1.   KI stops that by flooding the thyroid with normal iodine, leaving no room for any more.

2.   One safeguard is iodine pills, based on the principle that normal iodine will get to the thyroid first and block the radioactive iodine.

3.   Potassium iodide works by saturating the human thyroid gland with normal iodine so it cannot absorb radioactive iodine.

4.   Released from a reactor, it can be absorbed directly by people, where it is naturally concentrated in the thyroid, just as normal iodine is.