JAEA finally tracks contamination in immediate vicinity of Fukushima plant

September 24, 2013

By HISASHI HATTORI/ Senior Staff Writer

The Japan Atomic Energy Agency used an unmanned helicopter to crack a missing piece of the puzzle regarding radiation contamination close to the wrecked Fukushima No. 1 nuclear power plant.

It has finally come up with the first detailed study of radiation levels within a 3-kilometer radius of the center of the plant. The zone has been inaccessible to scientists because of the three reactor meltdowns triggered by the 2011 earthquake and tsunami disaster.

The JAEA measured radiation levels within 3 km of the stricken plant between August and October last year, and detected readings of more than 19 microsieverts per hour south and west of the nuclear plant.

The government-affiliated research body said radiation levels remained relatively high northwest of the facility as well.

It also investigated radiation levels above ground within an 80-km radius of the plant, and found that the extent of contamination has fallen 36 percent from a previous survey.

The agency regularly complied contamination maps inside an 80-km radius of the plant based on cumulative doses at 10,000 locations.

As of last December, cumulative doses in the area had declined by 36 percent from June 2011. This was largely due to a sharp decrease in levels of cesium-134, which has a half-life of two years, according to the JAEA.

The research institute said 29 percentage points of the observed 36-percent fall in cumulative doses was due to the decrease in cesium-134 levels, while cumulative doses declined a further 8 points because rain washed away radioactive substances on the ground and helped them seep into the soil.

However, annual radiation levels of 1 millisievert or more were detected in 60 percent of the area.

Cumulative doses on streets among houses and other buildings decreased to 35 percent of the previous measurement, although cumulative doses on roads in forest areas fell to 44 percent of the previous survey. That is apparently because decontamination work has been more intensively carried out in residential areas than in woodland regions.

By HISASHI HATTORI/ Senior Staff Writer
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