a regional simulation study on dispersion of nuclear pollution from the damaged fukushima nuclear...

SCIENCE CHINA Earth Sciences

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• RESEARCH PAPER • July 2014 Vol.57 No.7: 1513–1524

doi: 10.1007/s11430-013-4811-2

A regional simulation study on dispersion of nuclear pollution from the damaged Fukushima Nuclear Power Plant

FEI JianFang1*, WANG PengFei1,2, CHENG XiaoPing1, HUANG XiaoGang1 & WANG YiBai3

1 College of Meteorology and Oceanography, PLA University of Science and Technology, Nanjing 211101, China; 2 Dalian Naval Academy, Dalian 116018, China;

3 61741 Troops of PLA, Beijing 100094, China

Received February 25, 2013; accepted August 1, 2013; published online May 12, 2014

A nuclear accident involving the leaking of radioactive pollutants occurred at the Fukushima Nuclear Power Plant in Japan, following an earthquake and subsequent tsunami on March 11, 2011. Using official Japanese data on pollutant emissions dur-ing the accident, this study simulates the dispersion of nuclear pollutants. The source term of the nuclear leakage of radioactive material is designed using PM2.5 as the tracer of radioactive pollutants, and the study considers dry and wet deposition pro-cesses. A coupled-model system is constructed from the air-quality model Models-3/CMAQ and the Weather Research and Forecasting atmospheric model. The transport path and distribution of radioactive pollutants over long and short distances are simulated with different model horizontal resolutions of 30 and 4 km respectively. The long-distance simulation shows that, following the Fukushima nuclear accident, under the effect of westerly winds, radioactive pollutants are transported generally towards the eastern Pacific and reach the American continent after 5 days, but their concentration is only about 10–7 times the concentration near the Fukushima Nuclear Power Plant. The time required for pollutants to reach the United States is basically consistent with measurements made in California on March 18. Because the upper westerly wind is faster than the lower west-erly wind, the distribution of pollutants tilts eastward in terms of its vertical structure. The short-distance (local) high- resolution simulation indicates that strong winds and precipitation associated with a cyclone can accelerate the deposition, dif-fusion and transport of pollutions, and local cyclonic circulation can change the transport path of pollutants, even resulting in repeated effects of pollution in some areas. Pollutants disperse to southeastern Honshu, Japan, on March 14, 2011, agreeing well with the timing of local observations of increases in the absorbed dose rate. Results also show that radioactive pollutants from the Fukushima nuclear accident are mainly transported and diffuse eastward, resulting in a relatively short-term impact on the Japanese mainland even under the influence of the cyclone system. Therefore, in terms of atmospheric conditions, the location of the Fukushima Nuclear Power Plant is appropriate and could serve as a reference to site selection and protection of other nuclear facilities.

radioactive pollutant, Fukushima, dispersion and transport, numerical simulation

Citation: Fei J F, Wang P F, Cheng X P, et al. 2014. A regional simulation study on dispersion of nuclear pollution from the damaged Fukushima Nuclear Power Plant. Science China: Earth Sciences, 57: 1513–1524, doi: 10.1007/s11430-013-4811-2

A M9.0 earthquake occurred at 05:46 (all times UTC) on March 11, 2011, centered in the Pacific Ocean east of the

Japanese island of Honshu (38°N, 142.9°E). The earthquake triggered a tsunami, which seriously damaged the Fukushi-ma Nuclear Power Plant and led to leakage of radioactive pollutions (e.g., 131I and 137Cs) that pose a serious threat to public health and the environment. By the end of March,

1514 Fei J F, et al. Sci China Earth Sci July (2014) Vol.57 No.7

according to measurement data released by the Chinese Ministry of Environmental Protection (http://www.mep. gov.cn/gkml/hbb/qt/201104/t20110419_209414.htm), the radionuclide 131I had been detected in a number of provinces and cities in China. On the morning of April 12, the Fuku-shima nuclear incident was ranked at the highest level 7 by the Nuclear and Industrial Safety Agency, making it only the second level-7 nuclear incident internationally.

