The document discusses nuclear accidents and their severity levels according to the International Nuclear Event Scale (INES). It describes seven levels of severity, ranging from minor accidents with no safety impact (Level 1) to major accidents resulting in widespread health and environmental effects (Level 7). Only three events have been rated Level 7 - Chernobyl in 1986, Fukushima in 2011, and the Kyshtym disaster in 1957. Accidents are measured based on their impacts on people, the environment, and radioactive damage and exposure.