Radioactive cesium has formed hotspots on the seabed east of the Fukushima No. 1 nuclear power plant, the site of reactor meltdowns following the earthquake and tsunami disaster of March 2011, scientists said.
Researchers from the University of Tokyo, the National Maritime Research Institute and other entities said Aug. 7 the hotspots are likely a result of highly radioactive water that leaked from the crippled nuclear plant in April and May 2011 and subsequently sank into seabed depressions. They say they are the first to have measured the distribution of radioactive substances on the seabed off the hobbled plant.
Continual seabed monitoring could provide clues to contamination mechanisms and cesium migrations, said Blair Thornton, a project associate professor of ocean perception systems at the University of Tokyo's Institute of Industrial Science.
The researchers developed equipment for measuring seabed gamma ray levels, which they lowered from aboard a vessel, to study the distribution of radioactive substance levels on a continual basis on the seabed off the stricken plant.
They detected about 10 times the background cesium-137 concentrations in seabed depressions located 5.9 kilometers and 3.2 km off the coast hosting the nuclear plant.
Cesium levels measured about 500 becquerels per kilogram of seabed soil on average in one rocky area 1.6 km off the coast, but several spots in that area showed readings in excess of 5,000 becquerels, with the maximum as high as about 40,000 becquerels, the scientists said.
The hotspots are unrelated to the recent findings of radioactive water leaking into the sea, which became a major issue following a June announcement that high radioactive levels had been detected in groundwater from a monitoring well on the stricken plant site, the scientists added.
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