Generally speaking, there are three channels that transport and diffuse leaked radioactive pollutants: atmos-pheric circulation, ocean surface circulation and thermoha-line circulation into the interior ocean at depth (Qiao et al., 2011). Among these channels, transportation through at-mospheric circulation is the fastest and is affected by com-plex and changeable weather conditions, including wind, precipitation and atmospheric vertical motion and stability (Zhang, 2005). Depending on the particle weight, nuclear materials can pollute regions near or far from a pollution source, and even globally depending on the atmospheric conditions. Rain and snow accelerate the settlement of ra-dioactive particles (Jiao, 2010), which could result in more serious surface pollution in a precipitation area and less pollutant in the air, thus affecting the dispersion of pollu-tants.

Because the Fukushima Nuclear Power Plant is located at a latitude at which westerly winds prevail, pollutants dis-perse eastward most of the time and have little impact on China. However, the complex and changeable weather situ-ations in spring complicate the dispersion of pollutants. In spring, frequent cyclone activity dominates weather pro-cesses in East Asia (Yao et al., 2003, Zhang et al., 2012). A cyclone often leads to high winds and precipitation, which can accelerate the dispersion and local deposition of pollu-tants.

The numerical model is an important means of quantita-tively assessing the concentration and transport path of ra-dioactive pollutants in the atmosphere. Starting in 1989, the International Atomic Energy Agency, the World Meteoro-logical Organization and other international institutions jointly organized “atmospheric diffusion model assessment activities”1), resulting in the establishment of atmospheric dispersion models widely used in emergency deci-sion-making and simulating radioactive pollutants. At pre-sent, there are two types of numerical models used in simu-lating radioactive pollutants: the Lagrangian model and Eu-lerian model. Lagrangian particle models compute trajecto-ries of a large number of so-called particles to describe the transport and diffusion of tracers in the atmosphere. Many international organizations have developed Lagrangian par-ticle dispersion models and used them to simulate radioac-tive pollutants; e.g., the FLEXPART model (Stohl et al., 2005), ParModel (Yao et al., 2005), and Monte Carlo multi- source model (Zhang et al., 1999). Atmospheric dispersion

models, such as the segmented plume model, puff model and particle-puff dispersion model, have also been rapidly developed (Cai et al., 2000). In forecasting the leakage of nuclear radiation in the Fukushima incident, Lagrangian particle dispersion models have been widely used. Many institutions, including the Central Institution for Meteorol-ogy and Geodynamics in Austria, Finnish Meteorological Agency, Norwegian Institute of Air Research and Environ-mental Emergency Response Center of the Beijing Regional Specialized Meteorological Center, have published short- term forecasts of the dispersion trend of nuclear radiation based on a Lagrangian model. However, a Lagrangian mod-el usually does not include air entrainment, vertical mixing due to clouds and other transport and mixing effects. In ad-dition, wet and dry deposition processes are relatively sim-ple in the Lagrangian model. A variety of physical and chemical processes are included in the Eulerian model in simulating the long-range transport of pollutants, which is impossible in the Lagrangian model (Gao et al., 1997). Eu-lerian transport models include the Community Multi-Scale Air Quality (CMAQ) model for the United States (Byun et al., 1999) and the Chemical Weather Forecast System (CFORS) for Japan. Additionally, the CFORS model (Uno et al., 2004) has been improved and used in simulations of the dispersion of a radioactive plume in a nuclear test (Zheng et al., 2000), and has provided satisfactory simula-tion results for high-and low-altitude nuclear tests (Zhang et al., 2008a, 2008b). The CMAQ model can accurately simu-late the long-range transport of pollutants and consider fully wet and dry deposition processes and physical processes, such as processes of diffusion and advection, cloud and aerosol effects, and plume treatment. Therefore, the CMAQ model can simulate the dispersion and deposition of pollu-tants during a nuclear accident more accurately.

By designing the source term of the nuclear leakage of radioactive material and considering dry and wet deposition processes, this paper builds a coupled model system from the air-quality model Models-3/CMAQ and Weather Re-search and Forecasting (WRF) atmospheric model. Using this system, the transport path and distribution of radioac-tive pollutants over long and short distances are simulated with different model horizontal resolutions of 30 and 4 km respectively. The effects of nuclear radiation on areas in Japan under different atmospherically conditions are also analyzed. The validity of the coupled model is tested and preliminary results are acquired.

1 Simulation and forecast system for nuclear pollutants

A coupled model system is built from the Models-3/CMAQ

1) IAEA, WHO, ECE. 1989. ATMES Technical Specification Document.

