Radiation levels have dropped by 40 percent on average in each of the four types of evacuation zones around the crippled Fukushima No. 1 nuclear power plant, the government's nuclear watchdog said.
The Nuclear Regulation Authority said June 5 it used airborne radiation measurement data to estimate variations in air dose rates over the 17-month period from November 2011 to March 2013. It is the first time that the NRA has released such estimates by zoning category.
NRA officials attributed the decrease partly to physical decay and partly to rainwater washing radioactive materials into the ocean.
The Fukushima nuclear disaster was triggered by the Great East Japan Earthquake and tsunami in March 2011.
In the "difficult-to-return zones," where evacuees are not allowed to return for at least five years counting from March 2012, the average radiation level fell from 14.5 microsieverts per hour to 8.5 microsieverts per hour. The annual dose estimates in November 2011 exceeded 100 millisieverts in 27 percent of the land area of the difficult-to-return zones, but that ratio had dropped to only 6 percent by March 2013.
The average dose rate declined from 5.7 microsieverts per hour to 3.4 microsieverts per hour in the "no-residence zones," where evacuees are expected to be able to return in a few years.
In the "zones preparing for the evacuation order to be lifted," where evacuees are expected to be able to return sooner, that figure dropped from 2.0 microsieverts per hour to 1.1 microsieverts per hour.
The average radiation level fell from 2.7 microsieverts per hour to 1.5 microsieverts per hour in the "planned evacuation zone," a high-risk zone to the northwest of the plant and outside the 20-kilometer radius that is yet to be reclassified into any of the three other categories.
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