The explosion at the Fukushima No.1 nuclear power plant in 2011 emitted a large of contaminated materials that covered most of Fukushima Prefecture with radioactive substances.In this case,this catastrophe is referred to as the “nuclear disaster” to distinguish it from the related tsunami and earthquake.As a result of the explosion,radioactive substances of more than 5 mSv/year were observed,which forced the inhabitants within the 20 km zone to evacuate into surrounding areas.