Abstract—In this study, the local and

regional scale transport and

dispersion of radionuclides released

during the 2011 Fukushima Daiichi

Nuclear accident was modeled using

the HYSPLIT model.

Cesium-137 was chosen as the target

pollutant due to its long half-life

and its effects on man and the

environment at large. The transport

and dispersion was modeled for the

first week after the accident since

it was the period of peak emission

and deposition. The model outputs

show that peak deposition period was

March 14-15, 2011, with ground level

activities in the range of 103 – 106

Bq/m2. The affected parts of Japan

mainland include Fukushima, Ibaraki,

Tochigi, Saitama, Gunma, Chiba, and

Tokyo Prefectures. The area

northwest of the plant up to a

distance of about 50 km shows the

highest level of radioactive

Simisola Ogundugba is with EnvironmentalEngineering Department, Cyprus InternationalUniversity, Lefkosa via Mersin, Turkey (e-mail: simo2001ng@yahoo.com).

deposition. Using appropriate dose

conversion factor, the received dose

within the first year after the

accident was also estimated.

Keywords— Accident, Dispersion,Nuclear, Hysplit.

I.INTRODUCTIONue to the nature of radioactive

materials, a major reactor

accident in a nuclear power plant

can lead to a catastrophe for people

and the environment at large. By

definition, a major reactor accident

would involve the severe overheating

and subsequent melting of the

nuclear reactor core, which would

cause a release of substantial

quantity of radioactive materials,

after breaching several barriers,

into the environment. The major

health and environmental threat

would be due to the escape of the

fission products to the atmosphere,

from where they can be transported

over long distances across trans-

national boundaries.

D

Radioactive Pollution, Local and Regional Scale Transport and

Dispersion Modeling: A Case Study of the Fukushima Nuclear

Accident.

Simisola Ogundugba, and Oluwadara Oke

The disaster that struck Japan’s

FDNPP-1 on March 11, 2011, caused

the most significant release of

radioactivity since that of

Chernobyl, Ukraine 1986 and was far

worse than the 1979 Three Mile

Island accident in USA. Unlike at

Chernobyl and Three Mile Island, the

disaster at Fukushima was initiated

by natural occurrence rather than

equipment failure and human error

although study on the plant reveals

some subtle design errors. At 14:46

JST on March 11, 2011 a magnitude

9.0 earthquake hit the pacific

shores of North-Eastern Japan

causing a shutdown of FDNPP-1 units

1, 2 and 3. Units 4, 5 and 6 were in

their periodic maintenance stages

[16].

After shutdown, the emergency diesel

generators (DG) started

automatically but at 15:27 JST, a

14-15m tsunami hit the coast

followed by high waves until 15:41

on the same day. All the DGs except

one in the unit 6 reactor building

were lost. This is due to the fact

that the failed DGs were installed

in the basement of the turbine

buildings. Therefore, the core

cooling function was basically lost

for units 1, 2 and 3 as well as the

fuel pool cooling function for these

units.

Because heat removal systems failed,

pressure built up to high levels in

the primary containments of units 1

through 3. The containments were

vented to the atmosphere to relieve

the pressure, allowing the escape of

radioactivity leaking from the

reactor cores.

A chemical reaction between the

fuel’s zirconium cladding and high-

temperature steam is believed to

have generated large amounts of

hydrogen in the containments of

units 1 through 3.

(1)

The hydrogen leaked from the

containments or the venting systems

into the reactor buildings and

caused large explosions in units 1,

3, and 4 [16].

The loss of cooling also affectedthe plant’s spent fuel pools whichhold fuel rods that have beenremoved from the reactors aftertheir ability to sustain a nuclearchain reaction has greatly reduced.Although much of the radioactivityin the spent fuel has been decayingfor many years, the large volumes ofspent fuel in the pools represent a

significant source of radioactivity.If water in the spent fuel poolsboils away or leaks out, the spentfuel rods may overheat and releaseradioactive material into the air.However, the explosions of reactorbuildings 1, 3, and 4 exposed thosepools to the atmosphere, and debrisfrom the explosions may have falleninto the pools and damaged thestored fuel [27].Cesium-137 is among the most common

heavy fission products. Fission of

various isotopes of thorium,

uranium, and plutonium all generate

about 6% Cesium-137 [18]. This high

fission yield results in an

abundance of cesium-137 in spent

nuclear fuel, as well as in regions

polluted by fission byproducts after

nuclear accidents [19]. The large

quantities of cesium-137 produced

during fission events pose a

persistent hazard. Its half-life of

about 30 years is long enough that

objects and regions contaminated by

cesium-137 remain dangerous to

humans for a generation or more, but

it is short enough to ensure that

even relatively small quantities of

cesium-137 release dangerous doses

of radiation (its specific

radioactivity is 3.2 × 1012 Bq/g)

[19], [15].

Fig. 1: Decay Scheme of Cesium-137

II. ATMOSPHERIC DISPERSION ANDRADIONUCLIDE RELEASE

A. Theoretical and Physical Basis for DispersionAtmosphere is the most important

pathway for the long-range transport

of radio-nuclides released from a

nuclear accident. IAEA -Tecdoc 379

(1986) explains atmospheric

dispersion phenomenon, atmospheric

dispersion model and features of the

atmosphere affecting the dispersion

in this way: Atmospheric dispersion

is the transport of the effluent by

winds and the subsequent diffusion

by atmospheric turbulence. An

atmospheric dispersion model is a

mathematical relationship between

the rate of effluent release and the

distribution of its concentration in

the atmosphere. The generic equation

for isotropic dispersion is given

as:

(2)

Where: C is downwind concentration (Bq/m3)Q is emission rate (Bq/hr)U is wind speed (m/s) σy, σz are dispersion coefficients dependent on atmospheric stabilityH is the effective emission height (m)x, y, z are the plume coordinates

The factors contributing the

dispersion may be divided into

transport and trajectory process

(advection); diffusion by turbulent

eddies; and modifying process e.g.

depletion.

A. Transport and Trajectory Process

Most models consider the source to

be an ideal point source that is not

accompanied by energy release and

which does not have a significant

effect on atmospheric conditions.

However real sources are of finite

size and have momentum and buoyancy.

There is the initial kinetic energy

due to initial discharge energy and

the thermal energy when the

temperature of the effluents is

above normal atmospheric

temperature. This causes the

pollutant air mass to rise above its

release point while dispersing at

the same time. These effects are

important in regions near to the

source. For longer distances, the

ideal point source assumption is

more accurate for radionuclide

releases.

Puff is a term, which is used in

models that follows the movement of

air masses. A puff of a pollutant

released to the atmosphere travels

along the wind direction and

develops into a continuously

expanding cloud due to turbulent

eddies. The centers of mass of a

series of continuous puffs define a

plume trajectory. Its trajectory is

determined by the wind field and its

variation with time [19].

B. Diffusion by Turbulent Eddies

Wind speed and direction varies

continuously with time in three

dimensions. A long-term wind

direction record shows a summation

of rapid changes. This fluctuation

is referred to as turbulence and is

a basic feature of the atmospheric

transport responsible for eddy

diffusion. The part of the eddy size

spectrum taking part in the

diffusion process depends upon the

size of the cloud of dispersing

material. Eddies much smaller than

the cloud or plume size cause a

minor redistribution of the effluent

within the plume, while eddies much

larger than the plume or cloud cause

it to be bodily shifted without

altering the concentration

distribution inside the plume. As

the cloud travels downwind, the

scale of eddy motion responsible for

atmospheric diffusion increases

continuously [11].

C. Modifying Process (Atmospheric

Removal)

Process removing radionuclides from

the atmosphere and interaction of

nuclides with the earth’s surface

are very important for modeling

atmospheric transport and

consequences of nuclear accidents.

Especially for long-term

consequences, the amount of

deposited radioactivity contributes

more to the total dose received by

humans than the direct exposure from

the plume [1]. Three basic removal

processes contributing to further

depletion of activity are dry

deposition, wet deposition and

radioactive decay.

Dry deposition plays an important

role for most of the radionuclides

except noble gases. Different

materials have different dry

deposition velocities on different

surfaces. Dry deposition rate also

depends on weather conditions in

terms of wind speed and atmospheric

stability. Gravitational settling

has a significant effect on dry

deposition especially for heavy

particles (radius >1 μm) [1].

