When there is radioactive fallout from a nuclear power plant emergency (or from a nuclear bomb), the fallout will have a number of different radioactive isotopes, including cesium, iodine, and strontium. The radioactive isotope of iodine (‘radioiodine’) in question here is iodine-131. This isotope can accumulate in the thyroid gland.
“The thyroid is a small gland, measuring about 2 inches across, that lies just under the skin below the Adam’s apple in the neck…. The thyroid gland secretes thyroid hormones, which control the speed at which the body’s chemical functions proceed (metabolic rate)…. To produce thyroid hormones, the thyroid gland needs iodine, an element contained in food and water. The thyroid gland traps iodine and processes it into thyroid hormones.” 
The accumulation of iodine-131 in the thyroid can cause cancer: “the thyroid gland is very sensitive to radiation.” 
“The Chernobyl reactor accident resulted in massive releases of 131-I and other radioiodines. Beginning approximately 4 years after the accident, a sharp increase in the incidence of thyroid cancer among children and adolescents in Belarus and Ukraine (areas covered by the radioactive plume) was observed. In some regions, for the first 4 years of this striking increase, observed cases of thyroid cancer among children aged 0 through 4 years at the time of the accident exceeded expected number of cases by 30- to 60-fold. During the ensuing years, in the most heavily affected areas, incidence is as much as 100-fold compared to pre-Chernobyl rates….” 
Non-radioactive isotopes of iodine are commonly found in food. And iodine is an essential micronutrient.  However, the human body cannot distinguish between the radioactive and non-radioactive isotopes of iodine. So if radioactive iodine is in your body, the thyroid gland will tend to take it up. But if you consume a daily supplement of potassium iodide (KI), flooding the body with plenty of non-radioactive iodine, any radioactive iodine is diluted, and is much less likely to be taken up into the thyroid.
The use of KI to prevent thyroid cancer is safe and effective.
“The effectiveness of KI as a specific blocker of thyroid radioiodine uptake is well established … as are the doses necessary for blocking uptake. As such, it is reasonable to conclude that KI will likewise be effective in reducing the risk of thyroid cancer in individuals or populations at risk for inhalation or ingestion of radioiodines. Short-term administration of KI at thyroid blocking doses is safe and, in general, more so in children than adults.” 
The dosage recommendations of the FDA are relatively simple  —
Adults over 40 years of age do not need KI supplements, unless their predicted thyroid exposure is greater than or equal to 500 rads (500 cGy), in which case the dosage is 130 mg of KI once each day. (A radiation dosage of 500 rads is very high, and is not likely to occur except close to the location of the nuclear power plant in question.)
Adults over 18 through 40 years of age should receive 130 mg of KI once each day (two 65 mg KI tablets per day).
Adolescents over 12 through 18 years of age should receive 65 mg of KI once each day, unless they are approaching adult size (approaching 70 kg of body weight which is 154 lbs), in which case they should receive the full adult dose: 130 mg of KI once each day.
Children over 3 years through 12 years should receive 65 mg of KI once each day.
Young children over 1 month through 3 years should receive 32 mg of KI once each day (half of a 65 mg KI tablet).
Infants from birth through 1 month should receive 16 mg of KI once each day (one quarter of a 65 mg KI tablet).
“The protective effect of potassium iodide (KI) lasts approximately 24 hours.” 
Radioactive iodine (I-131) has a short half-life of only 8 days. After 8 days, half has decayed into non-radioactive elements; after 80 days, 99.9% of the radioactive iodine has decayed. So the danger from iodine is short-term. 
Caution: Please do not take potassium iodide (KI) if there is no danger of radioactive fallout in your area. KI can have side effects and can be harmful to your health. KI tablets provide what is basically a massive dose of iodine; this should be taken only in cases of an actual radiation emergency. Do not act out of fear. Use prudent judgment based on reason and knowledge.
But if you wish to take precautions in advance of possible fallout, you can supplement your diet with kelp capsules. This product is widely available as a health supplement. Kelp is a type of seaweed that is naturally high in iodine. This will provide you with the U.S. RDA for iodine, which is 150 mcg (micrograms, not milligrams) for persons ages 14 through adult.
by Ronald L. Conte Jr.
14 March 2011
 Merck Manual of Medical Information, Second Home Edition (unabridged), Mark Beers, editor; Section 13, Hormonal Disorders, Chapter 163, Thyroid Gland Disorders; (New York: Simon & Schuster, 2003), p. 948.
 Ibid., p. 954.
 FDA, ‘Guidance: Potassium Iodide as a Thyroid Blocking Agent in Radiation Emergencies’, U.S. Food and Drug Administration, December 2001; p. 4, PDF file.
 Ibid., FDA, p. 5.
 Ibid., FDA, p. 6.
 National Cancer Institute, ‘I-131’s Rapid Breakdown’, www.cancer.gov.