Shigekazu Hirao

Estimation of release rate of iodine-131 and cesium-137 from the Fukushima Daiichi nuclear power plant

The atmospheric release rates of I-131 and Cs-137 from the Fukushima Daiichi nuclear power plant in March 2011 were estimated by comparing environmental monitoring data of air concentration and deposition rate on a regional scale with calculated values from an atmospheric dispersion model. Although the release rates were not estimated for all days after 11 March, because of lack of monitoring data, temporal changes in the release rates were reasonably estimated with estimated uncertainties in a factor of 3.3 and 2.9 for I-131 and Cs-137, respectively. A large release was estimated from the night of 14 March to at least the afternoon of 15 March, with maximum values of 7.2 × 1015 Bq h−1 for I-131 and 1.5 × 1014 Bq h−1 for Cs-137. The release rates during other periods were estimated at one- to two-orders of magnitude smaller than the largest release rate on 15 March. Uncertainty in the estimated release rate for 15 and 20 March was larger than for other periods. The significant release during 14 and 15 March and the trend of the release rate by the end of March were consistent with previous reports. This agreement, despite using different datasets, shows robustness of the temporal changes estimated in the studies.

Quantification of emission estimates of CO 2 , CH 4 and CO for East Asia derived from atmospheric radon-222 measurements over the western North Pacific

ABSTRACT Emissions of CO, CH4 and CO2 in East Asia were estimated using the atmospheric concentration of 222Rn measured at Minamitorishima (MNM) and Yonagunijima (YON) with a high-precision 222Rn measuring system. The 222Rn measurements showed a distinct seasonal variation, as well as enhanced 222Rn (ERN) events associated with high-frequency synoptic variations. The enhancements of CO, CH4 and CO2 coincided with the ERN events at MNM and YON. The enhancement ratios ΔCH4/ΔRn, ΔCO2/ΔRn and ΔCO/ΔRn calculated for the ERN events and corrected for the radioactive decay of 222Rn during the transport were used to estimate the emissions of CO, CH4 and CO2 by multiplying a constant 222Rn exhalation rate of 1.0 atom cm−2 s−1 and the associated catchment area estimated by a 3D transport model. By modifying the categorisation of the sources, the 222Rn-derived emissions of CO2 and CH4 were found to be in agreement with the published emission databases. As for the CO emission, results of our study were consistent with the REAS CO emission database but showed that the EDGAR database underestimates the CO emission over East Asia by about 44%. It is demonstrated that the 222Rn tracer method used in this study could provide a useful and independent tool to verify the trace gas emissions in East Asia.

Inverse modeling of Asian 222Rn flux using surface air 222Rn concentration

When used with an atmospheric transport model, the (222)Rn flux distribution estimated in our previous study using soil transport theory caused underestimation of atmospheric (222)Rn concentrations as compared with measurements in East Asia. In this study, we applied a Bayesian synthesis inverse method to produce revised estimates of the annual (222)Rn flux density in Asia by using atmospheric (222)Rn concentrations measured at seven sites in East Asia. The Bayesian synthesis inverse method requires a prior estimate of the flux distribution and its uncertainties. The atmospheric transport model MM5/HIRAT and our previous estimate of the (222)Rn flux distribution as the prior value were used to generate new flux estimates for the eastern half of the Eurasian continent dividing into 10 regions. The (222)Rn flux densities estimated using the Bayesian inversion technique were generally higher than the prior flux densities. The area-weighted average (222)Rn flux density for Asia was estimated to be 33.0 mBq m(-2) s(-1), which is substantially higher than the prior value (16.7 mBq m(-2) s(-1)). The estimated (222)Rn flux densities decrease with increasing latitude as follows: Southeast Asia (36.7 mBq m(-2) s(-1)); East Asia (28.6 mBq m(-2) s(-1)) including China, Korean Peninsula and Japan; and Siberia (14.1 mBq m(-2) s(-1)). Increase of the newly estimated fluxes in Southeast Asia, China, Japan, and the southern part of Eastern Siberia from the prior ones contributed most significantly to improved agreement of the model-calculated concentrations with the atmospheric measurements. The sensitivity analysis of prior flux errors and effects of locally exhaled (222)Rn showed that the estimated fluxes in Northern and Central China, Korea, Japan, and the southern part of Eastern Siberia were robust, but that in Central Asia had a large uncertainty.

Continuous Observation of Atmospheric 222Rn Concentrations for Analytic Basis of Atmospheric Transport in East Asia

The observation network of atmospheric radon-222 (222Rn) concentration established in East Asia region is introduced, and the characteristics of the observations at two continental sites Beijing and Seoul and three remote sites Cape Ochiishi, Hachijo Is. and Hateruma Is. are discussed in this paper. Higher levels of 222Rn concentrations with typical diurnal variation with early morning maxima were observed on the continent, and lower levels with no diurnal variation at remote islands. Seasonal variations with summer minima and winter maxima were commonly obtained at all five observatories, and they suggested contribution of 222Rn originated from the continent to atmospheric 222Rn over the remote islands isolated in the ocean. A backward trajectory analysis showed clear relationship between variation in wind field and in 222Rn concentration at Hachijo Is., and proved availability of the observation for analysis of atmospheric transport in East Asia.