Radioactive materials can also be

deposited by the action of rain and

it is generally known as ‘wet

deposition’. Two separate processes,

termed washout and rainout, may be

considered. Washout is the

deposition of material by raindrops

falling through a plume (below-cloud

removal) while rainout is removal of

material embedded in raindrops

within the cloud (in-cloud removal).

Both rain and snow can remove

materials from the atmosphere. It is

shown that the washout coefficient

strongly depends on the size of the

particle. This factor, however, is

not included in most atmospheric

dispersion models [1]. Radioactive

decay effect is also treated for the

whole group of nuclides in the

formulation of radioactive chains

when the daughter nuclides are borne

and will grow in the plume with a

decay of the parent nuclides [11],

[20].

III.FEATURES OF THE LONG RANGETRANSPORTAs material dispersion takes place

over longer distances its motion is

affected by larger areas of the

atmosphere and features not

considered in short range modeling

have to be taken into consideration.

According to the IAEA Tecdoc-379

these include:

(i) Vertical changes of atmospheric

conditions encountered as plumes

grow, including wind direction shear

and the presence of elevated

inversions.

(ii) The variations in atmospheric

conditions, such as wind velocity,

stability and mixing layer depth

during the movement of the plume.

(iii) The spatial variation of

atmospheric conditions which means

that data obtained at a single

meteorological situation near the

release point may not quite

represent the conditions over the

region through which the plume is

dispersing.

IV.DESCRIPTION OF THE HYSPLIT MODELHySPLIT, Hybrid Single Particle

Lagrangian Integrated Transport

model was developed in NOAA Air

Resources Laboratory in the United

States for modeling the trajectories

of air parcels, or the transport,

dispersion, and deposition of

pollutants [4]. The inputs needed

for HySPLIT calculations are

pollutant species properties,

emission parameters, gridded

meteorological fields and output

deposition grid definitions. The

horizontal deformation of the wind

field, the wind shear, and the

vertical diffusivity profile are

used to compute dispersion rate.

Gridded meteorological data are

required for regular time intervals.

The meteorological data fields may

be provided on one of the different

vertical coordinate system:

Pressure-sigma, pressure-absolute,

terrain-sigma or a hybrid absolute-

pressure-sigma. The model has the

ability to treat the pollutant as

particles, or Gaussian puffs, or as

top/hat puffs. The term Hybrid

refers to the additional

capabilities of HySPLIT to treat the

pollutant as Gaussian or top/hat

puff in the horizontal while

treating the pollutant as a particle

for the purposes of modeling the

vertical dispersion. A key benefit

of this hybrid model is that the

higher dispersion accuracy of the

vertical particle treatment is

combined with the spatial resolution

benefits of horizontal puff

splitting.

V.METHODOLOGYA. Study PeriodRegarding the nature of this

accident, there was core meltdown

and it is certain that there will be

continuous release of radioactivity

to the environment over a long

period of time via the pathways of

air, water and land [11]. It is

however expedient to investigate the

periods of high and significant

release of radioactivity as this

will cause the highest radiological

consequences on the affected

population in the local, regional

and even global scale. Many of the

reviewed literature uphold the

premise that most of the significant

radioactive release occurred within

the first week after the accident.

it was within this period that

primary containment venting was

carried out in units 1 and 3, and it

was also this same period that the

hydrogen explosions took place in

units 1, 3, and 4 [8], [16], [21],

[25]. In lieu of the reliable facts

presented in the literature, the

first week after the accident from

12-19 March, 2011 is chosen as the

study period

B. Meteorological Input DataThere exist different meteorological

input fields available for use in

Hysplit. This study however uses the

National Weather Service’s National

Centers for Environmental Prediction

(NCEP)'s Global Data Assimilation

System (GDAS) in the Unites States

as the meteorological input data.

The upper air data field include

temperature in [ºK], u and w wind

speed component with respect to grid

in [m/s], pressure vertical velocity

in [hPa/s], geopotential height

[gpm] and vertical humidity [%]. The

surface data fields provided are 2 m

temperature in [ºK], 10 m u and w

components in [m/s], surface

pressure in [hPa], surface

temperature in [ºK], total

precipitation (6 hr accumulation) in

[m], momentum flux u-component at

surface in [N/m2], momentum flux v-

component at surface in [N/m2],

sensible heat net flux at surface in

[ W/m2], latent heat net flux at

surface in [W/m2], downward short

wave radiation flux at surface in

[ W/m2], relative humidity at 2 m

AGL in [ % ], volumetric soil

moisture content fraction of layer

0-10 cm below ground in [fraction],

total cloud cover for entire

atmosphere in [%].

D. Source Term

There are uncertainties surrounding

the actual release rates of the

radionuclides especially during the

early phase of the accident but were

assumed to be the same as the values

recorded at around 21:00 JST on

March 14 (Terada et al., 2012).

Several literature quotes various

source term estimates at different

periods after the accident for some

key radionuclides such as Cs-137, I-

131 and Xe-133 [8], [21], [22].

However the most comprehensive

estimate is seen in the works of

Stohl et al., 2012 which reports the

estimated activities in both the

reactor core units and the spent

fuel pools as at the time of the

accident, when the chain reaction in

the operating reactors was stopped.

The release rate of Cs-137 adopted

for the model run is calculated from

these estimated activities. HySPLIT

model is run using a point source

and as such the releases from the

reactor cores and the spent fuel

pools will be treated as a single

cumulative point source averaged

over the entire study period.

Table 1

Emission Rates Used in Actual

SimulationDate Time

(UTC)

Emission

Rate

(Bq/hr)

Release

Height

(m)14-03-

2011

00.00-

12.00

2.83*1013

(a)

120-150

14/15-03-

2011

12.00-

03.00

1.44*1015 (b) 120-150

15-03-

2011

03.00-

23.59

1.21*1014 (a) 120-150

VI.RESULTSTable 2 gives the values of all variables used in the actual simulation.

Table 2Actual Values Used in Simulation

Paramete

r

Model Run 1 Model Run 2 Model Run 3

Location

(Lat./Lo

n.)

37.233oN/

141.015oE

37.233oN/

141.015oE

37.233

141.015

Time

(JST)

00.00-12.00

(14/03)

12.00-03.00

(14-15/03)

03.00-23.59

(15/03)

Release

rate

(Bq/hr)

2.83E+13 1.44E+15 1.21E+14

Release

Height

(m)

120 - 500 120 – 500 120 - 500

Release

Duration

(hr)

12 15 21

Total

Run Time

(hr)

15 18 24

Pollutan

t

Cs-137 Cs-137 Cs-137

Dry

Depositi

on

Velocity

(m/s)

0.001 0.001 0.001

Henry’s

Constant

0 0 0

Wet

Depositi

on

4.0E+04/5.0E-

05

4.0E+04/5.0E-

05

4.0E+04/5.0E-

05

Radioact 10976 10976 10976

ive Half

Life

(days)

(Adapted from: Terada et al., 2012)

A. Trajectory Analysis and Deposition PlotsThe trajectory analysis depicts

the footprint of the polluted air

masses as they are being advected by

wind. As seen below, the emissions

during the early stages of the

accident were largely carried over

the north pacific due to the

prevailing wind direction in those

days. However between 14th to 15th

March, there was a change in wind

direction as evident in the plots.

It means that the plume was carried

over Mainland Japan, especially

towards the North-West and South-

West and central parts of Honshu.

Fig.2: Trajectory plots from March 12 to 16, 2011

The trajectory analysis for 14-15th

March was drawn on a Google map to

clearly illustrate the areas which

are most likely to be affected due

to the passage of the plume. The

areas affected includes Fukushima,

Ibaraki, Chiba, Utsunomiya, Sendai,

Yokohama, Tokyo; spanning a radius

of about 300km around the power

plant.

Fig.3: Google Earth Plot of March 14-15

Trajectory

Fig.4: Google Earth Plot of Model Run 2

Fig.5: Google Earth plot of Model Run 3

The radioactive plume emitted from

FDNPP-1 first flowed towards the

North Pacific Ocean. The high-

concentration emission due to the

hydrogen explosion which occurred in

Unit-1 on March 12 was carried in

the North-Northwest direction from

the plant and caused a high amount

of dry deposition.

Another hydrogen explosion which

occurred on 14 March led to the

release of high-concentration plume

which started to flow in the South-

Southwest direction from the plant,

causing dry deposition along the

south-eastern area of Fukushima, and

Ibaraki prefectures. This plume

successfully dispersed and caused

widespread dry deposition reaching

far to Tokyo, Saitama, and Kanagawa

Prefectures although the extent was

not as high as those in the former

areas.