Fei J F, et al. Sci China Earth Sci July (2014) Vol.57 No.7 1515

and WRF atmospheric model, and is mainly applied to the long-range and short-range simulation and forecast of the dispersion and deposition of nuclear pollutants. Figure 1 shows the system flow.

1.1 WRF atmospheric model

The WRF model was jointly developed by the National Center for Atmospheric Research, the National Centers for Environmental Prediction (NCEP) and other agencies and has been widely used in weather numerical simulation and forecasting. The model is fully compressible and non- hydrostatic and uses an Arakawa-C grid staggering. The control equations are in flux form. The WRF model con-tains a complete set of physical processes such as the radia-tion process, the boundary layer parameterization process, the convective parameterization process, the sub-grid tur-bulent diffusion process, and micro-physical processes. It is portable, easy to maintain, scalable and highly efficient. Results obtained with the WRF model are transformed by the Meteorology-Chemistry Interface Processor module and provide meteorological background information and the dry deposition velocity for the source-term processing module and CMAQ model.

1.2 Models-3/CMAQ air-quality model

Models-3 developed by the United States Environmental Protection Agency is an air-quality model that can simulate complex atmospheric physics and chemical reactions and be applied to environmental assessment and decision analysis. The CMAQ model is the core part of Models-3 and can simulate a variety of transport and transformation processes of pollutants. The main module in the CMAQ model is the chemical reaction mechanism processing module, which includes three categories of processes: (1) purely chemical reaction processes between all types of reactants, with the use of the CB-IV (Carbon Bond-IV) mechanism, RADM2 (Regional Acid Deposition Model) mechanism and other mechanisms; (2) diffusion and advection processes that are purely related to atmospheric conditions, including advec-tion, sub-grid-scale diffusion and turbulent diffusion; and (3)

Figure 1 Components and processes of the coupled-model system.

processes related to both chemistry and meteorology, which can be further divided into (a) optical decomposition pro-cesses related to radiation, (b) pollutant plume diffusion processes, and (c) chemical processes related to clouds. Equations for the dry-deposition velocity of gas and parti-cles in the model are Vd = (Ra + Rb + Rc) – 1 and Vd = (Ra + Rb + RaRbVg) – 1 + Vg, where Ra is the aerodynamic re-sistance coefficient, Rb is the border resistance coefficient, Rc is the surface resistance coefficient, and Vg is the particle sinking speed. In parameterizing resistance coefficients, the distribution of vegetation provided by the WRF model is fully considered. The amount of wet deposition in the model is calculated depending on the circumstances considering the absorption effect of cloud and water or by calculating the Henry balance equation. A variety of physical processes such as diffusion and advection processes, cloud and aero-sol effects, plume processing, and aerosol dry deposition simulation are fully considered in the model. The model can simulate the diffusion and deposition processes of radioac-tive particles under different meteorological and environ-mental conditions. The CMAQ model has been widely used in China and abroad in recent years and the authors of this paper also use it in the simulation of atmospheric pollutants such as ozone, dust storms and fog (Lu et al., 2005, Wang Y B et al., 2009, Wang R et al., 2009, Wang et al., 2012).

1.3 Designing a source term of nuclear radiation

In simulating the dispersion trend of pollutants from the Fukushima nuclear accident, the processing of an emission source of nuclear pollutants is important. The current de-fault source-term processing part of Models-3/CMAQ is the Models-3 Emission Processing and Projection System (MEPPS). According to weather conditions and socio- economic activities, the emission source is categorized as a point source, non-point source, mobile source or biological source, and each category has its corresponding processing module. This forecasting system has improved the MEPPS module, and the radionuclide emission points can thus be set according to the specific circumstances of the nuclear accident and the emission rate can also be set. The unit of concentration is becquerels per cubic meter (Bq/m3).

Considering that there was no jet of radioactive particles to the upper air (Stohl et al., 2012) during the Fukushima nuclear accident in Japan, radioactive particles are set to release from the ground at the location of the Fukushima Nuclear Power Plant (37.3°N, 141.0°E) according to the radionuclide leakage pattern. Various particles released in the nuclear accident have different weights. A substance suspended in air can easily adsorb radioactive particles and suspend the radioactive particles in air, thus forming radio-active aerosols. In this paper, we assume that the radioactive substances are adsorbed on aerosol particles, PM2.5 (aerosol particles whose diameter is less than 2.5 μm) is chosen as the tracer of the nuclear pollutants, and the radioactivity of


the particles is set proportional to the particle mass. From the start to the end of the simulation, particles are released at a uniform rate into the air. These assumptions will result in a slight difference in the pollutant concentration in the simulation from that in the actual situation, but will not af-fect the dispersion trend. Finally, according to the weather conditions and the state of the environment, the Emis-sion-Chemistry Interface Processor module is employed to process data into hourly and three-dimensional format, providing real-time emission data of the radioactive parti-cles for the CMAQ model.

