The information about the use of potassium iodine for radiation protection would be easier to understand with a few facts.
Radioactive iodine, specifically iodine-131 is a product of an operating nuclear reactor or a nuclear bomb. That’s about the only place you will find it. It has an 8 day half-life in which it decays to Xenon 131, a stable isotope. Keep that 8 day half-life in mind. It is a very important fact.
During a nuclear event, radioactive iodine is produced and iodine can easily go airborne. While airborne, the body can easily absorbs it. The body sends that iodine directly to the thyroid where it can concentrate and do much damage.
Now here’s where the potassium iodine (KI) becomes useful. The thyroid holds a maximum quantity of iodine, something like 5 milligrams. If a person takes a dose of 100 milligrams before the radioactive iodine is absorbed from the air, then there is no room for it in the thyroid. (All the parking spaces are filled.) All excess iodine is eliminated from the body through the urine. That’s why KI is called a ‘blocking agent’.
Now, about that 8 day half-life: The clock on the half life begins at the moment of the reactor shutdown or the bomb explosion. From that time on, the amount of iodine-131 drops in half every eight days. Notice that the packs of KI tablets includes only 14 tablets. After 14 days, most of the iodine is gone or been so diluted in the air that it is no longer a problem.
The accident at Three Mile Island did not release iodine-131 to the environment because the staff had added chemicals to the coolant water to catch it, also, silver control rods in the reactor combined with the iodine to form silver iodine. (They didn’t plan it that way—it was just a lucky coincident.) Chernobyl did release iodine. One week after the accident in Idaho we were able to measure it with sensitive instruments. We didn’t worry. The quantity was so small that exposure was insignificant. Besides, we were getting much higher exposures from naturally occurring radon gas, which we also monitored.
I was retired when the Fukushima accident occurred so I can only guess that the guys measured that iodine but I’m sure it wasn’t something to be concerned about. Now, three years later, you can be sure all the iodine-131 is gone.
There was one time when I did find a significant iodine-131 source. I happened to walk into the office of an operating nuclear reactor with my radiation meter under my arm. It started to make a lot of noise. One of the staff leaned over my direction and asked if anything was wrong. The rad level shot up and the other people in the room started to laugh. As it turned out this guy measured 85 millirem (850 microsievert) due to a medical injection of iodine-131 which he had received to treat a medical problem. That’s when I joined the laughter.
E R Haroldsen, retired Health Physicist (radiation control)