A photograph taken from an Asahi Shimbun helicopter shows the Edogawa river emptying into Tokyo Bay.
June 7, 2018
Radioactive cesium from the crippled Fukushima No. 1 nuclear power plant continued to flow into Tokyo Bay for five years after the disaster unfolded in March 2011, according to a researcher.
Hideo Yamazaki, a former professor of environmental analysis at Kinki University, led the study on hazardous materials that spewed from the nuclear plant after it was hit by the Great East Japan Earthquake and tsunami on March 11, 2011.
Five months after disaster caused the triple meltdown at the plant, Yamazaki detected 20,100 becquerels of cesium per square meter in mud collected at the mouth of the Kyu-Edogawa river, which empties into Tokyo Bay.
In July 2016, the study team detected a maximum 104,000 becquerels of cesium per square meter from mud collected in the same area of the bay, Yamazaki said.
He said cesium released in the early stages of the Fukushima disaster remained on the ground upstream of the river, such as in Chiba Prefecture. The radioactive substances were eventually washed into the river and carried to Tokyo Bay, where they accumulated in the mud, he said.
On a per kilogram basis, the maximum level of radioactivity of cesium detected in mud that was dried in the July 2016 study was 350 becquerels.
The government says soil with 8,000 becquerels or lower of radioactive cesium per kilogram can be used in road construction and other purposes.
The amount of radioactive cesium in fish in Tokyo remains lower than 100 becquerels per kilogram, the national safety standard for consumption.