2 Simulation of mid-range and long-range dis-persion and transport of nuclear pollutants

2.1 Set of model parameters

In the early morning of March 12, a strong earthquake and subsequent tsunami caused an explosion at the Fukushima Nuclear Power Plant. According to the available meteoro-logical background field, the radionuclide 137Cs is assumed to begin to release at 00:00 on March 12 from the Fukushi-ma Nuclear Power Plant. The simulation area covers the Pacific Ocean above 20°N, ranging from eastern China to western America. The model uses NCEP FNL (Final) glob-al reanalysis data as initial and boundary conditions. The data have global coverage and spatial resolution of 1° × 1°. The interval time of the data is 6 h, with data corresponding to times of 00, 06, 12 and 18 h. The data are available on the surface, at 26 mandatory levels from 1000 to 10 hPa. Parameters include surface geopotential height, temperature, potential temperature, wind, sea-surface temperature, pre-cipitation and tropopause pressure and other physical quan-tities. The release rate of radioactive substances refers to release amounts of 137Cs (0.14 TBq/h) published by Nuclear Safety Commissions of Japan on April 23. The daily release amount is 3.36 × 1012 Bq, the release resolution is 0.5° × 0.5°, and the release position is on the ground at the Fuku-shima Nuclear Power Station (37.3°N, 141.0°E). It is as-sumed that the release rate of radionuclides is 3.89 × 107

Bq/s, which is put into the model. The other specific param-eters are given in Table 1. The system uses a relatively coarse resolution (30 km) and we mainly analyze the long-

distance transport of nuclear pollutants under large-scale circulation conditions. This simulation is denoted as test 1.

2.2 Simulation of pollutants at mid-distance and long-distance

The simulation results show that starting from 00:00 on March 12, 2011, pollutants are transported in a northeasterly direction, reaching 160°E and 47°N (Figure 2(a)) and crossing the 180° meridian (Figure 2(b)) into the central Pacific at 12:00 on March 12. Subsequently, the dispersion of pollutants accelerates, and at 12:00 on March 15, pollu-tants reach the western United States and Russia’s Kam-chatka Peninsula to the north (Figure 2(c)). At 12:00 on March 16, the pollutants reach the western coast of North America and firstly enter the states of Washington, Oregon and California, although the concentration is a factor of about 10–7 times the concentration around the Fukushima Nuclear Power Plant (Figure 2(d)). The Comprehensive Nuclear-Test-Ban Treaty Organization measured the radia-tion level in California on March 18 as 100 times the usual level, or about 10 μSv/h. The data confirm that certain radi-oactive pollution dispersed from Fukushima to the United States across the Pacific Ocean (http://www.epa.gov/ japan2011/data-updates-march.html#18), but the level of radiation is still less than the levels of natural radiation from rocks and solar radiation (a factor of about 10–6), which means the radiation has only a subtle effect on public safety and life. Comparing the observation and simulation data, the simulated time is consistent with the monitoring time when pollutants reach the United States, which verifies the simu-lated dispersion trend and scope of radioactive pollutants output by the system.

A cyclone is a convergent rising system, and thus, the center of a cyclone is a concentrated area of pollutants, where updraft transports pollutants to the upper air and up-per-air current carries the pollutants over long distances. During the Fukushima nuclear accident, an extratropical cyclone from northeast China crossed the ocean of north-eastern Japan from west to east. Figure 3 is the surface weather analysis for 12:00 on March 14 released by the Korea Meteorological Administration, in which the cyclone center is located near 46.5°N, 151.5°E. Figure 4 is a simula-

Table 1 Model parameters in test 1

Category WRF CMAQ

The central location of domain (40°N, 142°E) (40°N, 142°E)

Number of grids, horizontal resolution 300×150, 30 km 297×147, 30 km

Vertical levels 28 levels with different distance 28 levels with different distance

Physical and chemical processes

Lin Microphysical scheme, RRTM Long-wave radiation scheme, Dudhia short-wave radiation scheme,

