To date, there has been one Level 6 accident: Significant release of radioactive material likely to require implementation of planned countermeasures.(Initially rated as Level 5, upgraded later to 7.) A temporary exclusion zone of 20 kilometres (12 mi) was established around the plant, and officials considered evacuating Tokyo, Japan's capital and the world's most populous metropolitan area, 225 kilometres (140 mi) away. Major damage to the backup power and containment systems caused by the 2011 Tōhoku earthquake and tsunami resulted in overheating and leaking from some of the Fukushima I nuclear plant's reactors. Fukushima Daiichi nuclear disaster, a series of events beginning on 11 March 2011.49,400) was completely abandoned, and a 30 kilometres (19 mi) exclusion zone around the reactor was established. 14,000) was largely abandoned, the larger city of Pripyat (pop.

As a result of the plumes of radioisotopes, the city of Chernobyl (pop. Unsafe conditions during a test procedure resulted in a criticality accident, leading to a powerful steam explosion and fire that released a significant fraction of core material into the environment, resulting in an eventual death toll of 4,000-27,000. To date, there have been two Level 7 accidents: