Abstract
Highly radioactive cesium-rich microparticles (CsMPs) were released from the Fukushima Daiichi nuclear power plant (FDNPP) to the surrounding environment at an early stage of the nuclear disaster in March of 2011; however, the quantity of released CsMPs remains undetermined. Here, we report a novel method to quantify the number of CsMPs in surface soils at or around Fukushima and the fraction of radioactivity they contribute, which we call "quantification of CsMPs" (QCP) and is based on autoradiography. Here, photostimulated luminescence (PSL) is linearly correlated to the radioactivity of various microparticles, with a regression coefficient of 0.0523 becquerel/PSL/h (Bq/PSL/h). In soil collected from Nagadoro, Fukushima, Japan, CsMPs were detected in soil sieved with a 114 μm mesh. There was no overlap between the radioactivities of CsMPs and clay particles adsorbing Cs. Based on the distribution of radioactivity of CsMPs, the threshold radioactivity of CsMPs in the size fraction of <114 μm was determined to be 0.06 Bq. Based on this method, the number and radioactivity fraction of CsMPs in four surface soils collected from the vicinity of the FDNPP were determined to be 48-318 particles per gram and 8.53-31.8%, respectively. The QCP method is applicable to soils with a total radioactivity as high as ∼106 Bq/kg. This novel method is critically important and can be used to quantitatively understand the distribution and migration of the highly radioactive CsMPs in near-surface environments surrounding Fukushima.
Highlights
The nuclear disaster that occurred after the great east Japan earthquake in March of 2011 caused serious damage to the nuclear reactor unit nos. 1−4 at the Fukushima Daiichi Nuclear Power Plant (FDNPP),[1] which released a total of ∼1019 Bq of radioactivity associated with radionuclides such as 133Xe, 132Te, 131I, and 137Cs.[2]
A linear correlation between the photostimulated luminescence (PSL) intensity and the exposure time was obtained for the measurement of the Cs-rich microparticles (CsMPs) (OTZ7) when the PSL intensity was less than 10 000 (Figure 2b); the PSL plots deviated from the regression line of the correlation when the PSL intensity exceeded 10 000, which is approximately consistent with the maximum counts of the dynamic range of the imaging plate as certified by the manufacturer
In Koarashi et al.,[36] assuming that all Cs is associated with clay fraction, the ∼60 kBq/kg maximum radioactivity corresponds to only 3−12 Bq/g clay based on their volume percentage of 5−20%
Summary
The nuclear disaster that occurred after the great east Japan earthquake in March of 2011 caused serious damage to the nuclear reactor unit nos. 1−4 at the Fukushima Daiichi Nuclear Power Plant (FDNPP),[1] which released a total of ∼1019 Bq of radioactivity associated with radionuclides such as 133Xe, 132Te, 131I, and 137Cs.[2]. 1−4 at the Fukushima Daiichi Nuclear Power Plant (FDNPP),[1] which released a total of ∼1019 Bq of radioactivity associated with radionuclides such as 133Xe, 132Te, 131I, and 137Cs.[2] Of these radionuclides, 1015−1016 Bq of 137Cs was released to the atmosphere and dispersed to the surrounding environment.[3−5] Even though the fraction of the released Cs was as much as ∼7% of the total inventory,[3] radioactive Cs is currently responsible for most the radiation dose in contaminated areas due to its half-lives of 2.065 years and 30.17 years for 134Cs and 137Cs, respectively.[6] The areas that were subject to Cs contamination span over 15 prefectures including the metropolitan city of Tokyo.[7−10] The chemical forms of Cs released from the reactor were identified to be soluble hydroxides and chloride[11] in addition to possibly. The intrinsic forms of soluble Cs and the soluble Cs adsorbed onto the aerosols were considered to have been deposited on the ground in the large area mentioned above by precipitation, primarily during the periods of March 15th and 16th in the northern Fukushima Prefecture and March 21st− 23rd in the Ibaraki, Chiba, Tochigi, and Saitama prefectures and Tokyo.[8,16]
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