High accumulations of radioactive cesium-134 and cesium-137 have been detected in the town of Minami-Aizu, which lies 138 kilometers west-southwest of the stricken Fukushima No. 1 nuclear power plant.
Reinhabiting the large exclusion zone around the accident site may have to wait longer than expected.
Radioactive cesium isn’t disappearing from the environment as quickly as predicted after Chernobyl, according to new research presented here Monday at the meeting of the American Geophysical Union. Cesium 137’s half-life — the time it takes for half of a given amount of material to decay — is 30 years.
In addition to that, cesium-137’s total ecological half-life — the time for half the cesium to disappear from the local environment through processes such as migration, weathering, and removal by organisms is also typically 30 years or less.
This may have dreadful implications for Japan as they deal with the Fukushima nuclear disaster.
Japan releases new radiation maps
MEXT has repeatedly found caesium levels above 550 kBq/m2 in an area some 45 kilometres wide lying 30 to 50 kilometres north-west of the plant.
The highest was 6400 kBq/m2, about 35 kilometres away, while caesium reached 1816 kBq/m2 in Nihonmatsu City and 1752 kBq/m2 in the town of Kawamata, where iodine-131 levels of up to 12,560 kBq/m2 have also been measured.
“Some of the numbers are really high,” says Gerhard Proehl, head of assessment and management of environmental releases of radiation at the International Atomic Energy Agency.
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.
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.
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 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.
Tokyo Hotspot soil analysis, September 14, 2011:
The contaminated area also 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.
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.
The government has not disclosed the size of the area contaminated with cesium 137 released from the crippled Fukushima No. 1 plant.
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 science ministry released maps on Sept. 12 indicating the ground surface concentrations of the elements measured by aircraft.
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.
High accumulations were also found in the town of Tadami, about 152 km west of the plant.
The maps evaluated airborne measurements across Fukushima Prefecture, including the Aizu district, the western part of the prefecture where no data had previously been available. The measurements were made between Aug. 16 and 28.
Concentrations of cesium-137 were found at 30,000 becquerels per square meter or more in Minami-Aizu, Tadami, Yugawa and other municipalities.
At that rate of disintegration, John Emsley wrote in “Nature’s Building Blocks” (Oxford, 2001), “it takes over 200 years to reduce it to 1 percent of its former level.”
Cesium-137 mixes easily with water and is chemically similar to potassium. It thus mimics how potassium gets metabolized in the body and can enter through many foods, including milk.
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