Development and Verification of Long-Range Atmospheric Radon-222 Transport Model

A three-dimensional Eulerian atmospheric long-range transport model of 222Rn and 210Pb coupled with meteorological model MM5 was developed. The transport model includes advection, diffusion and removal processes in a horizontal scale of several thousand kilometers. Performance of the developed model was evaluated with measured data of surface 222Rn concentration and surface 210Pb deposition. The refinements of calculation conditions and input data were also done with respect to the following points: 1) the sensitivity of spatial distribution of 222Rn exhalation and 2) the accuracy of spatial distribution of precipitation. The model reasonably calculated the surface 222Rn concentration at two solitary islands in Pacific Ocean. Introducing a realistic spatial distribution of 222Rn exhalation rate, the calculated surface 222Rn concentrations showed good agreement with measured concentration. The calculation of 210Pb deposition was in quite good agreement with measurement if realistic precipitation distribution was used. These results show that the developed model can realistically calculate atmospheric transport of 222Rn and 210Pb in a synoptic scale.

Indoor/outdoor radon decay products associated aerosol particle-size distributions and their relation to total number concentrations

The activity size distributions of indoor and outdoor radioactive aerosol associated with short-lived radon decay products were observed at Nagoya, Japan, for some periods from 2010 to 2012, following the indoor observation by Mostafa et al. [Mostafa, A. M. A., Tamaki, K., Moriizumi, J., Yamazawa, H. and Iida, T. The weather dependence of particle size distribution of indoor radioactive aerosol associated with radon decay products. Radiat. Prot. Dosim. 146: (1-3), 19-22 (2011)]. The tendency of smaller indoor activity median aerodynamic diameter (AMAD) after rainfalls showed in the previous study was not consistently obtained, while the consistent tendency of less indoor radioactive particles with diameters in the accumulation mode was observed again after rainfalls. The indoor aerosols showed activity size distributions similar to the outdoor ones. Non-radioactive aerosol particle concentrations measured with a laser particle counter suggested a somewhat liner relationship with AMAD.

Release Rate Estimation of Radioactive Noble Gases in the Criticality Accident at Tokai-Mura from Off-Site Monitoring Data

A method of release rate estimation of radioactive noble gases from an accidental nuclear facility was proposed. This method was applied to the criticality accident case at Tokai-mura to evaluate an uncertainty of results. The release rate is estimated by combining the gamma dose rates measured at off-site monitoring posts and those calculated using an atmospheric dispersion model. To analyze the dependence of uncertainty of estimated release rate on that of the wind field, two wind field data were used; one from the meteorological model MM5 and the other from the analysis of measured wind data. The results of comparison that the analyzed wind field was in better agreement with the measured wind data than the MM5 wind field led to the smaller uncertainty in the analyzed wind case than that in the MM5 wind case. A gradual decrease in release rate with time was clearly shown in the analyzed wind case, while it was obscured by a large scatter in the MM5 wind case. The total activities of the released radioactive noble gases evaluated by the present method were 3:0 × 102 and 2:1 × 102 TBq for the cases with the retention times of noble gases, that is, the time elapsed before the noble gases are released into the atmosphere, of 5 and 10 min, respectively. The retention time of 10 min was reasonably compared with that evaluated from the total number of the fission reactions with the assumption of the same retention time.

Air concentration estimation of radionuclides discharged from Fukushima Daiichi Nuclear Power Station using NaI(Tl) detector pulse height distribution measured in Ibaraki Prefecture

ABSTRACT Air concentrations of 133Xe, 131I, 132I, 133I, 132Te, 134Cs, 136Cs, and 137Cs in the early stage of the Fukushima Daiichi Nuclear Power Station (FNPS1) accident were estimated for six locations in Ibaraki Prefecture from pulse height distributions measured with NaI(Tl) scintillation detectors continuously operated as a part of monitoring station systems. Air concentration of 133Xe in Ibaraki was estimated for the first time, which showed the maximum value of 150 kBq m−3 in the plume arrived at the southern part of Ibaraki around noon of 15 March 2011. The plume was found to consist mainly of noble gases without other nuclides at significant level and to correspond to the hydrogen explosion at Unit 3. Spatiotemporal distributions of 131I and other nuclides were discussed for the plumes detected during the periods of 15–16 and 20–21 March. Variations in radionuclide composition among the plumes and within each plume were also discussed.

Estimation of infiltration and surface run-off characteristics of radionuclides from gamma dose rate change after rain

A method to estimate the infiltration and surface run-off characteristics of radionuclides on three types of ground surface from gamma dose rate change due to rain has been developed. We proposed the estimation methods based on the differences in the dose rate increases between monitoring stations caused by different attenuation of gamma ray due to infiltration and by the different run-off characteristics. The gamma dose rate data used for the estimation were measured at near-by monitoring stations in which the ground type around the detector differed from each other. Rain events were selected by the criteria in which 214Pb and 214Bi deposition amounts are considered to be uniform with in the area in which objective monitoring stations are deployed. We also calculated the surface concentration considering both infiltration and surface run-off processes. As a result, it was shown that the calculated surface concentration was about 40% larger than that considering neither processes of the infiltration and surface run-off.

Development and verification of long‐range atmospheric transport model of radon‐222 and lead‐210 including scavenging process

A three‐dimensional Eulerian atmospheric long‐range transport model of radon‐222 (222Rn) and lead‐210 (210Pb) coupled with meteorological model MM5 was developed. The model calculates advection, diffusion, radiation decay and deposition processes in a horizontal scale of several thousand kilometers. This model was applied to East Asia. Performance of the model was evaluated with measured hourly surface air 222Rn concentration and monthly 210Pb deposition. The model verification was done with respect to the following points: 1) the sensitivity of vertical distribution of turbulent diffusivity and 2) the accuracy of spatial distribution of precipitation. In this report, improvement of the model performance is also discussed.