March 2016

Modelling of the atmospheric transport and fallout of the releases
from the Fukushima Daiichi accident

In 2012, IRSN made an initial assessment of the atmospheric transport and fallout models for releases
emitted during the Fukushima Daiichi nuclear accident [IRSN (2012)]. The simulations lacked realism
due to the difficulty in reproducing several transport episodes of these releases, and the cause of thedifferences found between the modelling results and the measurements was sought. Since then, thework on atmospheric dispersion modelling by the international community focused on reducing theuncertainties, both those relating to the input data of the atmospheric transport models and thoserelating to the models themselves. This work is still underway.
The weather conditions and the quantification of the atmospheric releases are the main input data of
the atmospheric transport models. The most significant progress has been the result of efforts focusedon improving these data.

Weather conditions
Weather conditions (winds, atmospheric stability, etc.) determine the transport of the radioactive
plume through the atmosphere. Rainfall is responsible for soil contamination due to leaching of theplume. This data is provided by weather forecast models.
One feature of the Fukushima accident concerns the local geography. The accident took place on the
Pacific coast, in an environment with complex orography, with marked reliefs only a few kilometresinland. This configuration greatly penalises the quality of weather forecasts. Furthermore, the accident
followed the earthquake and tsunami that destroyed part of the infrastructure rendering a number of
meteorological observations unavailable. However, these are used to constrain the models. The
operating conditions in the weather models are consequently degraded from their nominal operation.
As from 2012, the difficulty of properly taking into account the impact of the orography on the weather
forecasts was raised by IRSN [Mathieu et al (2012), Korsakissok et al (2013)]. Rainfall forecasts do notalways accurately reflect reality. However, accuracy is essential to simulate soil contamination. The
rainfall fields resulting from "radar" observations can be used as input for dispersion models. Using thisset of data has long been considered as the best option; however, the use of these data remains
delicate. One of the difficulties identified concerns light rain. It is not detected by "radar" observations
but appears to have played an essential role in the ground contamination on 14-16 March.
Since 2011, many weather forecasts were produced. Most of the teams working on the simulation of thetransport and fallout of the releases from the Fukushima accident first used forecasts whose spatial
resolution was about ten kilometres, which is insufficient to properly take into account the influence ofthe relief. The time resolution was often 3h, which is a frequency that was too low to assess when thewind shifted or when rainfall had just begun. Efforts were therefore focused on the production of
weather forecasts with finer resolution. The first IRSN estimates were made using the forecasts of1

March 2016
Météo-France and those of the European Centre for Medium Range Weather Forecasting (ECMWF1)[Mathieu et al (2012), Korsakissok et al (2013), Saunier et al (2013)]. Within the framework of the
UNSCEAR report on the Fukushima accident, experts from the WMO2 compared several meteorologicalfields [WMO (2011), Draxler et al (2015)] and the rainfall fields were the subject of particular attention[Arnold et al (2015)], without however managing to clearly identify a preferred weather forecastingsource for the simulations. As part of this workgroup, IRSN had access to fields from the Japanese
weather forecasting centre JMA. Finally, the MRI studied the difficulties related to the influence of theorography and generated weather simulations of the month of March 2011 at a fine resolution, seeking
to minimise modelling errors [Sekiyama et al (2013) ; Sekiyama et al (2015)]. IRSN had access to thesesimulations as part of the SAKURA project. Observations of rainfall by radar were also used in thesimulations. Given the lack of meteorological fields without uncertainties, many meteorological data
sources were used in order to qualify the interpretation of the results.
Table 1: Description of the meteorological data used by the IRSN for its modelling of the Fukushima accident

Weather data source
used by the IRSN

ECMWF

JMA
(UNSCEAR)MRI

WRF(Winiarek (2014))

spatial resolution

0.125 °
(Approx. 12.5 km)