From the afternoon of March 15,

there was precipitation observed

widely across eastern Japan. This

precipitation encountered the

passing plume and this caused a high

amount of wet deposition mostly

concentrated around the area

Northwest direction from the FDNPP-

1.

As observed in fig.5, the Northwest

areas of the FDNPP is the most

polluted area with surface activity

values in the order of 106Bq/m2

(highest was 2.2*106 Bq/m2). Fig.6

shows a close-up of the area

indicating that the length of this

area stretches to about 50 km from

the plant and has a width of about

20 – 30 km. This is also highlighted

in some literatures as well [8]

[14], [17]. This unusually high

deposition was due to the cumulative

effects of both dry and wet

deposition, making this entire

region a “hot spot” in the pollution

episode. The other areas affected by

this deposition episode was Miyagi,

Fukushima, Ibaraki, Tochigi, Gunma

and parts of Saitama Prefectures

with ground level activities greater

than 1,000 Bq/m2.

Fig.6: ‘Hot Spot’ Northwest of FDNPP

B. Exposure

IRSN (2011) used a conversion factor

of 16.6mSv/year per MBq/m2 to

establish a relationship between the

Cesium deposit levels and expected

external doses for the first year

after the accident. The expected

dose values are shown in table 3.

Table 3

Cs-137 Deposits and Expected First-

Year DoseCs-137

Deposit

(Bq/m2)

>

300,000

>

600,000

> 1

Million

> 3

Million

6 – 30 Million

First

Year

Externa

l Dose

> 5mSv > 10mSv > 16mSv > 50mSv 100-500mSv

In Japan, the maximum annual

radiation exposure dose for an

individual was 1 mSv before the

Fukushima Daiichi nuclear accident.

The 1 mSv included exposure to

artificial sources of radiation,

such as medical exposure, not

consideration exposure to natural

background radiations [7]. After the

Fukushima accident, the regulatory

dose limit was initially raised to

20 mSv per year for all age groups

and was later lowered to 5 mSv per

year for children.

From the deposition episodes shown

the population in Fukushima, Gunma,

and Ibaraki Prefectures are likely

to receive the highest dose between

10-100 mSv in the first year after

the accident. This population

roughly represents 5.5% of the total

Japan population. It should be noted

however that the dose levels

presented in table 3 do not include

other exposure pathways such as

internal contamination due to

inhalation of particles in the plume

and internal contamination due to

ingestion of contaminated food.

According to the BEIR VII report of

the US National Academy of Science

Advisory Committee on the biological

effects of ionizing radiation, the

dose-response curve for radiation-

induced cancer is a no-threshold

type which suggests that there is a

very high probability of the

occurrence of many radiation-induced

health effects such as cancer,

leukemia, brain problems, etc. in

years to come.

VII.CONCLUSION

From estimates gotten from dust

sampling techniques, high emission

rates were reported within the range

of 1012 - 1015 Bq/h with the highest

rates corresponding to periods of

hydrogen explosion in some of the

units. This emission resulted in the

deposition of radionuclides at

different parts of Japan mainland.

The most contaminated areas are

Fukushima, Ibaraki, Saitama, Chiba,

Miyagi, Tochigi, Gunma, and Tokyo

Prefectures with ground level

activities ranging from 104 – 106

Bq/m2. The area northwest of the

FDNPP-1 up to a distance of about

50km is the most polluted area due

to the formation of “hot spots”

caused by wet deposition on March

15. This area is farther than the

initial 20km evacuation zone. As an

environmental engineer a good way to

verify these results would be taking

representative air and soil samples

in these areas for analysis but this

couldn’t be done because of many

constraints.

Using dose conversion factor

presented by IRSN, people living in

the affected areas will receive

annual external doses from 10 –

100mSv which is extremely higher

than the Japanese annual limit of

1mSv. The consequence of this would

be a high occurrence of radiation-

induced sicknesses such as cancer,

leukemia, brain damage, etc.

The results of the transport and

dispersion modeling show a high

degree of correlation with values

reported in literature. From the

results of the modeling it is worthy

to say that the HYSPLIT model is an

accurate tool for reproducing and

analyzing the transport and

dispersion of airborne pollutants

and can thus be used as an early

decision tool in emergency

situations.

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