, rivers, lakes or the sea.
A9. Tritium can enter the body through inhalation, ingestion or absorption through the skin. Most tritium leaves the body as tritiated water in urine, breath moisture and perspiration. Most inhaled tritiated hydrogen is exhaled immediately.Tritium taken in as tritiated water has a biological half-life of 10 days, which means half of the tritium is excreted in this time. However, a small amount does become organically bound (bound to proteins, fat and carbohydrates) with an average 40-day half life.
A10. Routine reactor operation and maintenance result in the release of small amounts of radioactivity from tritium. The CNSC imposes limits, called Derived Release Limits (DRLs) that restrict the amount of radioactive material that may be released to the environment. These limits represent an estimate of a release that could result in a dose of 1 mSv to an exposed member of the public. Releases must still be as low as reasonably achievable and as a result, some limits at facilities are even lower than DRLs. Actual releases of tritium from nuclear facilities have typically been less than 10% of the DRL.The CNSC requires all domestic nuclear operators to provide quarterly reports of results of monitoring of routinely discharged radioactive effluents and annual reports of environmental monitoring programs. The CNSC also requires reporting of any release of a nuclear substance into the environment at a quantity not authorized by acts, regulations or licences, or any unmeasured release of a nuclear substance into the environment.
A11. In Canada, the CNSC’s Radiation Protection Regulations set radiation dose limits for members of the public and workers.
For the general public, the Canadian radiation dose limit from regulated activities is 1 mSv per year over and above natural background levels which are on average 2-3 mSv.
The dose limit for nuclear energy workers is 50 mSv per year and 100 mSv over five years. All doses must be as low as reasonably achievable. The average radiation dose received by Canadian workers in nuclear facility operations is less than 3 mSv/yr, with about 20% of that resulting from tritium.
Most countries with similar technologies to those in Canada have the same radiation dose limits for workers and members of the public (as derived from the recommendations of the International Commission on Radiological Protection).