5 km

3 km

5 km

temporal resolution

3h

3h

1h

1h

Characterisation of the releases
The source term, i.e., the evolution over time of the rate of each radionuclide released into theatmosphere, is essential input data for atmospheric dispersion models. Five years after the accident, its
accurate assessment still remains uncertain. There are two main families of methods for estimating
releases.
1. An approach based on reactor physics and the knowledge of the initial state of the system. It
consists in modelling the evolution of the state of the power plant and the events that led to
the radioactive releases. This approach requires a lot of data. The accuracy of the estimatedsource term is directly dependent on the uncertainties in the modelling of the state of thereactors, the precise knowledge of the events that occurred in the facility and the effectivenessof some of the release mitigation means implemented. To date, there is no complete source
term (including release kinetics for various radionuclides) obtained from this approach. Only
estimates of total amounts were provided, immediately after the accident [NISA (2011), NSC(2011)].
2. Methods coupling the environmental measurements and the atmospheric dispersion simulations
to infer release rates likely to explain the measurements. Inevitably, the quality of the source
term correlates to the accuracy of the weather fields used as input for the dispersion modeland the quantity and relevance of the measurements, since an event can only be reconstructed
if it is observed. There are methods called "simplified" and "inverse" methods. The first are
manual or semi-automatic and are based on a limited set of measurements. The inverse
modelling techniques are more operational automatic methods based on mathematicallyrigorous approaches.
12European Centre for Medium-range Weather Forecast
World Meteorological Organisation2

March 2016
Since 2011, various estimates of the source term have been published in scientific journals. They are all
the result of methods coupling environmental measurements and atmospheric dispersion models.
Simplified approaches are preferred by JAEA3 [Chino et al (2011), Terada et al (2012), Katata et al(2015)]. Initially, IRSN also adopted them [IRSN (2012), Mathieu et al (2012)]. The inverse modelling
methods that existed before the Fukushima accident enabled a source term to be reconstructed on thebasis of activity concentrations in the atmosphere. They have been applied to the case of Fukushima in
the work by Stohl et al (2011) and Winiarek et al (2012), of which IRSN is co-author. However, theapplication of these methods was limited by the low number of measurements of activity
concentrations available at the time. The IRSN team was the first to develop an inverse modelling
method to use the dose rate measurements [Saunier et al (2013)]. The same team was also the first to
invert the activity concentration measurements of Tsuruta et al (2014).
According to estimates, the total activities of 137Cs released to the atmosphere varies between 8 and 20
PBq (Table 1). The first source terms overestimated the releases. The released activities whose effects
were measured on Japanese territory vary between 5.5 and 12.3 PBq. It is especially the evolution overtime of the release rates that differentiates the most recent source terms. The rates vary substantiallyfor some time sequences. There has been no clear consensus to identify a source term that is more
realistic than the others. The differences are due to the weather conditions used as input data for theatmospheric dispersion model used and the types of measurements (activity concentration, dose rate or
total deposition); they reflect the uncertainties that persist in the estimation of the source term andwith regard to the meteorological fields.

3Japan Atomic Energy Agency

3March 2016
Table 1 : Main source terms estimated from measurements in the environment. In blue, the total quantities
estimated by CSNI and NSC on the basis of information from the facility. In green, the first source terms
constructed from inverse or simplified methods (combining radiation and meteorological measurements) in
2011-2012. In purple, recent source terms estimated using inverse or simplified methods. The total quantitiesof the releases whose consequences have been observed on the Japanese territory are specified.

source term

Total quantity of 137Cs
released (PBQ)
Over
Japan
and theocean

Methodobservations

Over Japan
only

CSNI (2011)

15

Facility

Total quantity alone (not kinetics)

NSC (2011)

12

Facility

Total quantity alone (not kinetics)

Chino et al (2011)

13

7.2

simplified

Activities in the atmosphere, dose
rates in Japan and facility events

Stohl et al (2012)

34.9

23.5

inverseActivities in the atmosphere overthe northern hemisphere andfacility events

Winiarek et al(2012)

