What do the numbers indicate?

Cracked by Bad Ass Thambi and aamaadmi!! Brilliant!Answer : International Nuclear Event Scale (INES)
There are 7 levels on the INES scale; 3 incident-levels and 4 accident-levels. Each increasing level represents an accident ten times more severe than the previous level.
7 – Major Accident (Chernobyl, 1986)
6 – Serious Accident (Kyshtym disaster at Mayak, 1957)
5 – Accident With Wider Consequences (Three Mile Island accident, 1979)
4 – Accident With Local Consequences (St.Laurent plant, France, 1980)
3 – Serious Incident (Paks plant, Hungary, 2003)
2 – Incident (Forsmark plant, Sweden)
1 – Anomaly (SOCATRI, France, 2008)