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By Ole Hendrickson
“In allowing large HT releases in an uncontrolled setting very close to homes, businesses, gardens, etc., the CNSC has made Pembroke into a prime destination for researchers studying the environmental fate of tritium.”
Hydrogen (H) occurs naturally in three forms. The most common form, sometimes called protium, consists only of one proton and one electron. Deuterium (D), which comprises less than 2 ten-thousandths of naturally occurring hydrogen, is twice as heavy, as it contains a neutron as well as a proton in its nucleus. Tritium (T), with two neutrons and a proton in its nucleus, is unstable and radioactive. It is produced in minute quantities when cosmic radiation interacts with nitrogen and oxygen in the upper atmosphere.
Tritium decays when one of its neutrons emits a beta particle (essentially an electron) and becomes a proton, thereby converting a hydrogen atom to helium. Physicists thought that because tritium has a low-energy beta particle, and is the weakest naturally-occurring radioisotope, its health effects could be ignored. But tritium is hydrogen: chemically and biologically highly reactive. It replaces a hydrogen atom in ordinary water to form tritiated water (HTO), binds to a wide range of organic molecules, and persists in the body as organically-bound tritium (OBT) compounds for months to years.
When in the body, all of tritium’s decay energy is deposited within the organism, where it damages cells and genes, causing DNA repair mechanisms to work overtime. For many years it has been known that tritium creates irreversible immune system damage: sub-lethal exposure of rainbow trout embryos to tritium “produced measurable, dose-dependent, and irreversible suppression of immune capacity in affected fish”. Risks of cancer and other diseases from tritium exposure have long been a topic of intense debate in the scientific community (see the Tritium Awareness Project). The International Commission on Radiological Protection, a quasi-official radiation standard-setting body based in Ottawa, Ontario, has been strongly criticized for ignoring these risks.
Figure from Fievet et al. (2013), Environmental Science & Technology 47: 6696−6703
Atmospheric nuclear weapons testing that occurred from 1945 to 1980 increased tritium levels in the environment several hundred fold. As tritium has a 12.3-year half-life, weapons-derived tritium has now largely decayed away. Today, nuclear reactors are the main source of tritium in the environment. CANDU reactors, which use deuterium oxide (heavy water, D2O) to moderate the nuclear chain reaction, yield prolific amounts of tritium. Neutrons derived from fissioning of uranium bombard the deuterium atoms in heavy water, some of which acquire an extra neutron and become radioactive.
Tritium build-up in heavy water shortens reactor life and poses a radiation hazard to workers and members of the public. Ontario Power Generation operates a Tritium Removal Facility at its Darlington nuclear station to extract some of the tritium from reactor heavy water, convert it to pure tritium gas (T2), and store it in stainless steel containers in a concrete vault.
Most, but not all, of the tritium extracted from CANDU reactor heavy water remains in the vault at the Darlington station. Tritium is expensive and in high demand. It is a major component of nuclear weapons (today’s weapons derive more of their explosive force from the fusion of tritium atoms than from the fission of uranium or plutonium atoms, though both processes occur simultaneously).
Currently, the main customer for Darlington’s tritium is a Pembroke, Ontario-based company, SRB Technologies (Canada) Inc., which fills phosphor-coated glass tubes with tritium gas, seals the tubes, and puts them in glow-in-the-dark devices such as exit signs (though LED lights are replacing tritium lights for most purposes). If the International Thermonuclear Experimental Reactor – the fusion reactor being built in France – comes on line, it will create a large new demand for Darlington’s tritium (and will release large quantities of tritium to the environment compared to fission reactors).
SRB Technologies’ tube-filling activities unavoidably release quantities of tritium similar to a nuclear reactor (quantities were much higher before the company was forced to shut down for 18 months). But whereas nuclear reactors release HTO, SRB’s releases are mostly in the form of tritiated hydrogen gas, HT, which is formed almost instantaneously when the T2 purchased by SRB from Ontario Power Generation comes in contact with the environment.
Current regulatory standards assume that the HT emitted by SRB can be ignored as an environmental or health risk. The Canadian Nuclear Safety Commission (CNSC) carried out a Tritium Studies Project with the intent of assuring the public that tritium releases are “safe” – a Synthesis Report recommended certain actions by CNSC “to make the regulation of tritium even safer.”
CNSC hired a consulting firm to produce a document entitled Investigation of the Environmental Fate of Tritium in the Atmosphere which claims (p. 57) that SRB “…releases HT; however, this is primarily of interest due to the small portion that converts to HTO. The HT itself has a much smaller dose coefficient and does not transfer to the human food chain.” In simple terms, the CNSC assumes that if a person gets caught in the plume of radioactive gas coming from SRB’s stacks, the HT they inhale will simply pass in and out of their lungs. The radioactive plume itself will largely be blown away from the SRB facility, and tritium will not persist in the local environment or transfer to the human food chain.
Unfortunately, this is not the case. Research published in 2015 in the Journal of Environmental Radioactivity by Patsy Thompson, Director General of the CNSC’s Directorate of Environmental and Radiation Protection and Assessment and colleagues, indicates that tritium has accumulated in large amounts in the vicinity of the SRB facility, mostly as OBT, in both soils and vegetation. CNSC dismisses risks of consuming local vegetables and fruit contaminated with these unexpectedly high amounts of OBT, even making the absurd claim on its web site that “radioactivity measured in water, air, soil, vegetation, milk, wine, fruits and vegetables samples … are within natural background levels”. The new research concludes that environmental transfer models should be revisited to ensure that they are adequately protective.
CNSC actions have also been counter to findings in another 2015 study in the Journal of Environmental Radioactivity, showing that soils and vegetation near SRB are continuously off-gassing radioactive tritium, as plants and microorganisms break down OBT compounds and release HTO. This creates a particular hazard to young children who play outdoors and lie on the grass.
In my report for the May 2015 CNSC hearing on relicensing of SRB Technologies, I explain why OBT accumulates near facilities that release large quantities of HT, and why this finding was unexpected. Whereas higher animals cannot directly convert HT to OBT, hydrogenase enzymes that readily carry out this conversion are widespread in bacteria and also found in certain green algae and fungi. For these organisms, hydrogen gas represents a valuable source of energy. They may have inducible hydrogenase enzymes that respond to an increased supply of hydrogen gas (e.g., where SRB’s radioactive plumes come in contact with microbes living on plants and in the soil, or in nuclear waste repositories). Current radiation protection models omit this direct pathway for conversion of HT to OBT.
The persistently and surprisingly high OBT levels near SRB are of considerable interest to French researchers. They traveled to Pembroke to carry out joint research with CNSC showing that airborne tritium levels near SRB are highest near the ground surface (and that soil is continuously emitting HTO). They are also funding tritium studies at the Canadian National Laboratories in Chalk River. Their interest is understandable. An operational International Thermonuclear Experimental Reactor would represent another large point source of HT, and they want to learn as much as possible about its environmental fate.
In allowing large HT releases in an uncontrolled setting very close to homes, businesses, gardens, etc., the CNSC has made Pembroke into a prime destination for researchers studying the environmental fate of tritium. As yet there appears to be no research on humans themselves, even though OBT could readily be measured (e.g., in fingernail and toe-nail clippings).
Ole Hendrickson is a researcher with the Concerned Citizens of Renfrew County. He has a Ph.D. in Ecology from the University of Georgia.
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