<p>The Fukushima Daiichi Nuclear power plant (FDNPP) accident occurred on 11 March, causing significant amounts of radionuclides released into the atmosphere and dispersed to the other part of the world. Among various physical and chemical processes, the wet scavenging process has a significant impact on the sophisticated spatial and temporal distributions of the released radionuclides and the corresponding health risks. Unfortunately, there are still large uncertainties in the modeling of the wet scavenging of Cs-137&nbsp;in the Fukushima accident, though numerical efforts have been made in the past ten years. To solve the problem, a new pair of in-cloud and below-cloud wet scavenging schemes have been integrated into the Weather Research and Forecasting-Chemistry (WRF-Chem) model for on-line coupled simulation of both the meteorology and radionuclide air dispersion in this study. The in-cloud scheme is Environ, whereas the below-cloud scheme is Mircea. Both are exponential functions of rain intensity. But the Mircea is more sophisticated in the sense that it uses different parameters for different types of rain. The new WRF-Chem model is comprehensively evaluated against the Cs-137 air concentrations measured at 99 suspended particulate matter (SPM) monitoring sites in the east of Japan and the Cs-137 ground deposition measured by an air survey. The new model is also compared with a previous WRF-Chem model, which only has the below-cloud wet scavenging scheme. For the spatial distribution of deposition density, the correlation coefﬁcient (CC), fractional bias (FB), ﬁgure of merit (FM) and Kolmogorov-Smirnov parameter (KSP) and a metric RANK considering the four metrics above are used. For atmospheric concentration, CAPTURE, OVERESTIMATE, threat score, factor of two (FAC2), factor of ﬁve (FAC5) and RANK2 are used. The spatial pattern of the deposition density and temporal profiles of typical plumes are also discussed in detail. According to the results, models with both in-cloud and below-cloud schemes present more possibilities in allocation between the atmosphere and ground. The results demonstrate that the new model considering the in-cloud scavenging shows a substantial improvement in cumulative deposition simulation compared to the previous WRF-Chem model with the below-cloud scheme alone and the in-cloud scavenging play a more important role than the below-cloud. With respect to the atmospheric concentrations, the new model effectively corrects the overestimation during the precipitation from 14:00 to 17:00 UTC 15 March, but it shows underestimations from 23:00 UTC 20 March to 11:00 UTC 21 March. The quantitative metrics suggest that the performance of the new model is quite good for both deposition and air concentration simulations.</p>

Improved Wet Scavenging Schemes for Air Dispersion Modeling of Cs-137 in the Fukushima Accident

Paper Proceedings: https://asmedigitalcollection.asme.org/ICONE/ICONE28/volume/85246

2021 28th International Conference on Nuclear Engineering (ICONE28)

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