Nuclear accidents tend to progress in sequence gradually and can be categorized by their possible effects. There are 3 categories of the accidents have been defined for the purpose of screening activation timing of the various response mechanisms. In addition, for the public and the international community having a better understanding of the accident situations, the seven-level International Nuclear Event Scale is also adopted to explain its severity.

Category	Explanation
Alert	A worsening of the safety functions of a nuclear reactor facility but not yet requiring public protective action.
Site Area Emergency	A major loss, or possible major loss, of safety functions of a nuclear reactor facility with possible public protective action taken.
General Emergency	A serious, or possibly serious, deterioration of the reactor core of a nuclear facility with the possible loss of integrity of the containment structure requiring public protective actions.