|
Canada vs. U.S. Tritium Standards
in Drinking Water
(A Primer on Tritium)
What is tritium? Tritium is a radioactive form or isotope of hydrogen, with a half-life of 12.3 years and a biological half-life (the amount of time the body requires to excrete one half of the tritium absorbed) of 10 days to 2 years. Tritium decays to non-radioactive helium by emitting a negatively charged beta particle (electron); this is why tritium is referred to as a “beta-emitter.” What are the uses of tritium? Tritium is used commercially as a light source in flares, emergency lights, exit signs and luminous dials (watches and clocks); tritium is also an essential fuel for experimental nuclear “fusion” and has been used for nuclear weapons production. What are the chemical properties of tritium? Hydrogen has three isotopes: “Protium” (normal hydrogen) is the simplest atom, is abundant, and has no neutrons; two normal hydrogen atoms when combined with an oxygen atom create an ordinary (“light”) water molecule, or H2O. “Deuterium” is the same as a protium atom, but has one neutron and has twice the mass of a protium atom; deuterium is referred to as a “heavy hydrogen” atom; when two deuterium atoms are combined with an oxygen atom, a molecule of deuterium oxide (D2O) or “heavy water” is created; deuterium occurs naturally: for every 7,000 molecules of ordinary “light” water, there is one molecule of “heavy” water; approximately 100,000 gallons of ordinary water are needed to produce a single gallon of pure heavy water. “Tritium” atoms are created when a deuterium atom absorbs an additional neutron; deuterium oxide, or “heavy water” than becomes “tritiated” water and is radioactive. What are the sources of tritium? Less than 1% of tritium occurs naturally (e.g. through the interaction of cosmic rays with molecules of certain elements in the upper atmosphere). Most of tritium is man made; fallout from thermonuclear weapons testing, begun in the 1940s, is a source of tritium in the global environment; nuclear power reactors are also a large source of tritium. Canadian Deuterium Uranium (CANDU) nuclear reactors are the largest Canadian point source of tritium; CANDU reactors use deuterium oxide or “heavy water” as both a moderator and a coolant, and effectively “breed” tritium as the nuclear fission process releases free neutrons (i.e. tritium atoms are created when a deuterium atom absorbs an additional neutron). How is tritium released to the environment? At the Bruce nuclear complex, tritium is released to the station cooling water effluent stream (Condenser Cooling Water) which discharges into Lake Huron. There are two potential operational sources of tritium into this discharge stream: 1. Radioactive Liquid Waste Management System Process effluents are collected in holding tanks for controlled release to the effluent stream; leaks resulting from certain process failures can also be collected in these tanks. 2. Process Leaks Various heat exchangers in the plant can develop leaks; these heat exchangers use station cooling water (lake water) to cool process fluids; in the event of leaks, tritium escapes directly to the cooling water and ultimately to the effluent discharge stream. Tritium can also be released to the air by venting of the plants and incineration of low level waste. Once released to the environment, tritium exposure can occur from a variety of sources, namely via water, air and food; exposure to water-borne releases can occur through the consumption of drinking water supplies or through other water contact (e.g. swimming, bathing, showering). How do Canadian CANDU reactors compare to U.S. nuclear reactors with respect to generation and emissions of tritium? The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) has noted that Canadian CANDU heavy water reactors generate about 30 times the amount of tritium of a light water U.S. reactor. According to UNSCEAR, a CANDU reactor also normally releases over 20 times the amount of tritium to the environment (water and air) than a U.S. light water reactor. The majority of tritium in Lake Huron and Lake Ontario is a byproduct of CANDU nuclear operations. Ontario Power Generations owns 20 (of Canada’s 22) CANDU reactors; 8 are on the shoreline of Lake Huron at the Bruce nuclear complex; 12 are on the shoreline of Lake Ontario at the Pickering and Darlington nuclear sites. Are there potential health hazards related to exposure to tritium? Yes. The U.S. EPA has classified tritium as a human carcinogen. In addition, there is a growing body of evidence which suggests that tritium is mutagenic (i.e. mutates genes causing hereditary defects) and teratogenic (i.e. causes malformations of an embryo or fetus). The most sensitive populations to tritium are considered to be fetuses, young children and women of childbearing age. Tritium can be taken into the body by inhalation, absorption through the skin or ingestion; tritium entering the body by inhalation is normally distributed uniformly among all the soft tissues in the body. Following ingestion, tritiated water is absorbed immediately from the gastrointestinal tract and then mixes rapidly with the total body water; in addition, ingestion of contaminated foods is a potential exposure pathway from air-borne tritium emissions; the ingestion of contaminated fruits, vegetables or food animal products raised near nuclear generating stations may be a significant source of organically bound tritium (i.e. organic compounds that