A good friend asked me the title question for this blog during the week. The question intrigued me enough to encourage some digging and computations. I would be interested in any questions that you have about the assumptions.
Based on reading a number of different articles and checking through the tables provided by the Vermont Department of Health, the fluid that was leaking into the ground contained tritium at a concentration of approximately 2.5 million picocuries per liter. That is equal to 2.5 x 10^-6 curies per liter. The rate that it was leaving the pipe was roughly 100 gallons (370 liters) per day. If the leak had been going on for a year before being detected and stopped, the total quantity of fluid that left the pipe would equal 138,000 liters. The total activity released would be 0.35 curies.
If a single person consumed every drop of that water, their whole body radiation dose would equal roughly 30 rem. According to a 1977 UNSCEAR study, the LD-50 (lethal dose for 50% of the population receiving the exposure) for tritium in adult rats was determined to be 1000 Rad. For the kind of low energy beta emissions that are produced by tritium, a rem is equal to a Rad. A dose of 30 rem received over a 1 year period would be unlikely to cause any immediate health effects, though it might add an additional risk of developing cancer sometime during the person’s life. The magnitude of that risk could be computed using the conservative linear, no-threshold dose assumption.
Of course, a person who tried to drink 378 liters per day for a year would have problems more immediate the possibility of increasing their lifetime risk of cancer.
I also was asked to put this discharge into some kind of perspective, so I decided to compare it to the allowable and measured releases from a well operated and safe CANDU reactor in Ontario. Pickering B has a Derived Release Limit (DRL) for tritium of 490,000 terabecquerels each year. That is 4.9 x 10^17 Bq or 13 million curies. Here is how the Canadian regulators determine the DRL for a facility:
The CNSC requires nuclear facility operators to closely monitor and report tritium releases into the environment. The releases of tritium into the environment are controlled, and do not pose a health risk.
The CNSC sets the limit for the radiation dose for the public arising from the activities of nuclear facilities at 1 mSv per year. In addition, to minimize the environmental impact of tritium releases, the CNSC uses the ALARA principle, which means that licensees are required to keep exposures “As Low as Reasonably Achievable”.
All authorized releases to the environment – known as Derived Release Limits (DRLs) – are calculated using a methodology that considers the various ways in which radionuclides, such as tritium, can impact members of the public.
For instance, the methodology must take into account the impact of tritium releases on locally-grown food, water supply and air quality. The methodology used is conservative and assumes, for instance, that individuals live, work and stay in their community for extended periods of time.
DRLs are specific to each facility, and take into account various regional factors such as meteorological and geographical conditions, as well as the population density near the facility.
Through careful management and application of ALARA (as low as reasonably achievable) actions, the plant managed to keep its releases in 2008 to just 200 TBq or 5,400 curies, just 0.04% of its allowed limit. At that rate, Pickering B released almost 15 curies per day, about 40 times as many curies as Vermont Yankee’s infamous leak released in an entire year.
Now can you see why I so strongly believe that the protesters and politicians in Vermont have been stirring up a tempest over a shot glass sized swirl? As one of my mentors used to say – no sense reacting if you cannot overreact.