Published on 22 August 2016
The Fukushima Daiichi nuclear power reactor units that generated large amounts of airborne discharges during the period of March 12–21, 2011 were identified individually by analyzing the combination of measured 134Cs/137Cs depositions on ground surfaces and atmospheric transport and deposition simulations. Because the values of 134Cs/137Cs are different in reactor units owing to fuel burnup differences, the 134Cs/137Cs ratio measured in the environment was used to determine which reactor unit ultimately contaminated a specific area. Atmospheric dispersion model simulations were used for predicting specific areas contaminated by each dominant release. Finally, by comparing the results from both sources, the specific reactor units that yielded the most dominant atmospheric release quantities could be determined. The major source reactor units were Unit 1 in the afternoon of March 12, 2011, Unit 2 during the period from the late night of March 14 to the morning of March 15, 2011. These results corresponded to those assumed in our previous source term estimation studies. Furthermore, new findings suggested that the major source reactors from the evening of March 15, 2011 were Units 2 and 3 and that the dominant source reactor on March 20, 2011 temporally changed from Unit 3 to Unit 2.
Since 2011, we have been estimating the source term—temporal changes in atmospheric release rates (Bq/h) of radionuclides—caused by the Fukushima Daiichi nuclear power station (FDNPS) accident using a reverse estimation method that combines atmospheric dispersion simulation and environmental monitoring data1,2,3,4,5. Many international researchers have also tried the source term estimation and model simulation of atmospheric dispersion of radionuclides during the accident. The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) summarized sixteen results on source term estimation (Table B2 of UNSCEAR 2013 Report6). It described that the source term estimated by Terada et al.3 (which is the one from our previous study) provided a sound basis for estimation of the levels of radioactive material in the terrestrial environment where prior measurements did not exist and actually the dispersion and deposition of released material modeled by the World Meteorological Organization (WMO) based on the source term by Terada et al.3 could replicate the broad pattern of deposition density of 137Cs over the Japanese land mass. We also summarized a number of international papers lately that have carried out the source term estimation and numerical analysis of atmospheric dispersion process of radionuclides released during the accidents (Table 1 of Katata et al.5).
The accuracy of our previous study’s latest source term increased with gradual increases in the number of monitoring data after the accident and improvement of our team’s numerical simulation model that included a sophisticated atmospheric deposition scheme5. The calculated ground-shine due to the large deposition event of March 15–16, 2011 agreed with observed data within a factor of 2 at most of the monitoring points, and the model also reproduced the spatial distribution of the airborne survey’s air dose rate and 137Cs surface deposition within a factor of 5. Therefore, the simulation results of the spatiotemporal patterns of 137Cs surface deposition have enough accuracy to compare with the observed 134Cs/137Cs ratio, though some discrepancies between simulation and observation occurred because of model simulation uncertainties. Using the latest source term in Katata et al.5, several atmospheric dispersion simulations by the National Oceanic and Atmospheric Administration (NOAA), USA, Canadian Meteorological Centre (CMC), and Met Office, UK, were successfully able to reproduce the measured surface contamination distribution and time series in air concentrations of radionuclides regardless of model structure and meteorological input data5. UNSCEAR also reported for Katata et al.5 that in any further or updated assessment, the committee would recommend the use of the latest estimate as “preferential”7.
While the timing and quantities of major atmospheric releases during the FDNPS accident had been estimated, the relationships between these releases and their specifically correlated reactor units have still not been clarified. During the period of March 12–15, 2011, the temporal rises in air-dose rates measured by a monitoring car at the FDNPS boundary were partially connected to the events that occurred in the reactors8. However, after March 15, 2011, although only a few studies investigated the potential reasons why the atmospheric releases continued for such a long period afterward9,10, the precise rationale behind the event still has not been verified definitively.
Therefore, this paper focuses on the reactor units that generated large 137Cs atmospheric releases during the period of March 12–21, 2011.
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