Potassium Chlorate

Chlorine bleach must be diluted or it will eat holes in clothing. - 121

The U.S. Centers for Disease Control says that potassium iodide pills will keep the thyroid gland from absorbing radioactive iodine.
It cannot protect other parts of the body from radiation.
Side effects can include nausea, upset stomach, rashes, inflammation of the salivary glands and possibly severe allergic reactions. - 482