Tuesday, March 15, 2011

Uranium 101

kw: observations, radioactivity

In the wake of the likelihood that a lot of radiation might be released from the damaged nuclear reactors in Japan, I'd like to address some common fears.

First, the danger is real, but it will probably be limited. If a total meltdown occurs, let us remember that Uranium metal is twice as dense as Lead, and nearly all of it will melt its way downward, until it is dissipated to the point that it can't sustain a chain reaction. Then it will cool off. However, some fraction of a percent of the total core will react with air and steam in the early stages, and this amounts to several grams of Uranium. Plus, because the reactor has been running for some time, the breakdown products of Uranium fission could amount to a few percent of the total, and these are more volatile. The most risky is an isotope of Iodine, which is why the Japanese government has been giving people Iodine pills. Having extra Iodine in the body will reduce the amount of radio-Iodine that the body can absorb, reducing long-term risk.

Uranium itself is only weakly radioactive. The half life of U238 is 4.6 billion years. Compare this with the most common isotope of Radium, with a half life of 1,200 years. It is four million times as radioactive as Uranium. Enriched Uranium has a little stronger activity, but still in the billion-year range, because U235 has a half life of 0.7 billion years.

What few people know is that Uranium is found in small amounts almost everywhere. Common granite (got a granite countertop?) contains 3 parts per million Uranium. That is three grams per tonne. It is present at about one part per million in ordinary Gypsum board, AKA drywall, or a gram per tonne. An ordinary room contains a tenth of a tonne of drywall in its walls. However, the paper and paint on the wall completely stop the main radiation the Uranium emits.

The Uranium in your environment is the largest component of the background radiation of 3 millisieverts (3 mSv) per year that we all absorb. How does this compare with other exposures? A full-mouth set of dental x-rays gives you a quick dose of 0.15 mSv, or some 5% of the yearly background. A CT scan, by contrast, produces a dose of about 10 mSv, or three years of background all at once. This is considered a safe dose if not repeated more often than once every two years. During the five years after my cancer operation, I had a CT scan yearly. The early detection was considered an acceptable risk.

What kind of dose would a meltdown produce? For those nearest the event, it could be truly grave: hundreds of mSv per day. This is why evacuation is going on. For the next continent downwind (America), the worst case is expected to be an extra mSv or two per month for a year or more, which is like getting a yearly CT scan, or two. However, the more likely scenario from a partial meltdown that gets contained, similar to the Three Mile Island experience, is an extra mSv or two total during the first year, then nothing more. Even people who are most susceptible to radiation would not be considered at any great risk from that. It would be like moving to Denver, where there is an extra half to one yearly mSv from cosmic rays because of altitude.

An even better scenario, assuming there is some kind of meltdown and release, is for very rainy weather to wash most of the stuff from the skies before it spreads around the globe. I'm glad March and April are rainy months in the Northern hemisphere!

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