To assess the radiological dose to the public resulting from the Fukushima Daiichi Nuclear Power Station (FDNPS) accident in Japan, especially for the early phase of the accident when no measured data are available for that purpose, the spatial and temporal distributions of radioactive materials in the environment need to be reconstructed through computer simulations using the atmospheric transport, dispersion, and deposition model (ATDM). For the ATDM simulation, the source term of radioactive materials discharged into the atmosphere is essential and has been estimated in many studies. In the present study, we further refined the source term estimated in our previous study and improved the ATDM simulation with an optimization method based on Bayesian inference, which used various measurements such as air concentration, surface deposition, fallout, and newly released hourly air concentrations of 137Cs derived by analyzing suspended particulate matter (SPM) collected at air pollution monitoring stations. This optimization improved not only the source term but also the wind field in meteorological calculation, which led to the reduction of discrepancies in plume passage time at monitoring points to less than 3 h between calculations and measurements, by feeding back comparison results between the dispersion calculations and measurements of radionuclides. As a result, the total amounts of 137Cs and 131I by the present study became 1.0 × 1016 and 1.2 × 1017 Bq, respectively, and decreased by 29% and 20%, respectively, in comparison with those by previous study. The ATDM simulation successfully reproduced both the air concentrations at SPM monitoring points and surface depositions by airborne monitoring. FA10 for total samples of air concentrations of 137Cs at SPM monitoring points increased from 35.9% by the previous study to 47.3%. The deposition amount on the land decreased from 3.7 × 1015 Bq by the previous study to 2.1 × 1015 Bq, which was close to the measured amount of 2.4 × 1015 Bq. We also constructed the spatiotemporal distribution of some major radionuclides in the air and on the surface (optimized dispersion database) by using the optimized release rates and ATDM simulations. The optimized dispersion database can be used for comprehensive dose assessment in tandem with behavioral patterns of evacuees from the FDNPS accident by collaborating research group in the Japanese dose assessment project. The improvements in the present study lead to the refinement of the dose estimation.

中文翻译：

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在福岛第一核电站事故期间改进放射性核素的源项和大气弥散模拟。

为了评估日本福岛第一核电站事故（FDNPS）事故对公众造成的辐射剂量，特别是在事故的早期阶段（当时没有可用于此目的的测量数据）时，日本放射性物质的时空分布必须通过使用大气传输，扩散和沉积模型（ATDM）的计算机模拟来重建环境。对于ATDM模拟，排放到大气中的放射性物质的来源是必不可少的，并已在许多研究中进行了估算。在本研究中，我们进一步完善了先前研究中估计的源项，并使用基于贝叶斯推断的优化方法改进了ATDM仿真，该方法使用了各种测量方法，例如空气浓度，表面沉积，沉降，以及通过分析在空气污染监测站收集的悬浮颗粒物（SPM）得出的每小时释放的137Cs的最新空气浓度。这种优化不仅改善了源项，而且还改善了气象计算中的风场，通过反馈色散计算之间的比较结果，从而将监测点的羽流通过时间差异减少到了3小时以内。和放射性核素的测量。结果，与先前的研究相比，本研究的137Cs和131I的总量分别变为1.0×1016和1.2×1017 Bq，分别减少了29％和20％。ATDM模拟成功地再现了SPM监测点处的空气浓度和通过机载监测产生的表面沉积物。SPM监测点的137Cs空气总样本中的FA10从之前的研究的35.9％增加到47.3％。陆地上的沉积量从以前的研究的3.7×1015 Bq减少到2.1×1015 Bq，接近于实测值2.4×1015 Bq。我们还通过使用优化的释放速率和ATDM模拟，构建了空气和表面（优化的色散数据库）中一些主要放射性核素的时空分布。通过与日本剂量评估项目的研究小组合作，可以将优化的分散数据库与FDNPS事故撤离人员的行为模式一起用于综合剂量评估。在本研究中的改进导致剂量估计的改进。