Rain to blame for March 15 radiation spike in Fukushima

September 08, 2011


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 contamination levels would have been much lower if it had not been for the rain, the researchers said.

The belt extends about 40 kilometers to the northwest of the nuclear plant and runs through the town of Namie and Iitate village. 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.

Areas 40 percent the size of Tokyo, or 800 square km, were contaminated above the threshold level for compulsory evacuation following the 1986 Chernobyl accident.

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 a.m. and 11 a.m. and between 1 p.m. and 3 p.m., 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.

It is thought that rainfall then deposited radioactive substances onto the ground and created the belt of high contamination. According to the Japan Meteorological Agency, precipitation of 0.5-1.5 millimeters per hour was recorded for a duration of about half a day starting at 5 p.m. in Iitate village.

"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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