16 December 2011 Last updated at 10:11
How does Fukushima differ from Chernobyl?
Japanese authorities have raised the severity rating of the nuclear crisis at the damaged Fukushima Daiichi power plant to the highest level, seven.
The decision reflects the ongoing release of radiation, rather than a sudden deterioration. Level seven previously only applied to the 1986 Chernobyl disaster, where 10 times as much radiation was emitted.
But most experts agree the two nuclear incidents are very different. Explore the table below to find out how they compare.