Terada Hiroaki, Nagai Haruyasu, Tsuduki Katsunori, Furuno Akiko, Kadowaki Masanao, Kakefuda Toyokazu
Japan Atomic Energy Agency, 2-4 Shirakata, Tokai, Ibaraki, 319-1195, Japan.
Japan Atomic Energy Agency, 2-4 Shirakata, Tokai, Ibaraki, 319-1195, Japan.
J Environ Radioact. 2020 Mar;213:106104. doi: 10.1016/j.jenvrad.2019.106104. Epub 2019 Dec 16.
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 Cs 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 Cs and I by the present study became 1.0 × 10 and 1.2 × 10 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 Cs 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 × 10 Bq by the previous study to 2.1 × 10 Bq, which was close to the measured amount of 2.4 × 10 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.
为评估日本福岛第一核电站事故对公众造成的辐射剂量，特别是在事故早期阶段，由于缺乏相关实测数据，需要通过使用大气传输、扩散和沉降模型（ATDM）进行计算机模拟来重建环境中放射性物质的时空分布。对于ATDM模拟，排放到大气中的放射性物质源项至关重要，许多研究都对其进行了估算。在本研究中，我们进一步细化了先前研究中估算的源项，并采用基于贝叶斯推断的优化方法改进了ATDM模拟，该方法利用了各种测量数据，如空气污染监测站收集的悬浮颗粒物（SPM）中分析得出的空气浓度、地表沉积、沉降物以及新释放的每小时铯空气浓度。这种优化不仅改进了源项，还改进了气象计算中的风场，通过反馈放射性核素扩散计算与测量之间的比较结果，使监测点羽流通过时间的计算值与测量值之间的差异减少到3小时以内。结果，本研究中铯和碘的总量分别变为1.0×10和1.2×10贝克勒尔，与先前研究相比分别减少了29%和20%。ATDM模拟成功再现了SPM监测点的空气浓度和机载监测的地表沉积情况。SPM监测点铯空气浓度总样本的FA10从先前研究的35.9%提高到了47.3%。陆地沉积量从先前研究的3.7×10贝克勒尔降至2.1×10贝克勒尔，接近实测的2.4×10贝克勒尔。我们还利用优化后的释放率和ATDM模拟构建了空气中和地表一些主要放射性核素的时空分布（优化扩散数据库）。优化扩散数据库可与日本剂量评估项目中的合作研究小组结合福岛第一核电站事故撤离者的行为模式用于综合剂量评估。本研究中的改进使得剂量估算更加精确。