incorporate radioactive tritium in place of normal hydrogen). Once ingested through air, food or water, living organisms may incorporate radioactive elements into their tissues. Tritium mimics normal hydrogen in water and tritiated water mimics normal water. Tritium can harm living organisms by emitting beta particles, which can break chemical bonds and damage living cells, particularly through damage to DNA molecules, which encode genetic information. Damage to the DNA of sperm or egg cells can result in damage to future generations. Is there any evidence to suggest that populations around Canadian nuclear facilities might be concerned about adverse health effects due to tritium? Food and water around Canadian nuclear plants show elevated levels of tritium (e.g. an apple at a farm near the Bruce complex tested at over 900 times normal background radiation). There are statistically credible increases in childhood leukemia deaths and Down’s Syndrome around the Bruce and Pickering nuclear facilities. Are there national standards for the maximum concentration of tritium in drinking water? Yes. Canada (Guidelines for Drinking Water Quality) has recently lowered the limit for tritium in drinking water from 40,000 becquerels per litre (Bq/L) to 7,000 Bq/L. In the U.S., the Environmental Protection Agency (EPA)* sets a limit under the Safe Drinking Water Act of 740 Bq/L, almost 10 times as restrictive as Canada’s recently reduced limit. * The limit for “beta-emitters,” in the case of tritium, is 20,000 Picocuries (a Picocurie is one-trillionth of a Curie). One Curie equals 37 billion becquerels. What is a becquerel? A becquerel is an international unit of measure of the activity of a radioactive nuclide. Tritium decays to non-radioactive helium by breaking down and emitting a negatively charged beta particle (electron). One becquerel represents one of these atomic transformations or disintegrations per second. Drinking water with 7,000 becquerels per litre concentration would mean that 7,000 of these breakdowns or transformations happen per second per litre of water. Are there any risk estimates for drinking water with a concentration of tritium at 7,000 Bq/L? Using risk estimates from the International Commission on Radiological Protection, the World Health Organization, and the assumption of a linear dose response curve (i.e. the total lifetime dose is the sum of annual doses), lifelong exposure (70 years) to 7,000 Bq/L would result in approximately 340 excess fatal cancers per million people exposed, or just under 1 in 3,000. This estimate does not include non-fatal cancers or potential hereditary defects. Can the Bruce nuclear plant discharge concentrations of tritium into Lake Huron water greater than the Canadian drinking water limit of 7,000 Bq/L? Yes. Canadian federal regulators allow discharges of tritium well in excess of national safe drinking water standards, based on the dilution effect that occurs when “tritiated” water mixes with lake water. It is from this mixture that officials obtain “Derived Emission Limits” or “Derived Release Limits.” Have there been any serious accidental spills of tritium from the Bruce nuclear complex into Lake Huron? Yes. In a noted past accidental release into Lake Huron from the Bruce complex, spanning over a 7 day period, concentrations of tritium in drinking water at Port Elgin, a town 15 miles north, reached in excess of 1,500 Bq/L, over twice the U.S. EPA limit. The Michigan shoreline is 50 miles across Lake Huron from the Bruce nuclear complex. Acknowledging that notification and action protocols were inadequate, an agreement was subsequently reached between Bruce representatives and provincial health officials to strengthen and formalize procedures for notifying nearby Canadian municipal water supply plants. The Bruce nuclear complex also has two radioactive waste storage sites that are leaking tritium into groundwater monitoring wells. Groundwater sampling holes from Site 1 have in the past exceeded 203,500 Bq/L for tritium. The waste at Site 1 is being moved to Site 2 (now called the Western Waste Management Facility - WWMF). One groundwater sampling hole at the WWMF has recently exceeded 12,000 Bq/L. Have any other organizations recommended a lower drinking water standard for tritium? Yes. In 1994, an (Ontario) appointed “Advisory Committee on Environmental Standards” recommended that the maximum permissible concentration of tritium in drinking water be immediately reduced 70 fold to 100 Bq/L, grading to 20 Bq/L over 5 years. Their recommendations were rejected by the Ontario government. The International Joint Commission, created by the 1909 Boundary Waters Treaty, has identified tritium as a persistent toxic substance, and a candidate for zero discharge. References/Sources A Standard for Tritium: A Recommendation to the (Ontario) Minister of the Environment and Energy (Advisory Committee on Environmental Standards, 1994) “Notification/Action Protocols for Abnormal Tritium Releases at BNPD” (1996) UNSCEAR (United Nations Scientific Committee on the Effects of Atomic Radiation). Sources and Effects of Ionizing Radiation. Report to the UN General Assembly. United Nations, New York: 1977. U.S. EPA Safe Drinking Water Act 42 U.S.C. s/s 300f et seq. (1974) Health Canada Summary of Guidelines for Canadian Drinking Water Quality (March, 2001) Canadian Nuclear Safety Commission Canadian Coalition for Nuclear Responsibility British Columbia Institute of Technology |