An extensive area of more than 8,000 square kilometers has accumulated cesium 137 levels of 30,000 becquerels per square meter or more after the accident at the Fukushima No. 1 nuclear power plant, according to Asahi Shimbun estimates.
The affected area is one-18th of about 145,000 square kilometers contaminated with cesium 137 levels of 37,000 becquerels per square meter or more following the 1986 Chernobyl accident in the former Soviet Union.
The contaminated area includes about 6,000 square kilometers in Fukushima Prefecture, or nearly half of the prefecture. Fukushima Prefecture, the third largest in Japan, covers 13,782 square kilometers.
The government has not disclosed the size of the area contaminated with cesium 137 released from the crippled Fukushima No. 1 plant. Cesium 137 has a long half-life of about 30 years.
The Asahi Shimbun calculated the size of the contaminated area based on a distribution map of accumulated cesium 137 levels measured from aircraft, which was released by the science ministry on Sept. 8.
The estimated size may increase in the future because the distribution map will be subject to corrections and because it currently covers only five prefectures.
The contaminated area includes about 1,370 square kilometers in northern Tochigi Prefecture, about 380 square kilometers in southern Miyagi Prefecture and about 260 square kilometers in Ibaraki Prefecture.
In the Chernobyl accident, a vapor explosion occurred at the No. 4 reactor during an experiment, spewing radioactive materials contained in fuel up thousands of meters into the atmosphere, which spread over Europe and other areas in the Northern Hemisphere.
The amount of radioactive materials released from the Fukushima No. 1 plant was relatively limited because the reactors were damaged but still sealed to some extent.
After the Chernobyl accident, cesium 137 levels of 37,000 becquerels per square meter or more were accumulated in an area about 145,000 square kilometers, or equivalent to about 40 percent of Japan, according to the International Atomic Energy Agency.
The science ministry's distribution map only shows the area contaminated with 30,000 becquerels of cesium 137 or more, not detailed breakdowns such as 37,000 becquerels.
The no-entry zone and the planned evacuation zone around the Fukushima No. 1 plant total about 1,100 square kilometers, affecting about 85,000 residents.
In the planned evacuation zone, the government has called on residents to leave on the grounds that radiation levels will exceed 20 millisieverts a year.
After the Chernobyl accident, a highly contaminated area with cesium 137 levels exceeding 550,000 becquerels per square meter was designated as a forced migration zone.
It stretched over about 10,300 square kilometers in the current Belarus, Russia and Ukraine, and an estimated 400,000 residents evacuated, including those outside the zone.
According to the science ministry's distribution map, about 600 square kilometers around the Fukushima No. 1 plant was contaminated with cesium 137 levels of 600,000 becquerels or more. The area is one-17th the forced migration zone around the Chernobyl plant.
Radiation levels are still high in the area. Measurements in six locations in the town of Namie, Fukushima Prefecture, which are 24 to 31 kilometers from the Fukushima No. 1 plant, ranged between 4.5 and 32.6 microsieverts per hour on Sept. 9.
The accidents at the Fukushima No. 1 plant and the Chernobyl plant are both rated the worst level 7 on the International Nuclear Event Scale because the quantities of radioactive materials released exceeded several tens of thousands of terabecquerels.
The amount of radioactive materials released into the atmosphere from the No. 1 to 3 reactors at the Fukushima No. 1 plant is estimated to be 770,000 terabecquerels.
The amount is about one-seventh the 5.2 million terabecquerels released into the atmosphere from the Chernobyl plant over 10 days.
Of various radioactive materials, the amount of cesium 137 was 15,000 terabecquerels in the Fukushima accident, about one-sixth the 85,000 terabecquerels in the Chernobyl accident.
Cesium 137 levels of the atomic bomb dropped on Hiroshima were far lower, at only 89 terabecquerels.
An official at the Nuclear and Industrial Safety Agency said a nuclear plant accident is substantially different in nature from an atomic bomb.
Radiation accounts for only 15 percent of the energy of an atomic bomb, with the remaining 85 percent resulting in the blast and heated air.
After the Fukushima accident, people will be exposed to low levels of radiation over a long term, whereas atomic bomb victims were immediately exposed to high levels of radiation and suffered acute symptoms, such as diarrhea, fever and hair loss.
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