Simulation of atmospheric radiocesium (¹³⁷Cs) from Fukushima nuclear accident using FLEXPART-WRF driven by ERA5 reanalysis data

 results from Katata et al., (2015) [27]. Heavy 
pressure gradient followed Northwest- rain, from 5 mm to 10 mm per 6 hours, 
Southeast axis are reproduced well in the WRF occurred over broad area around Fukushima 
model (Fig. 5). It’s worthy to note that, for region. Due to the impacts of earthquake and 
facilitating “eyeball” verification, the number tsunami, almost all the meteorological 
 5 
SIMULATION OF ATMOSPHERIC RADIOCESIUM (137Cs) FROM FUKUSHIMA NUCLEAR ACCIDENT 
observation equipments were inoperable after research groups such as Kinoshita et al., (2011) 
the nuclear accident. Therefore, it’s difficult to [30], Stohl et al., (2012) [10] and Sugiyama et 
obtain good quality meteorological observation al., (2012) [31]. In fact, rain occured over the 
in this case. Large-scale meteorological north area of Fukushima prefecture from 17:00 
information during the occurrence of JST March 15 to 04:00 JST on the March 16 
radioactive material emissions into the air was [30]. On the 20 to 22 March, sustainable low 
presented in the 2013 World Meteorological pressure caused moderate rainfall in the 
Organization (WMO) report by previous vicinity of Tokyo. 
 (a) (b) 
Fig. 5. Simulation of geo-potential height (shaded color) and wind field (barbs) on level of 850 mb, at 12h00 
 UTC 15/03/2011 from the WRF model in Exp 1 (a), in comparison with the ERA5 reanalysis data (b). 
 Notice: Factor of 10 is applied to thin the number of wind barbs in (a) 
Fig. 6. Accumulated simulated precipitation from the WRF model in experiment Exp 1, from 09:00 to 15:00 
 on March 15, 2011 
 6 
 KIEU NGOC DUNG et al. 
B. Evaluation of atmospheric radiocesium resolution. In this paper, high resolution of 
 The Futaba station, an observatory 01 km is very suitable for considering the 
station in the town of Futaba, is very close to geographical location of Futaba station, as 
the Fukushima NPP. The distance between well as other neighboring stations (e.g. 
Futaba town and the plant is only about 3.2 Naraha station). Calculation results of the 
km, where was severely affected by both concentration of atmospheric radiocesium 
 137
earthquakes, tsunamis and the effects of Cs for every hour at Futaba and Naraha 
radiation [28]. For the other researches of station are displayed in Fig. 7 and Fig. 8, 
global radioactive dispersions, the vicinity respectively. The observation data displayed 
areas of the plant are often not taken into in these figures are retrieved from Tsuruta et 
account, because of the limitation of the grid al., (2011) [28]. 
 Fig. 7. Hourly accumulated concentration of 137Cs at Futaba station from observation (red dashed line) and 
 Exp 4 (blue solid line) with range of simulation results from all experiment (shaded light blue) 
 From Fig. 7 and Fig. 8, it can be seen Fig.7 and Fig. 8) can be recorgnized, 
that simulation results have a good agreement especially for concentrations of less than 102 
with the observed data, especially from 12 to Bq.m-3 per hour. The uncertainty in simulation 
14/03/2011 at Futaba station and from 15 to or the sensitivity of calculation results to 
16/03/2011 at Naraha station. Peak values of different micro-physics option is more clear in 
137Cs concentration on 12 and 19/03/2011 at the case of Futaba station than in Naraha 
Futaba station are reproduced well in all station. This can be seen on simulated range 
experiments. Peak values on 15, 16 and of 13-14/03/2011 and 19-21/03/2011 in Fig. 
19/03/2011 at Naraha station are also captured 7. The Exp 4 was displayed due its better 
well by the FLEXPART-WRF model. The performance, in comparison with others 
range of simulated values from four experiments, which is confirmed by statistical 
experiment (i.e. shaded light blue area in verification shown in Fig. 9. 
