The Carnival is Up
Before I begin the somewhat heavy work of summarizing the Canadian tritium study, I want to note that that the Sixth Carnival of Nuclear Energy is up at NEI Nuclear Notes. Where else can you read about climate, AND read an excellent deconstruction of the nuclear-equals-proliferation argument AND see a recent picture of Chernobyl's control room? Follow the link to the Carnival. Carnivals are fun.
Also, I want to include a link to a recent Nuclear Fissionary post, comparing the results of coal environmental effects (like ash pond spills) with tritium discharges at nuclear plants. Thanks, Fissionary!
The Scope of the Canadian Study
This was a compendium of studies, including air, water and food studies of both power reactors and waste disposal sites. The DOE and NRC dose limits for ionizing radiation for the public is 100 millirem/ year (from nuclear facilities) and that is the same as the Canadian dose limit of 1.0 mSv/year. (Canadians, like Europeans, use Sieverts, causing American bloggers to keep multiplying by 100 to get to our own familiar terms of millirems.) In comparison, the background radiation from natural sources in the U S in 3 mSv per years, or 300 millirems. (Data derived from a standard handout prepared by NF Meeting, from data contained in the DOE/BER database.)
Page 12 of the report shows that dosages to members of the public from people living near Canadian nuclear generating facilities varied from 0.00045 to 0.000236 mSv/year, well below 1 mSv. Some other installations, such as Chalk River Laboratories, had higher dosages for people in the area, due to old spills. All dosages were orders of magnitude below any doses known to cause health effects.
Background on Canadian Reactors
As a bit of background, one should note that tritium releases from Canadian types of reactors (which use heavy water) are orders of magnitude higher than releases from U.S. reactors. Rod Adams post on the tritium release from Vermont Yankee has the most readable description of the overall situation. Adams notes that Pickering B (a Canadian power plant) is allowed to release 13 million curies of tritium (liquid emissions) a year, though the plant only releases about 5000 curies. In contrast, an estimate of the entire tritium-leakage from Vermont Yankee was 0.35 curies.
In the Canadian case, the relatively high tritium release allowances do show up as measurable tritium in drinking water of 2 to 20 Bq/L (54 to 540 picocuries/L) while background is about 2 Bq/L (54 picocurie/L) (page 14). Some groundwater monitoring wells at facilities (not power plants) are much higher, up to 3,000, 000 Bq/L. (page 11) These waters are not used for drinking, and they are not causing similarly high numbers in local aquifers.
(1 Bq is 27 picocuries. If you choose the read the report yourself, multiply the Bq numbers by 30 in your head while you are reading it. That's what I do.)
Organically Bound Tritium
There was some new information about organically bound tritium. Organically bound tritium in soils and vegetables was higher than expected, as far as I can tell from reading the report. Page 22 of the report says that near nuclear sites, the ratio of organically bound tritium (OBT) to HT (tritium in water) in plants was 2 to 3, and for animals it was 10. This ratio also showed a lot of variation.
The report also notes (page vii) that people consuming local produce (near Pembroke) received less that 0.004 mS per year from produce, a negligible amount, even taking into account organically bound tritium.
As the report concluded (page 25)
In conclusion, tritium exposures are highly unlikely to cause adverse health effects in the public or in workers. The doses to which these groups are exposed are far below doses where radiation effects have been shown:
• In Canada, doses to the public from tritium releases from nuclear facilities are far below the public dose limit. Doses from tritium exposures among people living near Canadian nuclear facilities are in the range of 0.0001 to 0.1 mSv/year... These doses are not only well below the limit, but also are negligible compared to natural background radiation, including that from radon (approximately 2 to 3 mSv/year depending upon geographic location).
However, the Canadians also recommended that the drinking water limits on tritium be changed to 100 Bq/L, or 2700 picocuries per liter (p ix).
Conclusions for Vermont Yankee
There will undoubtedly be those who will seize on the lower tritium limit suggested by the Canadians and think it is relevant to something-or-other about Vermont Yankee. However, no tritium has been found above background or detection level in any water near the VY plant, so limits of 2700 picocuries per liter (suggested by the Canadian study) or 20,000 picocuries per liter (current NRC and Canadian rules) don't matter, in terms of plant operation.
Vermont Yankee is a zero-discharge plant. Even during the leak, it discharged only undetectable amounts of tritium. Therefore, guidelines of 2,700 picocuries or 20,000 picocuries per liter allowed in drinking water make no difference. The Canadian guidelines are interesting, but irrelevant to people in Vermont.
Conclusions for Canada
I believe the Canadians did this study and reached these conclusions because of historically high tritium discharges at some of their research facilities. If you read the report, table by table and graph by graph, you can see that there are some facilities that have discharged too much tritium and need some attention. Of these facilities, the Chalk River research area leads the list.
I think this study was the Canadian Nuclear Safety Commission's way of insuring attention to these areas.
I think you summed the report up nicely. It should be noted that the lowering of discharge values was motivated more by a desire to coordinate standards with those of various health departments across the country. These were, or are about to be brought in line with the EU numbers in the near future.
It's not just research reactors that lead to higher tritium releases. The CANDU reactor uses heavy water as a moderator, which since it absorbs fewer neutrons than regular light water, allows for the use of natural (unenriched) uranium. Unfortunately, that means what neutrons are absorbed by the water form tritium. Hence, the tritium level in the coolant is way higher, and any coolant loss has way more tritium.
Not that there's anything wrong with that. When the absorbed doses from reactors gets anywhere close to the absorbed doses from bananas (potassium 40), then I'll get worried.
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