19

12

inverseActivities in the atmosphere overJapan, the USA and Canada

Mathieu et al(2012)

20.6

12.6

simplified

Activities in the atmosphere, dose
rates close to the facility andfacility events

Terada et al (2012)

8.8

5.5

simplified

Activities in the atmosphere, dose
rates close to the facility andfacility events

Saunier et al (2013)

15.5

12.3

inverse

Dose rates in Japan

Winiarek et al(2014)

11.6 to
19.3

7.4

inverseActivities in the atmosphere anddeposition measurements in Japan

Katata et al (2015)

12.4

8.7

simplified

Activities in the atmosphere, dose
rates close to the facility,
measurements at sea and facility
events8

inverseActivities in the atmosphere overJapan (including Tsuruta et al,2014)

Unpublished 2015
IRSN

A complete source term estimated by modelling the state of the reactors would be complementary
since it would be free of uncertainty sources related to the meteorology and environmental
measurements, and would enable progress with regard to some release episodes.
Since 2012, the NEA4 launched a major international project 5 to improve severe accident models andanalyse the progression of the Fukushima accident. The assessment of the source term of the accident
4

Nuclear Energy Agency of the Organisation for Economic Cooperation and Development (OECD)

4

March 2016using modelling of the reactor physics is among the stated objectives of this project, in which IRSN isheavily involved.

Atmospheric dispersion modelling
The source term and meteorological fields are used as input for atmospheric dispersion models tosimulate the transport of radioactive plumes in the atmosphere and their impact on the ground. The
quality of the simulation is evaluated by comparison with the radiation measurements in theenvironment.
Since 2011, the simulations have become more realistic and now the model - measurement differences
have been significantly reduced. Decisive progress was made through the use of various weather
forecasting sources, more realistic source terms and a better understanding of environmental
contamination episodes [Tsuruta et al (2014)]. However, all studies still report weaknesses and often
face the same difficulties in modelling certain sequences [Morino et al (2013) ; Terada et al (2012) ;Korsakissok et al (2013) ; Saunier et al (2013) ; Katata et al (2015) ; Draxler et al (2015)].

5

The BSAF project: https://www.oecd-nea.org/jointproj/bsaf.html

5March 2016(a) Episode of 12 March - Instant activity on the of 12th ofMarch at 19 :00(b) Episode of 14-16 March - Instant activity on Marchat 6:00

Activity(Bq/m3)■(c) Episode of 18 March - Instant activity on the 18th ofMarch at 10 :00 am(d) Episode of 20-21 March - Instant activity on the20th of March at 20 :00

Rain(mm/h)

Figure 1 : Instant activities of 137Cs for the simulated plume compared with the measurements (coloured
circles). Rainfall is represented by the blue palette and is transparent on the maps. Each map represents an
instant of the main contamination episodes. 12 March at 19:00 (a); 15 March at 6:00 (b); 18 March at 10:00 am(c) and 20 March at 20:00 (d). The Idate location is indicated by means of a white square in Figure (b).

The episode of 12 March was correctly simulated in the first phase but the models fail in the second
one, in which the strongest activities are measured. Whatever the weather data source, the plume does
not move along the coast enough (compared to the facts) but rather is transported towards the Pacific
Ocean in the Northeast direction. Figure 1 illustrates this behaviour, which highlights the lack ofrealism of the wind along the coast.

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March 2016
The episode of 14 to 16 March is now significantly better modelled, but progress still needs to bedone.
The simulation of the strong deposits area between the nuclear site and the Fukushima basin is much
more realistic but its location is still slightly off and the leaching of the plume starts at best one to twohours later than in the measured reality. The lack of precision of the rainfall data is directly responsible
for these assessment faults (Figure 1b6).
A difficulty that is common to all of the models is the simulation of the fallout in the Nakadori Valley.
Indeed, the vertical distribution of the activity of the plumes and the direction of the wind are notrealistic enough due to insufficient consideration of the influence of the relief.
The episode of 18 March has now been well modelled (Figure 1 c).
The episode of 20 to 21 March has, recently, been modelled much better and the various phases ofthis sequence have now been properly reconstructed. For example, due to the weather forecasts
provided by the MRI [Sekiyama et al (2013) ; Sekiyama et al (2015)], the modelling of plumes
transported toward the South has been significantly improved. So far, the delay of the plume did notallow the contamination of the Tokai region to be simulated, which also deteriorated the simulations inmetropolitan Tokyo. The MRI weather forecasts allow simulations of the coastal zone to be better
addressed. Thus, the simulations of IRSN have achieved a good agreement with the environmental
measurements, as shown in Figure 1 d.

