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Determination and Comparison of the Strontium-90 Concentrations in Topsoil of Fukushima Prefecture before and after the Fukushima Daiichi Nuclear Accident

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Abstract
To precisely understand the status of scattered strontium-90 (90Sr) after the 2011 accident at the Fukushima Daiichi Nuclear Power Plant (F1-NPP) of Tokyo Electric Power Company (TEPCO), the measurement of the soil samples collected both before and after the day of the accident from the same sampling locations is necessary. However, very few reports have investigated the background contaminant data before the accident even though several studies have been conducted to investigate the effects of the F1-NPP accident. To address the lack of the passed 90Sr information and reestablished baseline, this study focuses on the stored topsoil samples that are collected from the same sampling locations from the Fukushima Prefecture before and after the F1-NPP accident, which are analyzed for obtaining the 90Sr concentrations. The results of our investigation exhibited that the 90Sr concentrations in the Fukushima Prefecture soils ranged from 0.2 to 20.4 Bq/kg in the samples that were collected before the accident and from 1.37 to 80.8 Bq/kg in the samples that were collected after the accident from identical sampling locations. Further, the soil samples that were collected from 30 out of 56 locations displayed significant differences in terms of concentrations before and after the accident. In addition, the relations between the 90Sr concentrations and the soil properties of the samples (organic content, pH, water content, and composition) were investigated, and it was found that the organic content and water content had a positive correlation with 90Sr concentrations and, in contrast, the sandiness was shown to have a negative correlation with 90Sr concentrations. The depth characteristics were also investigated. The aforementioned results indicate that this tendency would be observed even in the future.
 
Introduction
 
A large amount of radioactive materials was scattered throughout the environment (ocean, atmosphere, land, and so on) because of the accident that occurred on March 11, 2011 at the Fukushima Daiichi Nuclear Power Plant (F1-NPP) that was owned by Tokyo Electric Power Company Holdings, Inc. (TEPCO).(1−3) Seven years have passed by since the accident, and research institutes around the world have been monitoring the influence of the environmental dynamics of radionuclides that have been released.(4−13) More specifically, there have been several environmental monitoring reports regarding β-ray-emitting nuclides, such as radioiodine and radiocesium, because multiple samples can be analyzed in a relatively short time using certain types of instruments such as a germanium semiconductor detector, a sodium iodide scintillator detector, and a lantern bromide scintillator detector.(14−19) Meanwhile, radiostrontium (90Sr) (half-life: 28.79 y(20)) is a pure β-ray-emitting nuclide that does not emit γ-rays, which makes it necessary to chemically isolate it for measuring β-rays because the β-ray spectra overlap. In particular, it is imperative to monitor 90Sr over a long period because it will require several decades to decommission F1-NPP. In Japan, instead of a few literature concerning the development of a rapid analytical means,(21−25) radiochemical analysis using milking-low background gas-flow counter (milking-LBC) is adopted as the official analysis method for analyzing 90Sr because of good sensitivity and/or high-precision analysis in low concentration levels in the environment.(26) This method requires considerable amount of time and effort to pretreat the analysis as compared to those required by the γ-ray measurement method. Although various studies have been vigorously conducted,(27−33) the study related to the scattering of 90Sr is not as advanced as compared to that related to the γ-ray-emitting nuclides such as radiocesium.
To precisely understand the status of scattered 90Sr after an incident of nuclear accident, the samples collected both before and after the day of the accident should be measured, thereby distinguishing from the fallout of atmospheric nuclear tests (20th century’s) that have been conducted in the past. So far, the survival ratios of nuclides with short half-lives in samples have been employed in several studies.(34) However, this technique cannot track the long-term process because it becomes difficult to evaluate the nuclides that exhibit a short decrease in half-lives. The optimal method for addressing these issues is to measure the radioactive concentrations of 90Sr in soil that is collected at identical locations before and after the accident. However, few examples exhibited the presence of 90Sr in soil before the F1-NPP accident, which was completely unexpected. Fortunately, we already possessed analytical data related to the 90Sr concentrations in soil samples that were collected before the accident with precise sampling locations throughout the Fukushima Prefecture (not published). Therefore, in this study, we succeeded in estimating the exact amount of 90Sr deposition before and after the F1-NPP accident. When performing the long-term observation, understanding the background level of 90Sr before the accident was observed to be considerably important for understanding the environmental radioactivity and the environmental dynamics or the usage of 90Sr as a tracer.
In this study, we measured the radioactivity concentrations of 90Sr in the topsoil at the same locations in the Fukushima Prefecture before and after the accident and obtained the background levels of 90Sr before the F1-NPP accident. Thus, we revealed the deposition status of 90Sr before and after the accident. We also investigated the correlation between the soil properties and 90Sr to determine the status of deposition of 90Sr on the topsoil in Fukushima prefecture (Figure 1).
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December 27, 2018 Posted by | Fukushima 2018 | , , , | Leave a comment