FAQ: Investigation of the Environmental Fate of Tritium in the Atmosphere

Q1. What is tritium?
Q2. What is the Tritium Studies Project?
Q3. What is the Investigation of the Environmental Fate of Tritium in the Atmosphere report about?
Q4. What does the report conclude?
Q5. How do CNSC-licensed facilities determine tritium concentrations in the atmosphere?
Q6. How does the model compare to actual readings?
Q7. How do active and passive air samplers work?
Q8. The report recommends resolving the discrepancies between active and passive samplers. What action will the CNSC take?
Q9. The report also recommends conducting near field studies of air, soil water and groundwater to better understand tritium behaviour. How will the CNSC proceed?

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Q1. What is tritium?

A1. Tritium is a radioactive form of hydrogen that occurs both naturally and as a by-product in nuclear reactors and tritium processing facilities. Tritium exposure can pose a health risk if it is ingested through drinking water or food, inhaled or absorbed through the skin.  Releases of tritium by nuclear reactors and tritium processing facilities are regulated by the CNSC to ensure the health and safety of workers and the public, and the protection of the environment.

Q2. What is the Tritium Studies Project?

A2. In January 2007, the Commission Tribunal directed CNSC staff to initiate research studies on tritium releases in Canada, and to study and evaluate tritium processing facilities exercising the best practices around the globe.  In response, the CNSC has undertaken several research projects under the banner of the Tritium Studies Project. This research will enhance the information used in the regulatory oversight of tritium processing and tritium releases in Canada.

Q3. What is the Investigation of the Environmental Fate of Tritium in the Atmosphere report about?

A3. The report is primarily a literature review about how tritium behaves in the atmosphere and how it is distributed in the environment. It looks at natural and man-made sources of tritium and how tritium gas transforms into tritiated water. It also describes the behaviour of tritium in the natural cycle of water in the environment, i.e., from air to soil water and surface water, and from soil water to groundwater as well as its behaviour in lakes and rivers.

Beyond this literature review, this report compares predictions from a standard model of tritium environmental behaviour to observed concentrations in air and water for a few representative nuclear facilities releasing tritium. 

Q4. What does the report conclude?

A4. The report makes several general conclusions:

• Tritium behaviour in air and water is reasonably well understood and can be modeled with reasonable accuracy.
• The standard air models used for verifying releases are somewhat conservative and therefore suitable to estimate annual public dose.
• There is a need to resolve discrepancies between air monitoring results from active and passive samplers to reduce uncertainties in measured values.

Q5. How do CNSC-licensed facilities determine tritium concentrations in the atmosphere?

A5. There are two main ways to determine tritium concentrations in the atmosphere: modelling and monitoring. Modelling estimates tritium concentrations based on existing science, whereas monitoring measures actual concentrations on-site. Tritium air monitors are required at all facilities as part of the licensee’s environmental monitoring program. These monitors are part of a larger program to verify that the facility is operating within its authorized environmental release limits.

Q6. How does the model compare to actual readings?

A6. Overall, the model performs much as expected, and the majority of the predictions were in agreement with the monitored results. The model is somewhat conservative, meaning that predicted results are higher than those measured. It is suitable for regulatory use, as it ensures that environmental exposures to tritium are not underestimated.

Q7. How do active and passive air samplers work?

A7. Tritium in the air is sampled continuously by two methods: the active sampler and the passive sampler. The active sampler collects water vapour by drawing air continuously at a steady rate through two molecular sieve canisters; it requires a continuous power source and careful maintenance. The passive sampler simply traps tritium molecules passing through a small hole in a vial that contains a mixture of water and ethylene glycol; this device is more practical for large-scale monitoring. Licensees analyze the information generated by the samplers to verify their releases against model predictions and use the most conservative reading (i.e. the highest one) when both samplers are used to estimate tritium dose to the public.

Q8. The report recommends resolving the discrepancies between active and passive samplers. What action will the CNSC take?

A8. Considerable research on this issue has already been completed after several years of study by the CANDU Owners Group (COG). This information has been shared with the CNSC. The CNSC is considering the implications of COG’s findings for improving the accuracy of routine environmental compliance air monitoring. Altogether, the diverse information compiled within the Investigation of the Environmental Fate of Tritium in the Atmosphere report will be considered as part of the CNSC’s overall Tritium Studies Project analysis and recommendations that will be presented to the Commission Tribunal in 2010.

Q9. The report also recommends conducting near field studies of air, soil water and groundwater to better understand tritium behaviour. How will the CNSC proceed?

A9. A major research study within the Tritium Studies Project is already addressing this recommendation at four sites: two tritium safety light manufacturing facilities and two nuclear power generating stations. This Environmental Fate of Tritium in Soils and Vegetation study is nearing completion after two seasons of monitoring and will be released in 2010, when all laboratory analyses are completed. Currently, licensees are required to routinely monitor groundwater where there are concerns about actual or potential tritium groundwater contamination.