Noah Land surface processs cheme, Betts-Miller-Janjic Cu-mulus parameterization scheme and YSU Boundary layer

parameterization schemes

CB4-Chemical mechanism

Simulated time From 00:00 on March 12, 2011 to 00:00 on March 17, 2011, a total of 5-day simulation


Figure 2 Simulation results for the concentration of radioactive particles 137Cs from the Fukushima Nuclear Power Plant in the near-surface atmosphere. (a)–(d) show the cases for 12:00 on March 13, 12:00 on March 14, 12:00 on March 16,and 12:00 on March 16, respectively.

tion of sea-level pressure, vertical velocity at 850 hPa and the wind field at 500 hPa. The simulated cyclone center position (46°N, 151°E) and the ground weather situation in East Asia are consistent with the observation.

Strong upward motion at the front of the cyclone (Figure 4, 46°N, 155°E–160°E) is conducive to the upward transport of pollutants. Figure 5 is a vertical cross-sectional plot of the simulated potential temperature (solid line), the


Figure 3 Surface weather chart for 12:00 on March 14, 2011 (Korea Meteorological Bureau). Source: http://web.kma.go.kr/chn/weather/images/ analysischart.jsp.

concentration of the radioactive plume (colored area), and the zonal wind and vertical wind (vector) along 46°N at 09:00 and 18:00 on March 14 from eastern Japan to the central Pacific (140°E–180°E). It is seen that the cyclone is associated with a cold front, and the upward motion brings nuclear materials to the upper air. This motion leads to a concentration of radionuclides greater than 103 Bq/m3 up to an altitude of 4 km and a center of pollutant uplift of about 1.5 km in height (Figure 5(a)). The rate of dispersion and the altitude of the radioactive pollutants are closely corre-lated. The rate of dispersion of pollutants in the troposphere is usually proportional to the altitude. Because the wind speed increases with altitude, the distribution of pollutants tilts eastward in the vertical structure. Affected by the wind at this height, the dispersion accelerates and the center of the pollution plume moves eastward about 10 degrees of

longitude in 12 hours. At 18:00 on March 14, the center of the cyclone has moved to the southern ocean of the Kam-chatka Peninsula (48°N, 159°E), which is accompanied by the slope of the isotherm at that latitude gradually decreas-ing, the vertical airflow weakening and the concentration of pollutants decreasing (Figure 5(b)). At 12:00 on March 15, the range of pollutant diffusion expands significantly com-pared with the previous day (Figure 2(b), (c)), which is clearly related to the updraft brought by the cyclone, espe-cially in the westerly jet belt in which the wind is faster than 40 m/s on March 12–17. If pollutants enter the westerly jet belt above a height of 5 km, assuming that the average westerly wind is 35 m/s (slightly less than the central wind speed), they are transported about 3000 km each day along the direction of the jet and enter the boundary layer through deposition and downdraft.


Figure 4 Simulated sea level pressure, vertical speed at 850 hPa, and wind field at 500 hPa at 12:00 on March 14. Contours: sea level pressure; color: vertical speed (m/s); vector: 500 hPa wind field.

Figure 5 Vertical cross-sectional plot of the simulated potential temperature (solid line), the concentration of the radioactive plume (colored area), and zonal wind and vertical wind (vector) along 46°N. (a) For 09:00; (b) for 18:00. The black triangle indicates the location of the cold front.

3 High-resolution simulation of the short-range (local) dispersion of nuclear pollutants

The local dispersion of nuclear pollutants is mainly deter-

mined by the mesoscale meteorological conditions in the accident area. The simulation area is too large to determine the local dispersion in test 1 owing to the limited computing power and coarse resolution. Therefore, the analysis in test


1 mainly focused on the effect of large-scale meteorological conditions. To study local dispersion, it is necessary to re-peat the simulation in the accident area with higher resolu-tion. Two nested horizontal regions are used. The coarse domain is centered at 38°N, 150°E. The coarse domain has 150 × 150 grid points with grid spacing of 12 km, corre-sponding to the region of Japan and its surrounds; the nested domain has 211 × 211 grid points with grid spacing of 4 km, corresponding to eastern Honshu. The model is then inte-grated from 00:00 on March 12 to 00:00 on March 17, 2011. Vertical levels, physical and chemical mechanisms and the intensity of the release source are the same as in test 1, but the resolution of the release source data is improved to 0.1° × 0.1°. This simulation is denoted test 2. The Fukushi-ma Nuclear Power Plant is located at altitude at which a westerly wind prevails, and pollutants therefore disperse eastward most of the time. However, a cyclone often leads to high winds and precipitation, which can accelerate the dispersion and local deposition of pollutants. Moreover, cyclonic circulation can change the transport path of pollu-tants, even resulting in a repeated effect on a local region.

