1In areas contaminated by radioactive materials, well-designed dose assessment is necessary in order to protect people from radiation exposure and manage the exposure situation appropriately. Probabilistic dose assessment is a useful method for providing a more complete characterization of information on dose distributions in the population, and requires statistically characterized data on pathway-relevant contributors. The objective of this paper is to determine statistical features of contributors to external exposures, as well as to identify causes of variabilities of individual doses to the populations living in areas contaminated by the Fukushima Daiichi Nuclear Power Plant accident. To achieve these objectives, measurements of individual doses and ambient dose rates, as well as surveys of behavioral patterns, were performed between February and April, 2012. These were made with the cooperation of indoor workers, outdoor workers, and pensioners living in Fukushima prefecture. On the basis of these results, statistical analyzes were performed in order to identify variabilities of contributors. In addition, a multi-regression analysis was made to explore a significant relationship between individual doses and relevant contributors. As a result, the ambient dose equivalent rate also distributed with lognormal form and it had the variabilities attributable to the spatial distribution of deposited radionuclides. The distribution forms of time spent outdoors depend on the characteristics of occupation, and those of indoor workers and outdoor workers were lognormal and normal, respectively. Results of a multiple-regression analysis suggested that such valiabilities of contributors give rise to significant differences of individual doses among the populations.
Radiation exposure is one of most important factors to manage following a nuclear emergency. Actual measurement is the best way to obtain information concerning the dose received by the people in terms of accuracy and reliability. However, in practice, it is difficult to collect measurements from all people affected by nuclear accidents over the whole period of exposure from past to future. Therefore, probabilistic assessment using a model is needed. An assessment model of radiation doses from external exposures was developed based on the actual measurement of individual doses and ambient dose equivalent rates inside and outside houses in Fukushima City. A survey of behavioral patterns was also performed for the same purpose. In addition to our measurement and survey, we took into account the latest insights from the experiences of the Fukushima Daiichi nuclear power plant accident. Comparisons between the assessed and measured results revealed that the time-dependence of doses and the distribution of doses obtained using the developed models agree well with the results of actual measurements. Thus, our probabilistic approach was validated. Based on both our assessment and on our actual measurements, no participants were observed to receive doses in excess of 1 mSv y−1 as of 8 y after the Fukushima Daiichi nuclear power plant accident in Fukushima City.
Dose assessment is an important issue from the viewpoints of protecting people from radiation exposure and managing postaccident situations adequately. However, the radiation doses received by people cannot be determined with complete accuracy because of the uncertainties and the variability associated with any process of defining individual characteristics and in the dose assessment process itself. In this study, a dose assessment model was developed based on measurements and surveys of individual doses and relevant contributors (i.e., ambient dose rates and behavior patterns) in Fukushima City for four population groups: Fukushima City Office staff, Senior Citizens' Club, Contractors' Association, and Agricultural Cooperative. In addition, probabilistic assessments were performed for these population groups by considering the spatial variability of contamination and interpopulation differences resulting from behavior patterns. As a result of comparison with the actual measurements, the assessment results for participants from the Fukushima City Office agreed with the measured values, thereby validating the model and the approach. Although the assessment results obtained for the Senior Citizens' Club and the Agricultural Cooperative differ partly from the measured values, by addressing further considerations in terms of dose reduction effects due to decontamination and the impact of additional exposure sources in agricultural fields, these results can be improved. By contrast, the measurements obtained for the participants from the Contractors' Association were not reproduced well in the present study. To assess the doses to this group, further investigations of association members' work activities and the related dose reduction effects are needed.
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