MINISTRY OF EDUCATION
AND
TRAINING

MINISTRY OF SCIENCE AND

TECHNOLOGY
VIETNAM ATOMIC ENERGY
INSTITUTE

PHAM KIM LONG

RESEARCH AND
APPLICATION OF
FLEXPART IN THE
LONG-RANGE
ATMOSPHERIC
DISPERSION OF
RADIONUCLIDES

Major: Nuclear and Atomic
Physics
Code: 9.44.01.06

SUMMARY OF


DOCTORAL
DISSERTATION OF
PHYSICS


1. The reason for choosing the thesis topic
Lessons learnt from the Chernobyl disaster in 1986 or the
Fukushima nuclear accident in 2011 show the importance of
environmental radioactivity monitoring, simulation, calculation and
evaluation of atmospheric dispersion of radionuclides from nuclear
power plants (NPP) in supporting the emergency preparedness in
response to nuclear reactor accidents. Among the NPPs near our
country, the Fangchenggang NPP is located near the border of our
country less than 50 km, about 250 km away from Hanoi capital. We
need to build an Environmental Radiation Warning and Monitoring
Network to continuously monitor artificial and natural radiation
levels, combined with simulation of atmospheric dispersion of
radionuclides in supporting the emergency preparedness.
Due to these urgent requirements, I chose the thesis topic
“Research and application of FLEXPART in the long-range
atmospheric dispersion of radionuclides” with my desire to
contribute a small part in the field of monitoring, warning and
responding to environmental radiation incidents. The aim of this
thesis is to provide a suitable model for evaluation of the long-range
atmospheric dispersion of radionuclides in supporting the emergency
preparedness.
2. Purpose of this study
To learn mathematical models and simulation programs
suitable for long-range dispersion of radioactivity.

1


To learn meteorological models that meet the requirements of
simulation programs and meteorological data analysis tools.

observations yields confidence regarding its application to assess
radiation impacts and support emergency planning in response to a
possible future nuclear accident in the region.
5. The layout of the thesis
The thesis consists of 100 pages of content, 18 tables, 57
figures, 03 published works (02 articles and 01 national nuclear
conference), 79 references, 6 annexes, to be allocated as follows:
Overview: Introducing the reasons for choosing the thesis
topic, purpose, objective, scope and methods of research,
significance of the thesis (4 pages); Chapter 1: Overview of nuclear
power plants in East Asia, atmospheric dispersion model and
meteorological characteristics (30 pages); Chapter 2: Evaluation
methods

in

atmospheric

dispersion

of

radionuclides

using

FLEXPART (37 pages); Chapter 3: Results and Discussion (25
pages); Conclusion and recommendations for future research (3
pages); Finally, the list of publications related to the thesis,
references, and annexes.


Within a distance of 1000 km from our country border, 18 units
are operating and 4 units are under construction belong to China [1].
The units include generations of II, II +, III and III + reactors. In
particular, the Fangchenggang NPP is located in Quangxi, China.
With a distance less than 50 km from our country border, about
250 km away from Hanoi capital (Fig. 1.5). A total of six reactors
are planned to operate at Fangchenggang NPP (2 units are in
operation, 2 units are under construction, and 2 units are in the
construction plan) [2].

Fig. 1.5. Satellite image of Fangchenggang NPP
(Source: Google Earth, updated on May 10, 2016)
In fact in East Asia, the accident happened at Fukushima NPP
in March 2011 [4]. A huge amount of radioactive material was
released into the atmosphere and dispersed across the northern
hemisphere. Japan raised the disaster at Fukushima Daiichi to Level
7 on the INES scale [10].

6


As we all know when the nuclear accident occurs, an
inevitable byproduct of nuclear fission is the production of fission
products which are highly radioactive released into the environment,
especially the atmosphere. Radioactive isotopes are very useful in
environmental research to assess the extent of accidents affecting the
environment and people.
In such an urgent situation, we need to build an Environmental
Radiation Warning and Monitoring Network to continuously monitor

FLEXPART (Stohl et al., 1998, 2005) to simulate the atmospheric
transport of radionuclides from NPPs. The block diagram is shown in
Fig. 2.1.

Fig. 2.1. The block diagram of FLEXPART

8


2.3. High performance computing
In this study, the simulations were designed to run on the
PARAM-HUST supercomputer at the Centre for High-Performance
Computing, Hanoi University of Science and Technology.
2.4. Source term estimation of the atmospheric release
There are two main contents in this study:
1) Validation of the atmospheric dispersion model through the
Fukushima nuclear power plant accident. The source term has been
published by the many studies, such as Chino (2011), Stohl (2012), Terada
(2012), and Katata (2015) [5, 34, 35, 37]. In this study, the source term for
131

I and 137Cs were used by referring to the study of

Katata et al. (2015) [27] and WMO report (2013) [33].
2) Application of FLEXPART for assessing the impart of
atmospheric transport of radioactivity from Fangchenggang nuclear
power plant with regional characteristics of meteorological. The
source term is assumed to correspond to level 7 of the INES scale.
2.5. Meteorological input data
2.5.1. Meteorological input data for FLEXPART

9


2.5.3. Meteorological data analysis
NASA's Panoply was used to analyze meteorological data such
as pressure, temperature, wind speed and wind direction. Thereby
statistical analysis of meteorological data to see the main effects of
atmospheric circulation affecting the dispersion of radionuclides in
the atmosphere. In addition, other meteorological data analysis
software is used, such as NCL, WGRIB.
2.6. FLEXPART postprocessing tools
The output format of FLEXPART is binary files, in this study
using Quicklook developed by Radek Hofman to create regional and
global concentrations maps. In addition, the output data processing
software for FLEXPART such as Quickdose, Pflexible, and
Reflexible was developed by FLEXPART‘s community.
2.7. Statistical evaluation methods
Procedures

for

evaluating

atmospheric

Cs from FNPP to TWP and SEA. The results are evaluated with
observational data from the region. We assess the contribution of the
northeast monsoon and its associated meteorological conditions to
regional transport of Fukushima-derived radionuclides because this
could inform the potential impacts of radiological emissions from
other NPPs in northeast Asia (NEA).
For the emission scenarios of 131I and 137Cs, source terms from
Katata et al. (2015) were adapted for this simulation (Fig. 3.2). There
were two sets of particle median diameter (d p) and particle density
(ρp), namely, dp = 0.6 μm, ρp = 2500 kg/m3 (Arnold et al., 2015;
Geng et al., 2017) in the simulation I, and d p = 0.4 μm, ρp = 2500
kg/m3 in the simulation II. The simulations were designed to run on
the PARAM-HUST supercomputer.

11


Fig. 3.1. The locations of radiation monitoring stations operating
during the FNPP accident in TWP and SEA (red points). The FNPP
is marked with a star in yellow.

Fig. 3.2. The emission rates of 131I and 137Cs during the FNPP
accident according to Katata et al. (2015).

12


Table 3.1. FNPP-derived 131I observed at monitoring stations in TWP
and SEA.


13


concentrations in the atmospheric columns from 0 to 2 km and 2–10
km. These represent the planetary boundary and troposphere layers,
respectively.

Fig. 3.4. The meteorological conditions (mean sea level pressure
(Pa), wind speed (m/s) and wind direction) driven the first regional
plumes on March 18 and the second regional plume on April 4
3.1.2.1. Hermispherical transport
After the first release of radioactivity at FNPP on March 12,
the radioactive cloud was transported towards the Pacific Ocean,
where it was captured by the extra-tropical low-pressure system
located over the Bering Sea (the Aleutian Low). The plume was
lifted up to the troposphere and then rapidly transported
northeastward via the jet stream (Fig. 3.9 and 3.10). Cyclonic and
frontal systems following the jet stream led to vertical mixing
resulting in surface detection of radionuclides across the northern
hemisphere (Mészáros et al., 2016). The hemispherical plume
progressively arrived from North America (16–18/3) to the north
Atlantic (19/3), Scandinavia (22/3), western Russia (23/3), NEA and
then the western Pacific (30–31/3) (Fig. 3.9).

14


Fig. 3.9. Mean airborne concentration (in Bq/m3) of 131I in the 0–20
km layer.
3.1.2.2. Regional transport


The second regional plume departed from Japan on
approximately April 4, when two eastward moving anticyclones
occurred over the western Pacific with one approaching Japan and
the other further east (Fig. 3.4b). The FNPP radioactive plume was
blocked from tropospheric transport for several days by the eastern
anticyclone, while the anticyclone approaching Japan forced the
plume to move in a southwest direction and subsequently traveled
under the influence of the northeast monsoon. The radioactive air
mass traveled almost entirely within the marine boundary layer (Fig.
3.9d) and was not observed at higher altitudes (Fig. 3.10d).

Fig. 3.11. 131I and 137Cs peak activity concentrations as a function of
distance from Fukushima (km) during the second regional plume
The peak activity concentration associated with the arrival of
the second regional plume decreased exponentially with distance
from FNPP and had decreased by half after a distance of 577 km for
131

I and 433 km for

of

137

137

Cs, as shown in Fig. 3.11. The faster decrease

Cs peak activity concentrations with distance indicates greater

131

(dp = 0.4 μm).

Fig. 3.12. Time series of observed (blue column) and simulated 131I
surface activity concentration in the simulation I (red line) and
simulation II (green line)

17

I


3.1.5. New findings of this study
Our

research

was

published

in

December

2018

as


monsoon winds blows not only in the northeast direction but also in
the southeast at the end of the phase and intermediate directions. This
makes it difficult to guess the direction of the radiation plumes. The
thesis only initially mentioned some typical scenarios in January

18


2018 when the Siberian was most active high pressure is the highest
level pressure.
The atmospheric dispersion of radionuclides released from
China’s Fangchenggang NPP to Vietnam was simulated by using the
Lagrangian particle dispersion model FLEXPART with the
meteorological data CFSv2 [5]. The input parameters for particle
size, dry deposition, wet deposition through the Fukushima study
(Long et al., 2019) were used for this simulation with level 7 of the
INES scale. Atmospheric dispersion of radionuclides was simulated
from January 01 to 31, 2018. At this time, there are two main
northeast monsoon winds coming to Vietnam. The cold front began
to appear above Fangchenggang NPP on January 8 and 28 and then
moved through it. The simulation results of the concentration of
in the atmosphere are shown in Fig. 3.18. The concentration of

131

131

I

I at


Cs

derived from the Fukushima Daiichi nuclear power plant (FNPP)
accident to the tropical western Pacific (TWP) and southeast Asia
(SEA). Measurement data at ten monitoring stations located in this
region were used for validation of the model results. There was good
agreement between the model and observations, and the following
conclusion can be drawn.
The airborne radioactivity observed in this region came from
both the hemispherical transport following the jet stream and the
regional transport in the boundary layer by the East Asian northeast
monsoon. Due to the late arrivals of both hemispherical and regional
plumes, the TWP and SEA region had recorded much lower airborne
radioactivity than other regions in the northern hemisphere, which
were affected earlier by the FNPP radioactivity. The regional
transport, however, was more important in contributing Fukushimaderived radioactivity to the regions. The

131

I and

137

Cs activity

concentrations in the regional plume decreased exponentially with
distance from Fukushima and had decreased by half after a distance
of 577 km and 433 km, respectively. The faster decrease of