Radiation Monitoring in the Residential Environment: Time Dependencies of Air Dose Rate and 137Cs Inventory

Yoshimura Kazuya,Nakama Shigeo,Fujiwara Kenso 대한방사선방어학회 2022 방사선방어학회지 Vol.47 No.1

Background: Residential areas have some factors on the external exposure of residents, who usually spend a long time in these areas. Although various survey has been carried out by the government or the research institutions after the Fukushima Daiichi Nuclear Power Plant accident, the mechanism of radiocesium inventory in the terrestrial zone has not been cleared. To better evaluate the radiation environment, this study investigated the temporal changes in air dose rate and 137Cs inventories (Bq/m2) in residential areas and agricultural fields.Materials and Methods: Air dose rate and 137Cs inventories were investigated in residential areas located in an evacuation zone at 5–8 km from the Fukushima Daiichi Nuclear Power Plant. From December 2014 to September 2018, the air dose rate distribution was investigated through a walking survey (backpack survey), which was conducted by operators carrying a γ-ray detector on their backs. Additionally, from December 2014 to January 2021, the 137Cs inventories on paved and permeable grounds were also measured using a portable γ-ray detector.Results and Discussion: In the areas where decontamination was not performed, the air dose rate decreased faster in residential areas than in agricultural fields. Moreover, the 137Cs inventory on paved surfaces decreased with time owing to the horizontal wash-off, while the 137Cs inventory on permeable surfaces decreased dramatically owing to the decontamination activities.Conclusion: These findings suggest that the horizontal wash-off of 137Cs on paved surfaces facilitated the air dose rate decrease in residential areas to a greater extent compared with agricultural fields, in which the air dose rate decreased because of the vertical migration of 137Cs. Results of this study can explain the faster environmental restoration in a residential environment reported by previous studies.

Validation of a Model for Estimating Individual External Dose Based on Ambient Dose Equivalent and Life Patterns

Sato Rina,Yoshimura Kazuya,Sanada Yukihisa,Sato Tetsuro 대한방사선방어학회 2022 방사선방어학회지 Vol.47 No.2

Background: After the Fukushima Daiichi Nuclear Power Station (FDNPS) accident, a model was developed to estimate the external exposure doses for residents who were expected to return to their homes after evacuation orders were lifted. However, the model’s accuracy and uncertainties in parameters used to estimate external doses have not been evaluated. Materials and Methods: The model estimates effective doses based on the integrated ambient dose equivalent (H*(10)) and life patterns, considering a dose reduction factor to estimate the indoor H*(10) and a conversion factor from H*(10) to the effective dose. Because personal dose equivalent (Hp(10)) has been reported to agree well with the effective dose after the FDNPS accident, this study validates the model’s accuracy by comparing the estimated effective doses with Hp(10). The Hp(10) and life pattern data were collected for 36 adult participants who lived or worked near the FDNPS in 2019. Results and Discussion: The estimated effective doses correlated significantly with Hp(10); however, the estimated effective doses were lower than Hp(10) for indoor sites. A comparison with the measured indoor H*(10) showed that the estimated indoor H*(10) was not underestimated. However, the Hp(10) to H*(10) ratio indoors, which corresponds to the practical conversion factor from H*(10) to the effective dose, was significantly larger than the same ratio outdoors, meaning that the conversion factor of 0.6 is not appropriate for indoors due to the changes in irradiation geometry and gamma spectra. This could have led to a lower effective dose than Hp(10). Conclusion: The estimated effective doses correlated significantly with Hp(10), demonstrating the model’s applicability for effective dose estimation. However, the lower value of the effective dose indoors could be because the conversion factor did not reflect the actual environment.

Benchmark analysis on PFM analysis codes for aged piping of nuclear power plants

Hiroto Itoh,Yinsheng Li,Kazuya Osakabe,Kunio Onizawa,Shinobu Yoshimura 대한기계학회 2012 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.26 No.7

Probabilistic fracture mechanics is a rational methodology in structural reliability evaluation and risk assessment for aged piping in nuclear power plants. Several probabilistic fracture mechanical analysis codes have been improved or developed in Japan. To verify the reliability and applicability of two of these codes, we did a benchmark analysis using their basic functions in consideration of representative piping systems in nuclear power plants and typical aging mechanisms. Based on the analysis results, we concluded that the analysis results of these two codes are in good agreement.

Comparison of Dose Rates from Four Surveys around the Fukushima Daiichi Nuclear Power Plant for Location Factor Evaluation

Yukihisa Sanada,Mutsushi Ishida,Kazuya Yoshimura,Satoshi Mikami 대한방사선방어학회 2021 방사선방어학회지 Vol.46 No.4

Temporal Change in Radiological Environments on Land after the Fukushima Daiichi Nuclear Power Plant Accident

Saito, Kimiaki,Mikami, Satoshi,Andoh, Masaki,Matsuda, Norihiro,Kinase, Sakae,Tsuda, Shuichi,Sato, Tetsuro,Seki, Akiyuki,Sanada, Yukihisa,Wainwright-Murakami, Haruko,Yoshimura, Kazuya,Takemiya, Hiroshi The Korean Association for Radiation Protection 2019 방사선방어학회지 Vol.44 No.4

Massive environmental monitoring has been conducted continuously since the Fukushima Daiichi Nuclear Power accident in March of 2011 by different monitoring methods that have different features together with migration studies of radiocesium in diverse environments. These results have clarified the characteristics of radiological environments and their temporal change around the Fukushima site. At three months after the accident, multiple radionuclides including radiostrontium and plutonium were detected in many locations; and it was confirmed that radiocesium was most important from the viewpoint of long-term exposure. Radiation levels around the Fukushima site have decreased greatly over time. The decreasing trend was found to change variously according to local conditions. The air dose rates in environments related to human living have decreased faster than expected from radioactive decay by a factor of 2-3 on average; those in pure forest have decreased more closely to physical decay. The main causes of air dose rate reduction were judged to be radioactive decay, movement of radiocesium in vertical and horizontal directions, and decontamination. Land-use categories and human activities have significantly affected the reduction tendency. Difference in the air dose rate reduction trends can be explained qualitatively according to the knowledge obtained in radiocesium migration studies; whereas, the quantitative explanation for individual sites is an important future challenge. The ecological half-lives of air dose rates have been evaluated by several researchers, and a short-term half-life within 1 year was commonly observed in the studies. An empirical model for predicting air dose rate distribution was developed based on statistical analysis of an extensive car-borne survey dataset, which enabled the prediction with confidence intervals. Different types of contamination maps were integrated to better quantify the spatial data. The obtained data were used for extended studies such as for identifying the main reactor that caused the contamination of arbitrary regions and developing standard procedures for environmental measurement and sampling. Annual external exposure doses for residents who intended to return to their homes were estimated as within a few millisieverts. Different forms of environmental data and knowledge have been provided for wide spectrum of people. Diverse aspects of lessons learned from the Fukushima accident, including practical ones, must be passed on to future generations.