It was introduce by the International Atomic Energy Agency or in short ( IAEA) in order to enable prompt communication of safety- significant information in case of nuclear accidents.

I don't know that there is a technical definition of a nuclear disaster. There are two accidents that are commonly referred to as disasters. One is the well known Chernobyl disaster, and the other is the Kyshtym disaster, both of which were in the former USSR. The Windscale fire, Three Mile Island Accident, and other problems are referred to as accidents.

There is an International Nuclear Event Scale, which rates events on a scale of 1 (least important) to 7 (most important). On this scale, the Chernobyl Disaster is rated at a 7, and the Kyshtym Disaster is rated at 6. Three Mile Island Accident is rated at 5, as is the Windscale Fire, and two other accidents.

There is a link below to an article on the International Nuclear Event Scale.

No, not according to an Forbes article that states Chernobyl disaster (level 7 on International Nuclear and Radiological Event Scale) was magnitudes worse than the 2011 Fukushima nuclear disaster in Japan, which was a level 4 and likely be upgraded to a level 5.

The International Nuclear Event Scale (INES) is a means for communicating in consistent terms the safety significance of nuclear and radiological events.

Events are classified on the scale at seven levels; the upper levels (4-7) are termed 'accidents' and the lower levels (1-3) 'incidents'.

The Chernobyl nuclear accident is widely regarded as the worst accident in the history of nuclear power. It is the only nuclear accident that has been classified a "major accident" by the International Atomic Energy Agency and rated 7 on the INES.

The Soviet Union was also home to the second-most disastrous nuclear accident, at the Mayak Nuclear Power Plant near the city of Kyshtym, classified as a Level 6 Disaster, which is a "serious accident."

international scale