■Is there risk for cancer?
When thyroid cells absorb too much radioactive iodine ― either through the air or through contaminated food ― it can increase the risk for thyroid cancer, says the American Thyroid Association. Babies and young children are at highest risk as their thyroid glands are most radiation-sensitive. People over 40 are at less risk.
The massive explosion at Chernobyl in 1986 caused an epidemic of thyroid cancer and increases in leukemia rates. By comparison, the partial core meltdown at Three Mile Island in 1979, which released about one million times less radiation, according to the New York Times, did not appear to impact cancer rates even decades later.
The radiation release in Japan appears to be much closer to that of the Three Mile Island incident than Chernobyl.
■Wikipedia "Potassium Iodide"
WHO does not recommend potassium iodide (KI) prophylaxis for adults over 40 years, unless inhaled radiation dose levels are expected to threaten thyroid function; ... Such radiation doses will not occur far away from an accident site. (from Wikipedia)
■Fukushima Daiichi No.3 reactor uses mixed uranium-plutonium fuel: further damage on it could cause a serious hazard.