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Cs/
Cs isotopic ratio in soil collected near Fukushima Daiichi Nuclear Power Station through mass spectrometryShimada, Asako; Tsukahara, Takehiko*; Nomura, Masao*; Kim, M. S.*; Shimada, Taro; Takeda, Seiji; Yamaguchi, Tetsuji
Journal of Nuclear Science and Technology, 58(11), p.1184 - 1194, 2021/11
Times Cited Count：5 Percentile：64.12(Nuclear Science & Technology)Determining the completeness of nuclear reactor decommissioning is an important step in safely utilizing nuclear power. For example, Cs from the Fukushima Daiichi Nuclear Power Station (FDNPS) accident can be treated as background radioactivity, so determining the origin of Cs is essential. To accomplish this, measuring the Cs/Cs isotope ratio can be useful, so this study optimized a solvent extraction method, with calix[4]arene-bis(t-octylbenzo-crown-6) [BOBCalixC6] in 1-octanol, to purify radioactive Cs, radiocesium, from a solution of major environmental soil elements and mass spectrometry interference elements. This optimized method was applied to Cs purification in soil samples (40 g), and the final solutions contained a total of 10
g/ml of the major soil elements and ng/ml concentrations at most of interfering elements. Soil samples collected near the FDNPS were then purified, and the Cs/Cs isotope ratios were measured, using both thermal ionization mass spectrometry (TIMS) and triple quadrupole induced coupled plasma mass spectrometry (ICP-QQQ). The results of each of these measurements were compared, and we found that Cs isotope ratios obtained by TIMS were more precise, by an order of magnitude, while the ICP-QQQ results possessed good abundance sensitivities. A slightly higher Cs/Cs ratio in the northwest area of the FDNPS was observed, while other areas exhibited similar values, all within the measurement error range, which indicated different origins of radiocesium. These results agreed with previously reported 
Cs/Cs activity distributions, suggesting that this ratio may be useful in identifying radiocesium origins for evaluating future nuclear reactor decommissions.
Shimada, Taro; Nabekura, Nobuhide*; Takeda, Seiji
no journal, ,
It is necessary to confirm that the public radiation exposure dose by residual radioactivity is less than the criteria after site release at the completion of decommissioning of nuclear facilities. In order to obtain the radioactivity distribution in the site to evaluate the public dose, we have been developing a computer code, ESRAD, which estimates the radioactivity distribution from limited measurement data of radioactivity of soil in the site using the kriging. In this study, we improved the code by introducing the external drift kriging where dose rate distribution obtained before final survey was used as supplement information. Some calculations were carried out supposing hypothetical radioactivity distribution. As a result, the external drift kriging using dose rate distribution may improve the estimation results rather than the ordinary kriging.
Shimada, Taro; Nabekura, Nobuhide*; Miwa, Kazuji; Takeda, Seiji
no journal, ,
It is required to confirm that dose of public using the land after site release for is less than the criteria by dose evaluation based on the contamination distribution obtained by site survey before the completion of the decommissioning of nuclear facilities. Therefore JAEA is developing the methodology to evaluate public dose. We studied estimating methods for contamination distribution based on the data from the scoping survey and the final sampling measurement with errors using external drift kriging. As a result, it is indicated that estimated distribution by external drift kriging using scoping survey results with measurement errors is the same as that without measurement error. Estimation variance was increased with measurement errors. Therefore the evaluation method will be applied to determine the radioactivity distribution with uncertainty.
Cs/Cs isotope ratioShimada, Asako; Tsukahara, Takehiko*; Nomura, Masao*; Kim, M. S.*; Shimada, Taro; Takeda, Seiji; Yamaguchi, Tetsuji
no journal, ,
Radiocesium from the Fukushima Daiichi Nuclear Power Station (1F) accident can be treated as background radioactivity at completion of decommissioning, so determining the origin of radiocesium is essential. Therefore, we studied identification of radiocesium origin using Cs/Cs isotope ratio. The ratio originated from 1F was 0.3628
0.0005. When the ratio originated from decommissioning facility was 0.05
0.2 larger than that originated from 1F, it is challenging to decrease the stand deviation less than 0.01 for 1 Bq/g of Cs because precision was decreased as decreasing the Cs concentration. If the ratio is able to be determined within the standard deviation of 0.0005 for 0.5 Bq/g of soil sample, the identification would be available.
Shimada, Taro; Nabekura, Nobuhide*; Miwa, Kazuji; Takeda, Seiji
no journal, ,
It is necessary for confirmation of completion of decommissioning of nuclear facilities to comply with dose criteria for public using the land after site release, based on the radioactivity distribution at the land of the site. We are developing a methodology of public dose evaluation based on the survey results of evaluation of the radioactivity distribution. In this report, the external drift kriging method combined with scoping survey and sampling measurement were applied to 1km square area, and a cross validation method was applied to validate the evaluated distribution. Index of Mean Absolute Error is appropriate to evaluate the difference between evaluation and measurement. In addition, upper limit of 95% single side confidence interval for the evaluation of non-measured point was suggested to input for dose evaluation.
Miwa, Kazuji; Namekawa, Masakazu*; Shimada, Taro; Takeda, Seiji
no journal, ,
The distribution of surface soil contamination on the site of nuclear facilities after confirmation of the decommissioning may be changed by rainfall. In this study, the radionuclides migration model is developed to evaluate the radiation dose to the public including radionuclides distribution change. The result of radionuclides migration estimation on virtual site condition suggests that radionuclides which is migrated by surface water and soil migration gather in depression area. It is also shown that radionuclides migration on surface is more significant path to the ocean than radionuclides migration in groundwater. Based on these findings, it is suggested that the radiation dose considering change of radionuclides distribution may be larger than that from initial radionuclides distribution.
Shimada, Taro; Miwa, Kazuji; Sasaki, Toshihisa*; Takeda, Seiji
no journal, ,
The authors are developing a methodology for confirming that public dose using the site after site release will be in compliance with the dose criteria based on the radioactivity distribution at the termination of the decommissioning of nuclear facilities. In a previous study, it was confirmed that kriging with external drift combined with scoping survey and representative measurement could evaluate the radioactivity distribution. However, the issue that evaluated distribution should be validated from the viewpoint of sufficiency on points of representative measurement remained. Therefore, we developed the method for the validation, that mean absolute error (MAE) between each measured value and evaluated value is less than a reference value of MAE between an average of measured values and each measured value. The method was applied to supposed distributions and it was confirmed that some of the evaluated distributions were validated.
Cs/Cs isotopic ratio in environmental samplesShimada, Asako; Nomura, Masao*; Tsukahara, Takehiko*; Takeda, Seiji
no journal, ,
Although identification of radiocesium in environmental samples is commonly performed by using Cs/Cs radioactivity ratio, measurement of Cs will become challenge in a few decade ago because of short half-life (2 y). Instead of the measurement of Cs/Cs radioactivity ratio, the measurement of Cs/Cs isotopic ratio will be effective in future because of long half-life of Cs (133 My). In order to measure Cs/Cs isotopic ratio, Cs must be separated from the matrix of soil. Since 1 Bq of Cs is equal to 312.5 fg, large amount of soil have to be treated to extract Cs for the measurement. In the present study, we improved chemical separation method using BOBCalixC6 by optimizing extraction time and reuse of the organic phase. Finally, approximately 170 g of soil sampled at Toka-mura was dissolved and Cs was separate with the developed method to measure Cs/Cs isotopic ratio.
