Duration of Iodine Emissions

Fission is not continuing uncontrolled. The reason the reactors remain at a high temperature is because all the radiation energy from the fission products produces a large amount of heat ("decay heat"), and particularly because the water cooling system failed which allowed the heat to accumulate. If the fission was continuing uncontrolled ("supercriticality"), the reactor would immediately explode from the tremendous release of energy of the uncontrolled chain reaction, like what happened at Chernobyl. Although there seem to be a lot of problems including fires and partial melting of fuel, these are caused by the decay heat and the lack of active cooling system.

(Disclaimer: Although I am a Nuclear Engineering grad student, reactors are not my expertise, so if I have misstated something please correct me.)