No abstract

For the assessment of Fukushima Daiichi Nuclear Power Plant accident, the applicability of the thermal ionization mass spectrometry (TIMS), which is a type of mass spectrometry, was studied. For the study of the recovery/analysis method of cesium and strontium, at first, the radioactive cesium and strontium were generated by the irradiation of natural uranium at KUR. After this study, the applicability of this method to the environmental samples obtained in Fukushima prefecture was verified.

To precisely understand the status of scattered strontium-90 ( 90 Sr) 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 90 Sr 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 90 Sr concentrations. The results of our investigation exhibited that the 90 Sr 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 90 Sr 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 90 Sr concentrations and, in contrast, the sandiness was shown to have a negative correlation with 90 Sr concentrations. The depth characteristics were also investigated. The aforementioned results indicate that this tendency would be observed even in the future.

A radiometric quantitative methodology of Sr in seawater was developed using a measurement of theY decay time following iron-barium co-precipitation. With calculations of its decay time, the radioactivity of Sr can be indirectly determined under conditional environmental samples. In addition, to avoid the interference of other radionuclide, the prepared samples were measured using a germanium semi-conductivity detector; then, the deposited radioactivity was subtracted from the actual measurement values of beta-ray counting. In this paper, the seawater samples were collected within 2 km around Fukushima Daiichi Nuclear Power Plants during the term from October 2011 to March 2012. This method showed good linearity between theSr concentration and the total beta counting following the proposed method, with a correlation coefficient of 0.99 in seawater sample analysis. No interference that was caused by other radionuclides, such as radioactive cesium, was not observed in the quantification of Sr. The whole process requires 12 h to quantifySr; this time is 1/40 shorter than traditional milking-low background gas-flow counting method. The lower limit of detection (average value n = 60) of the Sr radioactivity was shown to be 0.03 Bq/L (uncertainty 4.2%).

Cesium was recovered from soil samples obtained in Fukushima prefecture. Isotopic analysis of Cs was performed by γ -spectrometry to determine the activity ratio of 134 Cs/ 137 Cs and thermal ionization mass spectrometry was used to determine the isotopic ratios of 133 Cs/ 137 Cs and 135 Cs/ 137 Cs. The analytical results showed that both the activity ratio of 134 Cs/ 137 Cs and the isotopic ratio of 135 Cs/ 137 Cs were within the expected values for the Fukushima Daiichi Nuclear Power Plant estimated using the ORIGEN-II code, suggesting that most of the radioactive Cs in the soil sample originated from the Fukushima Daiichi Nuclear Power Plant. The concentration of 137 Cs and the contribution of radioactive Cs from global fallout were correlated to the distance from the Fukushima Daiichi Nuclear Power Plant, while the contribution of radioactive Cs from each reactor did not show any similar distance dependence.