In a news story about the Japanese nuke plant that was damaged in the recent earthquake it was mentioned that radioactive iodine was released. No explaination was given as to why there would be iodine in a nuke plant, nor why it was radioactive, so naturally I’m curious.
Iodine (especially iodine-131) is one of the major fission products you get when the uranium nucleus splits. It’s troublesome because iodine naturally makes its way to the thyroid gland in the human body, and this is not somewhere where you want any radioactivity going on (since the iodine isotope is itself unstable and will beta-decay quite soon).
You can protect yourself against radioiodine exposure by taking potassium iodide (which I guess tastes salty, being an alkali-metal halide), which will saturate your thyroid with all the iodine it can process for a while and stop it taking up any more.
ETA: Of course, normal iodine is not radioactive, but what’s being released here is an isotope - chemically nearly identical to ordinary iodine, but with a different, less stable, nucleus.
It can also be produced during Plutonium fission. It’s dangerous because it’s a Beta emitter that has a half life of just 8 days
The thing that I’m curious about is that, as others have pointed out, radioactive iodine is essentially only produced as a fission product.
The reason I bring that up is I wonder just how many Curies were relased of the isotope. While it’s not unsual for very trace quantities of fission products to end up in the reactor coolant, it is only a trace contaminant. Even by radiological standards. (For the PWRs I’m most familiar with, there’s some trace contamination in the cladding metal by fissionable atoms. It’s, by no means, a large amount, nor a flaw in the manufacturing - just a quirk that means that fission products will be in the coolant. But the concentrations are… phenomenally low.)
And as Tapioca Dextrin mentioned it’s a very short half-life.
So, what was going on at the plant to have it accumulate signifigant amounts of radioactive Iodine? Or, is this simply a matter of the people reporting the the story going with a partial clue - i.e. they’ve heard that Iodine is an isotope of concern for nuclear catastrophes, and so focused on the presence of the isotope in the spill without understanding the concentrations involved?
http://www.straightdope.com/classics/a1_239a.html
The Master’s column on anti-nuke iodine pills.
Normal operations - That’s what’s going on.
As pointed out above, it’s a very common fission daughter product. With a half life of a mere 8 days, it doesn’t last too long once released - Effectively, released I-131 will decay to negligible amounts in roughly six weeks, but any time the reactor is up and running, you’ll begin to accumulate more radio-iodine. Eventually, provided you don’t monkey with reactor power levels too often, you’ll reach a steady state where new radio-iodine production balances out with decay of existing radio-iodine. And since this was a civil power plant, you can generally assume it will be operating at a steady power level most of the time.