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Cesium hotspots in seabed

August 8, 2013


Cesium hotspots found in seabed east of Fukushima






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.



Levels of radioactive substance on seabed surveyed



Researchers say the concentration levels of a radioactive substance are very high in the seabed near the crippled Fukushima Daiichi nuclear power plant that had an accident in 2011.

The researchers are from the University of Tokyo's Institute of Industrial Science. They spent a year, ending in July, measuring the concentration levels of radioactive cesium 137 in the mud in the seabed for 400 kilometers off Fukushima Prefecture.

They found that within 20 kilometers of the nuclear plant the radioactive cesium levels at 40 locations were more than 5 times the surrounding areas.

The researchers explain that these locations coincide with dips in the seabed.

They also say radioactive cesium levels near the mouth of the Abukuma River in Miyagi Prefecture, about 70 kilometers from the plant, were more than twice those of surrounding areas.

Researchers note radioactive cesium discharged by the nuclear plant disaster probably flowed into the river and was carried into the sea.

Project Associate Professor Blair Thornton at the Institute of Industrial Science says the researchers have been able to identify places where radioactive substances are likely to build up.

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