Improvement of the atmospheric dispersion models and the representation ofphysical processes
Beyond the objective of reaching a better understanding of the Fukushima accident, the challenge of
the work undertaken by the international community on atmospheric dispersion modelling is to improve
the representation of physical processes in the models. For IRSN, this means the improvement of theoperational assessment tools in order to improve the adequacy of the response of the Institute to anuclear accident.
From the perspective of the population exposure, modelling the deposition is a major challenge, bothdue to the doses induced by the deposition and by the depletion of the plume that it generates. The
realism of the depositions simulated is thus the subject of special attention and the difficulty inreproducing the depositions of the 14-16 March episode have led several teams, including IRSN, to study
the modelling of the wet deposition processes [Leadbetter et al (2015); Katata et al (2015) ; Quérel etal (2016) ; the MRI teams]. The dispersion models used in an operational context are generally based onvery simplified depositions models. Also, the challenge was to know whether complex models taking
into account the aerosol physics, particle size, rainfall, etc., would better simulate depositions.
Simulating the consequences of the Fukushima accident with a complex deposition model does not solve
the difficulties in reproducing some episodes. These difficulties seem mostly due to remaining
uncertainties with regard to the input data and the modelling of the plume behaviour in the context ofa complex orography influencing its vertical distribution. However, it is essential to model, even simply,

6

Figure 1 b illustrates the problem of accuracy of the rainfall data. The simulation is consistent with the activities of
Cs measured. At 6:00, the leaching of the plume should start in the Idate region (indicated by a white square).
The lack of rain in the simulation excludes this. The rainfall data used in this example are still radar observations.
137

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March 2016
the leaching process in the cloud as well as the leaching processes below the cloud [Quérel et al(2016)].
The investigation of this subject continues, particularly through model inter-comparisons. IRSN iscontributing actively to this work, particularly with the Science Council of Japan with regard to the
Fukushima accident.

Arnold, D., Maurer, C., Wotawa, G., Draxler, R., Saito, K. et Seibert, P. (2015). "Influence of themeteorological input on the atmospheric transport modelling with FLEXPART of radionuclides
from the Fukushima Daiichi nuclear accident." Journal of Environmental Radioactivity 139:
212-225.
Chino, M., Nakayama, H., Nagai, H., Terada, H., Katata, G. et Yamazawa, H. (2011). "Preliminary
estimation of release amounts of 131I and 137Cs accidentally discharged from the Fukushima
Daiichi Nuclear Power Plant into the atmosphere." Journal of nuclear science and technology
48(7): 1129-1134.
Draxler, R., Arnold, D., Chino, M., Galmarini, S., Hort, M., Jones, A., Leadbetter, S., Malo, A.,
Maurer, C., Rolph, G., Saito, K., Servranckx, R., Shimbori, T., Solazzo, E. et Wotawa, G.
(2015). "World Meteorological Organization's model simulations of the radionuclide dispersion
and deposition from the Fukushima Daiichi nuclear power plant accident." Journal ofEnvironmental Radioactivity 139: 172-184.
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http://www.nisa.meti.go.jp/english/press/2011/06/en20110615-5.pdf.
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depostion models applied to an accidental radioactive release." Int. J. of Environment and
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Grid Models." Journal of the Meteorological Society of Japan. Ser. II 93(1): 49-64.
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