Multi-model intercomparison of the global transport of radionuclides from Fukushima Dai-ichi nuclear power plant accident

The Science Council of Japan (SCJ) launched a model intercomparison project to review the capability of current numerical models to reproduce the transport of radioactive materials released to the environment as a result of the Fukushima Dai-ichi Nuclear Power Plant accident (FNPP1). The purpose of this initiative is to compare existing model results in order to assess model uncertainties, as this will be an important consideration when evaluating various applications and radioactivity dispersion assessments. The intercomparison has three foci, namely regional transport around the FNPP1, global transport and oceanic transport. This study presents the intercomparison of the global transport of radionuclides from the FNPP1 accident. In total, five transport models contributed to the long-range transport comparison, and 12 sets of simulation results were submitted. The source terms of the radionuclides were not specified. The simulations include source terms that are inversely analyzed by the Japan Atomic Energy Agency (Chino et al., 2011; Terada et al., 2012) and Stohl et al. (2012). We compared the transport and deposition of ¹³⁷Cs, ¹³³Xe, and ¹³¹I. The simulation results are both intercompared and to available observations. Generally, the contributed results show relatively good agreement of the deposition pattern of ¹³⁷Cs, indicating significant deposition of ¹³⁷Cs over the northwestern Pacific up to the Aleutian Islands, and reaching to the western part of North America. Differences are found in the long-range transport of ¹³⁷Cs to Europe and Russia. The models also exhibit differences in the transport to Southeast Asia by the Asian winter monsoon. Most of the models remove ¹³⁷Cs from the atmosphere mainly by wet deposition, which accounts for 88-100% of total deposition. The results indicate that the differences in the distribution and magnitude of wet deposition affect the lifetime of the ¹³⁷Cs.