BY TOMOYUKI YAMAMOTO STAFF WRITER (Asahi.com)
The level of radioactivity from contaminated water that leaked into the sea from the Fukushima No. 1 nuclear power plant is close to the highest levels ever recorded.
The water that leaked from a crack in a concrete wall near the No. 2 reactor building at the Fukushima No. 1 nuclear power plant is estimated to be 4,700 trillion becquerels, the Tokyo Electric Power Co. said April 21.
That figure is about 20,000 times higher than the permissible annual standard stipulated by the government. Officials said 520 tons of the contaminated water leaked into the ocean from the crippled Fukushima plant. The leakage of the highly contaminated water was discovered April 2 and was halted April 6.
The worst case in history took place in Sellafield on the coast of the Irish Sea in Cumbria, England, in the 1960s-70s. Cesium-137 with a radioactivity of 5,230 trillion becquerels a year was discharged into the Irish Sea from a nuclear fuel reprocessing factory at the peak year of 1975.
Although the radioactivity level in the water in the Fukushima disaster is lower than that of the Sellafield case, officials say it is still a serious situation because the radioactive substances were discharged into the sea over a much shorter period of time.
“Even if the sea contamination made through the air is factored in, the figure is still a little bit lower than that in Sellafield,” said Katsumi Hirose, former director of the Meteorological Research Institute’s Geochemical Research Department. “But this time, we have to pay attention to the fact that the radioactive substances were discharged in an extremely short period of time.”
As for the concentration of the contamination, about 200 becquerels per liter of water was recorded in the Irish Sea in 1974.
“In the sea 34 kilometers from the Fukushima No. 1 nuclear power plant, 186 becquerels per liter of water was recorded on April 15. The figure is close to the 200 becquerels found in the Irish Sea,” Hirose said.
Radioactive substances discharged into the ocean are diluted by seawater. However, they do not necessarily spread evenly. There is a possibility that a water mass with a high concentration of radioactive material still exists.
Hirose said the observation area should be expanded to sea areas in a radius of 100 kilometers, centering on the coast off Fukushima, and that observation points should be increased drastically.
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