An assessment on the environmental contamination caused by the Fukushima accident

The radiological releases from the damaged fuel to the atmosphere and into the cooling water in the Fukushima Daiich Nuclear Power Plant (FDNPP) accident are investigated. Atmospheric releases to the land and ocean mostly occurred during the first week after the accident whereas continuous release from the damaged fuel into the cooling water resulted in an accumulation of contaminated water in the plant during last six years. An evaluation of measurement data and analytical model for the release of radionuclides indicated that atmospheric releases were mainly governed by the volatility of the radionuclides. Using the measurement data on the contaminated water, the mechanism for the release of long-lived radionuclides into the cooling water was analyzed. It was found that the radioactivity concentrations of ⁹⁰Sr in the contaminated water in the Primary Containment Vessel (PCV) of unit 2 and unit 3 were consistently higher than that of ¹³⁷Cs and the radioactivity concentration of ⁹⁰Sr in the turbine building of unit 1 in year 2015 was higher than that in year 2011. It was also observed that the radioactivity concentration of long-lived radionuclides in the contaminated water in the FDNPP is still high even in year 2015. The activity ratio of ²³⁸Pu/^239+240Pu for the contaminated water was in the range of 1.7-5.4, which was significantly different from the ratios from the soil samples representing the atmospheric releases of FDNPP. It is concluded that the release mechanisms into the atmosphere and cooling water are clearly different and there has been significant amount of long-lived radionuclides released into the contaminated water.