From March 12 to 17, the weather systems affecting Ja-pan can be divided into two stages. The first stage is from 00:00 on March 12 to 12:00 on March 14, during which time consecutive extratropical cyclones that formed in Chi-na move eastward and affected northern Japan. Pollutants steadily disperse eastward. The second stage is from 12:00 on March 14 to March 17. In this stage, cyclones continue moving eastward and affected central and southern Japan. The cyclone system changes the dispersion trend of pollu-tants; pollutants directly disperse southwest and affect Japan's southern cities, including Tokyo. In this section, we focus on the analysis of the second stage.

At 12:00 on March 14, the northern cyclone that moved to northeastern Japan has little impact on the dispersion of radioactive pollutants. At the same time, to the south of Honshu, a mesoscale low-pressure system appears (Figure 6(a)), moves northeastward and rapidly strengthens. The Fukushima area is located between the two pressure systems, where the north and south winds converge, and therefore, the eastward dispersion trend of pollutants does not change (Figure 6(a)). As the northern cyclone gradually moves eastward and weakens, the region is mainly controlled by the southern cyclonic system (Figure 6(b)–(d)). The east of Honshu is affected by northeasterly winds so that pollutants directly disperse southwestward and reach inland Japan. At 06:00 on March 15, the farthest pollutants reach southern Japan, in the region west of 139°E (Figure 7(d)). At the same time, in the Sea of Japan, another cyclone appears and moves eastward, bringing much precipitation (Figure 6(d), (e)). Pollutants deposit on the ground and the concentration of pollutants in the atmosphere decreases (Figure 8(e)). Be-cause of a lack of mesoscale meteorological observations in the Japan region, particularly observations at sea, satellite cloud images (not shown) are compared with the simulated

weather situation, especially in terms of areas of precipita-tion and cloud. We find that the simulation results are rea-sonable. At 18:00 on March 15, with the eastward shift of the cyclone (Figure 6(f)), the westerly wind in this region weakens gradually, and then shifts to the north and eventu-ally to the northwest. The effect of pollutants on southeast-ern Honshu (Figure 7(f)) weakens as a result.

The simulation results are compared with observation data of the absorbed dose rate at a site in Onuma Hitachi (located in southeast Fukushima) obtained by the Japanese Ministry of Education. Because the absorbed dose rates take into account many factors, we focus on the comparison at the time when pollutants come into play.

Figure 8 shows the simulated 137Cs activity concentration in the lower atmosphere (blue line) and the observation of the absorbed dose rate (red line) at the observation site. Both simulation and observation indicate that the low- pressure system in southern Japan transports pollutants to southern Japan from 18:00 on March 14 to 09:00 on March 15, especially the times in the simulation and observation when pollutants reach Onuma Hitachi City (Figure 8) differ by only about 1 hour, it indicating consistent observation and simulation results. Although the simulated secondary peak value lags the observed peak by about five hours at 04:00 on March 15, and the simulation value tends to zero at 09:00 on March 15, the simulation results well present the overall trend of radioactive pollutants, confirming the good reliability of the simulation results.

4 Discussion

Using the coupled-model system established in this paper, the dispersion of nuclear pollutants is simulated. Since the simulation made certain assumptions, the results are rela-tively preliminary. Owing to the lack of observational data of nuclear pollutants, some of the simulation results are dif-ficult to verify. In the framework of the coupled model es-tablished in this paper, the key to improving the quantitative forecast is the design of the source term; e.g., emissions need to change over time, and radioactive particles with different radii should be treated separately to obtain a more accurate simulation of pollutant deposition. Such improve-ments can be made through the analysis of numerical simu-lations, laboratory tests and practical examples. This will be the next step in future work.

