Exposure to radiation short term and infrequently does not do significant harm. Chronic long term exposure to even lower levels can cause significant cellular damage leading to mutations and cancer.
There are certain nutrients which protect the cells from the compounds making up radiation such as: cesium-137 and radioactive iodine. Potassium Iodide (KI) is most well-known to protect the thyroid from radioactive iodine. Given that most Americans are Iodine deficient, supplementing with 200 mcg a day of Potassium Iodide may do the general population some good in general - even without radioactive fallout concerns. Over half of pregnant women in the USA are iodine deficient which sets up the newborn to mental retardation. However, potassium iodide is by far not the only nutrient needed to protect the body from radioactive fallout.
Complete trace minerals are needed. When the body is deficient in minerals, the more apt it is to absorb heavy metals and radiation. Sulfur compounds such as n-acetyl-l-cysteine and MSM are also useful in protecting the liver, lungs and kidneys.
Taking hot Epsom salt baths to help sweat and pull out heavy metals is good. Adding Moor Mud to enhance sweating and detoxification is even better. Make sure to replace electrolytes and additional minerals after profuse sweating.
Probiotics are known to protect the intestines from radiation-induced diarrhea and likey also help prevent absorption of heavy metals found in radioactive fallout. Supplementing with L-Glutamine to prevent damage and/or heal the small intestine may be necessary. Sodium Butyrate or Cal/Mag Butyrate may be needed to help heal the large intestine from radiation-induced intestinal and colon damage.
Greens such as spirulina, wheat grass, chlorella along with Miso, tempeh, kombu and other seaweeds are excellent detoxifiers and supporters of healthy iodine levels.
Questions about Radiation Exposure? Please post at the Radiation Exposure Forum and your questions will be answered.
The Radiation Exposure Forum has some excellent documents available for download to help you prepare further. Do visit this forum and download. Again, questions, do ask.