3 Avril 2012
April 3, 2012
Radioactive cesium up to 100 times pre-nuclear disaster levels has been detected in plankton inhabiting the sea far from the crippled nuclear plant following the March 2011 disaster, according to a survey conducted by Japanese and U.S. researchers.
The high concentration of cesium, which is believed to derive from the Fukushima No. 1 Nuclear Power Plant, suggests that radioactive substances that have leaked from the complex are spreading extensively in the sea.
Jun Nishikawa, research associate with the University of Tokyo's Atmosphere and Ocean Research Institute, underscored the need for a long-term survey on the contamination of marine creatures with radioactive substances.
"Even though radiation levels detected from the plankton samples were still low, there is a possibility that large amounts of cesium will accumulate in fish through the food chain in a phenomenon called biological concentration. We need to continue our survey," he said. "Each species of marine creatures that feed on animal plankton need to be monitored over the long term."
The results of the survey were published in the Proceedings of the National Academy of Sciences of the United States on April 3.
In the survey, Nishikawa and other researchers including those with U.S. Woods Hole Oceanographic Institution collected samples of sea water and animal plankton at about 60 locations in the sea some 30 to 600 kilometers off the crippled plant in June last year, and measured the levels of radioactive cesium in them.
Radioactive cesium was detected in at least one sample taken at each of the locations.
The largest amount of radioactive cesium in animal plankton was found in a sample collected at a location 300 kilometers from the power plant -- at 102 becquerels of cesium-134 and cesium-137 per kilogram in dry weight. This compares with the average amount before the accident, which stood at 0.1 to 1 becquerel of only cesium-137.
The small amount of cesium in plankton -- 0.3 becquerels per kilogram -- was found in a sample taken at a location 600 kilometers off the plant.
The largest volume of cesium in sea water was found in a sample collected 100 kilometers from the plant, at 7,733 becquerels per cubic meter.
In the survey, radioactive cesium was hardly found in samples south of the black current, which flows south of Fukushima and meanders eastward off the Boso Peninsula, leading researchers to believe that the current blocked the spread of radioactive substances south.