The belt of high-level radioactive contamination that extended to the northwest of the stricken Fukushima No. 1 nuclear power plant was the result of weather conditions on the afternoon of March 15, analysis by the Japan Atomic Energy Agency has shown.
Rainfall subsequently deposited large quantities of radioactive substances, discharged from the crippled No. 2 reactor, onto the ground.
The radiation dose survey results, released Sept. 1 by the government, also showed that zones of high doses were concentrated in the vicinity of the nuclear plant and to the northwest of the plant.
In the no-entry zone, the area within a 20-km radius of the nuclear plant, the maximum dose at a 1-meter height above ground was 139 microsieverts per hour in the Ottozawa district of Okuma town, about 1 km southwest of the nuclear plant.
In the planned evacuation zone, designated along the belt of high contamination beyond that radius, the maximum dose was 41.3 microsieverts per hour in the Hirusone district of Namie town, about 22 km to the northwest.
Haruyasu Nagai, leader of the Research Group for Environmental Science at the JAEA, estimated that large quantities of radioactive substances were discharged from the crippled No. 2 reactor over two periods: between 7 am and 11 am and between 1 pm and 3 pm, both on March 15.
During the second spell of discharge in the afternoon, a cloud of radioactive substances accumulated in gas form drifted first to the west and gradually changed direction to the northwest.
“Weak winds and slow plume movements contributed to the high concentrations of radioactive substances, but the largest contribution came from the rain,” said Hiromi Yamazawa, a professor of radiation-related environmental safety at Nagoya University.
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