After the Fukushima nuclear accident, many international institutions released numerical forecast products of the dis-persion of nuclear pollutants, mostly using a particle disper-sion model, such as the HYSPLIT-4 dispersion model, which is used by the Beijing Regional Specialized Meteor-ological Centre. However, the present study employed the CMAQ air-quality model, which includes a variety of phys-ical processes such as wet and dry deposition, and has cer-tain significance as a reference for building a nuclear-


Figure 6 Simulated sea-level pressure fields (black solid line) from 12:00 on March 14 to 18:00 on March 15 and hourly precipitation (color).


Figure 7 Simulated concentrations of 137Cs (color) and wind field (vector) in the near-surface atmosphere near the Fukushima Nuclear Power Plant.


Figure 8 Observation of the absorbed dose rate (red line) and simulation of 137Cs concentration in the lower atmosphere (blue line) at the Onuma Hitachi site from 00:00 on March 12 to 00:00 on March 17.

pollutant dispersion and forecast system. Although the Fukushima nuclear accident has not direct-

ly affected China, it has raised concern throughout the country. International worries about the security of nuclear power have quickly escalated. This study showed that radi-oactive pollutants from the Fukushima nuclear accident mainly dispersed eastward. Even under the influence of a cyclone system, the period during which nuclear pollutants affected the Japanese mainland was very short. Therefore, from the view of meteorological conditions, the authors consider that the location of the Fukushima Nuclear Power Plant is scientifically appropriate. Therefore, not only so-cio-economic development and geographical factors but also the characteristics of weather and climate should be fully considered when choosing the location of a nuclear power plant. Weather conditions and climate background directly affect the dispersion of pollutants, choice of emer-gency protective measures and emergency response action. Establishing a scientific and effective nuclear pollutant dis-persion and forecast simulation system is an important sci-entific approach for understanding the laws of development and variation of such dispersion, and it is also fundamental to implementing an environmental emergency response. At the same time, it is necessary to plan and establish a re-al-time environmental observation system around a nuclear power plant to provide the observations needed by the mod-el system. Therefore, in learning lessons from the Fukushi-ma nuclear accident, great importance should be placed on constructing environmental observation and forecasting capabilities for a nuclear power plant.

5 Conclusion

This study established a coupled-model system based on the WRF atmospheric model and Models-3/CMAQ air-quality model to investigate the Fukushima nuclear accident result-ing from an earthquake and subsequent tsunami in Japan in March 2011. The coupled model was used to simulate the transport path and dispersion of radioactive pollutants over long and short distances. The main conclusions of the study

are as follows. (1) By designing the source term of nuclear leakage of

radioactive materials, using PM2.5 as the tracer of radioac-tive pollutants, and considering the dispersion of nuclear pollutants and dry and wet deposition processes, a coupled model system was built from the air quality model Mod-els-3/CMAQ and the WRF atmospheric model. The simula-tion results for transport path, range and concentration of nuclear pollutants showed that the forecast capability of the coupled-model system is satisfactory.

(2) Under the influence of westerly winds, radioactive pollutants from the Fukushima Nuclear Power Plant were transported eastward over the Pacific Ocean most of the time. The pollutants reached the American continent after 5 days, but at a concentration that was only about 10–7 times the concentration near the Fukushima Nuclear Power Plant. The simulated time required for pollutants to reach the United States was basically consistent with observations made in California. At the same time, because wind speed is greater at higher altitude, the distribution of pollutants was tilted eastward in terms of its vertical structure.

(3) Cyclones occur frequently in spring in Japan and nearby areas. A cyclone is a convergent rising system and often brings heavy precipitation. Three cyclones affected Japan during the simulation period. Simulation results showed that the cyclone system caused rising movement of the air, which brought pollutants into the upper atmosphere. Hence, pollutants were transported more quickly. Cyclonic circulation affected the horizontal path of pollutants, even resulting in pollutants having repeated effects in some areas, and cyclonic winds and precipitation accelerated the disper-sion and local deposition of pollutants. Furthermore, precip-itation greatly diluted the concentration of nuclear pollu-tants in the atmosphere, and induced the partial deposition of pollutants on mainland Japan, resulting in a long-term impact of pollutants on mainland Japan.

The authors are grateful to the two anonymous reviewers for their thoughtful comments. This work was supported by the Special Funds of Public Welfare of China (Grant No. GYHY201306061) and the National Natural Science Foundation of China (Grant Nos. 41230421, 41105065 & 41275128).


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a regional simulation study on dispersion of nuclear pollution from the damaged fukushima nuclear...

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