The giant tsunami that hit the Fukushima power plant on March 11 damaged four of the six reactors at the site.
A series of fires and hydrogen gas explosions followed when the plant’s cooling systems failed.
Engineers were forced to use seawater in an effort to cool the overheated reactor cores but this did not stop the radioactive emissions.
The accident was first classified as level 4. It was later raised to 5 and from Tuesday moved to level 7 – the highest on the scale.
The International Nuclear and Radiological Event Scale measures the severity of radioactive incidents and accidents ranging from no safety significance at zero to a major incident – having widespread health and environmental effects at level seven.
The scale is designed so the severity of an event is about ten times greater for each increase in level on the scale.
On April 26 1986, one of four reactors exploded after an experiment at the Chernobyl power plant in central Ukraine.
About 200 tonnes of radioactive gas was released, 130,000 people were evacuated and 31 lost their lives. Even after 25 years no-one in Ukraine is allowed to live within 30km of the plant.
Rated as level 5 in 1979, the plant near the Pennsylvania state created worldwide headlines when one of its reactor cores melted, resulting in the release of radioactive gases. There were no deaths but 140,000 people were evacuated.
The incident made Three Mile Island synonymous with the dangers of nuclear power and helped slow the expansion of the American nuclear industry.
Another level five incident took place in the Brazilian city of Goiania in September 1987. Five people died and 240 were injured. This was caused after a radioactive treatment machine was left abandoned.
In 1999 the level four accident at the Tokaimura power plant in Japan had local consequences. Two workers died and 438 people were left exposed to radiation after a uranium leak.
In August 2008 the discharge of radioactive Iodine into a stream by a laboratory in Belgium effected fruit and vegetable gardens surrounding a 5km area.