Friday, November 20, 2009

Tritiated Steam, Smoke and Mirrors

One thing I learned in reading the news is to be aware of the source of your information. Often, special interest groups have a way of skewing the facts in a way that suits their purposes. You may think this rich coming from me, but I've been very open about my biases. This news article is not clear in the fact that its 'source' is an organization that is itself biased against nuclear power.

The facts in the article are disturbingly skewed in order to spread fear and misinformation. To set things straight, let me agree with what they say on the following:

Do Canadian nuclear reactors release tritium? Yes.

Can exposure to large quantities of radiation cause cancer? Yes.

Are levels of radiation around nuclear power plants higher than elsewhere? Yes. Five times in the case of tritium? Probably more in some cases.

Where I disagree with the article and Sierra club is in the danger posed by tritium. First a little bit of physics knowledge about tritium.

Tritium is an isotope of hydrogen, generally references to 'tritium' are usually to 'tritiated water' where a tritium atom has replaced one (or even both) hydrogen atom to form what is abbreviated as HTO, where T stands for tritium. Tritium, is radioactive with a natural half-life of about 12 years. When it decays it emits an extremely low-energy beta particle, also known as an electron. This electron, having an extremely low energy, is unable to pass through a piece of paper or a dead layer of skin and so, while external to your body poses absolutely no danger to your health.

Unfortunately, it behaves identically to water and will get into your body as easily as normal water, where it may pose a threat. This is both good news and bad news. The bad news is that it is inside of your body and can cause damage if you have long term exposure to large quantities. The good news is the tritium becomes spread out in your body and delivers what is known as a 'whole body does' rather than a local dose. On the surface, this sounds bad, but its actually good. Other radioactive isotopes, if ingested will accumulate in particular parts of your body, so even relatively small doses taken over a long period of time can cause serious and specific health problems. Tritium is first diluted by spreading across your whole body and then easily expelled by your body's natural processes. This means that if you ingest a quantity of tritium, it will have a half life in your body of approximately 8 days. After a little over a week, half the tritium will be gone. After a bit over two weeks, only a quarter will remain. After three and a half weeks, only one eighth the original amount will remain. And so on.

To give a similar comparison, it is like Vitamin C as opposed to Vitamin D. If you eat too much Vitamin D, then you will die because it is fat soluble and collects. Vitamin C, being water soluble, is diluted in your body and regularly flushed from your system and so no matter how much Vitamin C you eat, you will not die.

Alright, but I did say that levels of tritium around nuclear power plants can be five times the norm, isn't this dangerous? The answer is unequivocally, no.

The Canadian Nuclear Safety Commission regulates how much radiation a member of the Canadian public is allowed to receive as a result of the operation of a nuclear power plant. They, and the World Health Organization make certain assumptions concerning how much radiation a member of the public receives as a result of tritiated water. Using these assumptions they derived a limit of approximately 7700 Bq/L as a 'safe' limit. The WHO rounds up to 10 000 Bq/L, while the CNSC rounds down to 7000 Bq/L. These limits ensure that the public is exposed to less radiation from tritium than is normal to receive for medical operations and far less than a member of the public would receive from even naturally occurring background radiation! In Europe, after some arbitrary revisions of the initial assumptions concerning how much people should be allowed to be exposed to, they set a limit of 100 Bq/L. Sites around a nuclear power plant must be tested for their radioactivity in order to ensure that they are complying with these limits. Deviations are not tolerated.

Standard background levels range from 0 - 6 Bq/L, depending on where you are. Windsor, Ontario for example has a background tritium level of about 5 Bq/L.

Now, I said that the European standards were arbitrarily chosen, and I meant it. In my opinion, they chose limits not based on scientific analysis of the dangers posed by tritium itself, but on the assumption that any tritium in the water exceeding the limits set would be an indication that there was a release of other more dangerous radioactive particles. Why do they feel this way? Because their nuclear reactors don't produce significant quantities of tritium! So there shouldn't be large quantities of tritium in the water at all. In contrast, Canadian nuclear reactors produce tritiated water under normal operation so there is to be expected more tritium under normal circumstances.

That being said, Canadian nuclear power plants operate in a way that easily satisfies even the European standards. As this table shows.

Figure 1: Tritium levels around selected nuclear sites in Canada (see reference for all)

The highest value in that list there? Roughly 60 Bq/L. Which is well even the European levels. Only one site (Pickering) has any value that exceeds the European standards of 100 Bq/L. But even that one has a range of reported values from 1.9 Bq/L to 120 Bq/L.

Even still, that value is one hundredth the safety limit recommended by the WHO and easily an order of magnitude lower than that recommended by the CNSC. But "Canadian Nuclear Power Plants Easily Satisfy Stringent Safety Limits" wouldn't make a good headline.

Reference:

"Standards and Guidelines for Tritium in Drinking Water", Canadian Nuclear Safety Commission, January 2008. [pdf]

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