What is a nuclear accident?

A nuclear accident is an event in which there is a loss of control of radioactive materials with a possibility of radioactive poisoning (there is a related link below). It is not the same as a radiation accident, which is very limited in nature.
There are a number of causes of such events, ranging from equipment failure and human error to loss of coolant or spillage in an accident in transportation. A major nuclear accident happens when the core of a nuclear reactor is damaged by a meltdown or partial meltdown. But there are other types of nuclear accidents, including loss of radiological materials intended for medical use, as happened in the GoiĆ¢nia accident, in Brazil.
Nuclear accidents are rated according to an International Nuclear Events Scale (see related link below). This scale rates events on a scale of 1 to 7, 7 being worst, and numbers 4 through 7 are for accidents. An accident with local consequences is rated at number 4. Number 5 is an accident with wider consequences, and the Three Mile Island accident was of this type. Number 6 is assigned to what is called a Serious Accident. Number 7 is assigned to a Major Accident, and the Chernobyl Disaster was of this type.