I found this 1979 article from the Washington Post
which describes what happened on day 3 at Three Mile Island. A lot of very interesting data and it sounds errily similar to what were are presently seeing in Japan.
Because of the danger of possible radiation contamination, Watson's office authorized the Food and Drug Administration to contract for the manufacture, packaging and shipping to Harrisburg of 240,000 one-ounce vials of potassium iodide. It could be administered orally to collect in the thyroid, hopefully saturating the gland with this non-radioactive and non-cancer-causing agent before any radioactive iodine could reach it.
Falling coolant levels in the core exposed the top of the fuel rods. Unprotected by cooling water, the cladding on the outside of the fuel rods heated up rapidly. The zirconium in the cladding oxidized, releasing more heat, which in turn ballooned and split the cladding, allowing radioactive gases like xenon-133, krypton-85 and iodine131 to seep out through the cracks.
The gas bubble containing hydrogen, 1,000 cubic feet in size, at the top of the reactor. The reactor had become so hot that the coolant water had decomposed into its primary elements: oxygen and hydrogen.
The biggest danger was the possibility that the bubble would continue to grow, forcing all the coolant water out of the reactor, allowing the temperature of the fuel rods to build up until they reached 5,000 degrees.
At that heat, the uranium would begin to melt.
The lethal dose is described as 400 rems, but the sick, the elderly, young and unborn children could easily die from a dose of 150 rems. A dose that strong could begin to kill bone marrow so fast that death might follow in a matter of months.
To many scientists, the worst consequence of an overdose of radioiodine is not the lethal dose a few might get. It is the non-lethal dose which would concentrate in the thyroid gland in the throat, where radiation might produce tumors in thousands of people over a period of 30 years.