 7 
SIMULATION OF ATMOSPHERIC RADIOCESIUM (137Cs) FROM FUKUSHIMA NUCLEAR ACCIDENT 
 Fig. 8. Hourly accumulated concentration of 137Cs at Naraha station from observation (red dashed line) and 
 Exp 4 (blue solid line) with range of simulation results from all experiment (shaded light blue) 
 (a) (b) 
 Fig. 9. Taylor diagram compares simulation results from 04 experiments using Pearson correlation 
 coefficient and Normalized Standard Deviation for (a) Futaba and (b) Naraha station. Observation value is 
 depicted by black star 
 Fig. 9 demonstrates high sensitivity of atmospheric radiocesium retrieved from 
simulation results to different micro-physics four experiments. For example, at Futaba 
options of the WRF model. Scatter of station, simulation result from experiment 
experiment’s points on the Taylor diagram Exp 1 has CC value of 0.28 and normalized 
highlights the significant variations of not σ of 0.48. While respective verification 
only correlation coefficients (CC) but also metrics for Exp 4 are 0.77 and 0.36 which 
standard deviations (σ) of simulated means better capture of hourly observed 
 8 
 KIEU NGOC DUNG et al. 
release of 137Cs air concentration. At Naraha maps explain the peak of concentration 
station, the higher CC values can be seen, in shown in Fig. 7 and Fig. 8. At level of 100 
comparison with simulation results at m, atmospheric radionuclide propagated to 
station Futaba (i.e. 0.92 for Exp 4 or 0.89 the North on 12/03/2011 which plumes 
for Exp 1). Based on this Taylor diagram, spreaded widely to Southwest on 
the experiment Exp 3 show worse 15/03/2011. Smaller plumes in both area 
simulation results than Exp 1, Exp 2 and and intensity blowed along coastal line to 
Exp 4. Therefore, the configuration of Exp the South are simulated on 19/03/2011. 
1, Exp 2 or Exp 4 can be recommended for These results show a similarity to the results 
further study in the future. of Tsuyoshi et al., (2015) [1] in which 
 From Fig. 10, dispersion plume of different horizontal grid resolutions are used 
137Cs concentration at 100 m can be seen to calculate radioactivity concentration on 
for three different days. These distrubition 15/03/2011. 
 (a) (b) (c) 
 Fig. 10. Local-scale spatial distributions of accumulated concentrations of 137Cs at 100 meter from Exp 4 
 retrieved (a) from 00 UTC 12 to 00 UTC 13/03/2011, (b) from 00 UTC 15 to 00 UTC 16/03/2011 and (c) 
 from 00 UTC 19 to 00 UTC 20/03/2011. Unit: Bq.m-3 
 IV. CONCLUSIONS configured with two domains of 05 km and 01 
 This study investigates short-range km. Both meteorological conditions and 
 137
atmospheric transport of radionuclides after dispersion of atmospheric radiocesium ( Cs) 
Fukushima nuclear accident using a numerical are evaluated. In comparison with observation 
weather model and a Lagrangian particle at Futaba and Naraha station, all experiments 
 137
dispersion model. Four different experiments captured reasonably the variation of Cs 
were carried out using the FLEXPART-WRF concentration from 11/03 to 26/03/2011. 
model coupled with the WRF model. The Analysis on Taylor diagram confirm the 
ERA5 reanalysis data is used as initial and noticeable sensitivity of simulation results to 
boundary conditions for the WRF model with four selected micro-physics parameterizations. 
hourly update time step. The WRF model is The configurations of Exp 1, Exp 2 and Exp 4 
 9 
 SIMULATION OF ATMOSPHERIC RADIOCESIUM (137Cs) FROM FUKUSHIMA NUCLEAR ACCIDENT 
are recommended for further study due to their [5]. Morino et al., “Atmospheric behavior, 
better performance among all. deposition, and budget of radioactive materials 
 from the Fukushima Daiichi nuclear power 
 ACKNOWLEDGEMENT plant in March 2011”, Geophysical Research 
 Letters, VOL. 38, L00G11, 2011. 
 This article is supported by the project 
titled “Study the effects of the floating nuclear [6]. Yasunari et al., “Cesium-137 deposition and 
 contamination of Japanese soils due to the 
power plant on the sea and the nuclear power 
 Fukushima nuclear accident”, PNAS 
plants on Hainan Island on Vietnam’s marine 
 December 6, 108 (49) 19530-19534, 2011. 
environment by modelling and developing 
response plans for a nuclear accident occurred [7]. Katata et al., “Atmospheric discharge and 
 dispersion of radionuclides during the Fukushima 
over the sea”, project code: KC.AT, in the 
 Dai-ichi Nuclear Power Plant accident. Part I: 
framework of technical research program, Source term estimation and local-scale 
nuclear safety to ensure combat readiness for atmospheric dispersion in early phase of the 
the Army in the 2016-2020 period. This article accident”, Journal of Environmental 
is also supported by the project titled “Study Radioactivity 109, 103-113, 2012. 
and calculating the spread of atmospheric [8]. Le Petit et al., “Analysis of radionuclide 
radionuclides and modernization of stations releases from the Fukushima Dai-ichi nuclear 
for radioactive analysis” (PACT-1). power plant accident part I”, Pure Appl. 
 Geophys. , 2012. 
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