March 2016

Fukushima-Daiichi Accident:
Main contamination events
The contamination of Honshu Island
The ground contamination of Japan’s main island by the Fukushima-Daiichi nuclear accident wasessentially due to four episodes: that of 12 March, that of 14-16 March, that of 18 March and that of 2021 March. These episodes are analysed below in the light of the various measurements in theenvironment, particularly 137 Cs activity concentration measurements [Tsuruta et al (2014)].

Events of 12 March and 18 March (Figure 1a and 1 c)
On 12 March, some hours after the explosion of Reactor N°1 (at 6:36 UTC 1), a radioactive plume wasdetected and measured during its journey along the coast towards the North of Hamadori (Points J, Iand H). 25 km from the power plant (Point J), a maximum of 575 Bq/m 3 of 137 Cs was measured
between 12:00 and 14:00, consistent with the observed dose rate increases. In total, over the durationof the event, the activity concentration integrated at Point J was about 2660 Bq/m 3. During its journeyalong the coast, the plume did not leach, thereby limiting the contamination of the ground. North ofthe Fukushima Prefecture, the plume was no longer measured. It took a North-Northeast direction awayfrom the coast leading to the detection of 137Cs in Alaska on 18 March [Thakur et al (2013)].
The event of 18 March was comparable to that of 12 March, measured in terms of its duration andactivity. A maximum of up to 440 Bq/m3 was measured at Station J and values greater than 100 Bq/m 3
persisted there for 7 hours. The plume measured appears to be narrower than that of 12 March and did
not reach Station H, but their paths look similar.
These two events are responsible for the dry deposits on the eastern tip of the Miyagi prefecture. Adeposit of about 15 kBq/m2 of 137Cs was measured.

1All times are given in Universal Time (9 hours must be added to obtain the local Japanese time).

1March 2016(a) 12 March episode(b) 14-16 March episode

Activity(Bq/m3)(c)(d) 20-21 March episode

Deposition(kBq/m2)

18 March episode

Figure 1: Activity concentration measurements of 137Cs integrated over various time sequences (colouredcircles) superimposed on the total 137 Cs deposits measured. Episode of 12 March at 19:00 (a) - Episode of 1416 March (b) - Episode of 18 March (c) - Episode of 20 March (d). The black square in Figure (b) indicates thelocation of Idate.

2March 2016

Event of 14-16 MarchThis event is marked by turning winds and by a rainfall that generated significant contamination of theJapanese territory.
On the evening of the 14th of March, a first radioactive plume was transported by winds towards thesouthwest first along the coast (366 Bq/m3 measured at Tokai). It then came inland and reached theTokyo area. At Tsukuba, 153 Bq/m3 were measured between 23:00 on 14 March and 2:00 on 15 March.
This plume was not subjected to leaching by rain. Observations show that the wind then graduallyturned towards the west and then the northwest. Plumes probably travelled through the reliefs locatedto the west of the power plant (at an altitude of 900m on average, Mount Otakine reaches 1193 m) tobe measured in the Nakadori Valley (Stations A to G). The increase in the dose rate values at thestations located about twenty kilometres west of the plant, coupled with the analysis of the windsmeasured by the meteorological observation stations of the AMEDAS network, validate this assumption.
The South of the Valley was first struck on 15 March between 2:00 and 3:00 (Point E) and the maximum
value of 330 Bq/m3 was measured between 3:00 and 4:00. Plumes were detected in the north of thevalley at 6:00 (Point B). The activity concentrations were higher in the South of the valley than in thenorth, where they peaked at 138 Bq/m3 in Nihonmatsu (Station B). Very light winds probably favoured
the stagnation of the plumes, which were measured until noon in the southern part and up until 17:00
in the northern part. In the Fukushima Basin (Point A), the measured activities were much lesssignificant than elsewhere in the valley. They peaked at 33 Bq/m 3 at 13:00.
In the valley, precipitations were observed as from 8:00. The AMEDAS data indicate that light rainfall
started locally earlier without having been measured because they were below the detection thresholdof the pluviometer (0.5 mm/h). Rain likely started at 6:00 am at Idate (black square in Figure 1 b) andbetween 4:00 and 5:00 in Koriyama. They then progressed towards the Southeast, becoming generalised
and more intense. Rain generated deposits in the valley (of the order of 100-300 kBq/m2 of 137 Cs),
particularly along a strip located to the northwest of the plant, identified as one of the mostcontaminated areas of the Japanese territory (about 850 kBq/m 2 to 137Cs were measured at Idate,located at the edge of the maximum deposit zone).
At about 17:00, the wind turned again and pushed the plumes to the south, along the coast where they
were detected up until the 16th of March at 15:00.
The cross-analysis of all available measurements for this episode provided new perspectives and helped
to clarify the events.
The deposits in the valley occurred on 15 March: the plumes were transported there before those that
hit Fukushima and Idate. Leaching began at 5:00 in Koriyama and Shirakawa (Stations C and F). At
Idate, leaching took place between 6:00 and 9:00; at Fukushima (Station A), it took place between 7:00
am and 10:00 am.
It is interesting to note, for stations where all observations were available, that the deposits occurredat the beginning of the rainy episode when rainfall was sometimes too low to be measured. This
explains why this event has long been misunderstood.
Some Japanese teams (JAEA, MRI, etc.) hypothesised that the deposition in the valley was due todeposition by the fog. Yet both in the North and South of the Valley, the AMEDAS observations indicatethat there was no fog. At the centre of the valley, in Koriyama (Point C), the presence of fog has notbeen completely confirmed because the observations are not available. Nevertheless, the hypothesis ofa deposition generated by light precipitation is favoured.

3March 2016
The hypothesis of a deposition by orographic fog remains however credible to explain the contaminationon the side of the Nikko Mountains and on the mountains of the Gunma prefecture, as suggested by
Hososhima and Kaneyasu (2015), although there were no simultaneous fog and plume measurements inthis area.
The orders of magnitude of the activity concentrations measured near the ground, compared toincreases in the dose rate and deposits measured at the Fukushima (Point A) and Koriyama (Point C)
stations as well as other stations, suggests that the deposition is largely due to the leaching of theplume of which much of the activity would have been located at high altitude. This hypothesis cannot
be corroborated by observations, but it raises the delicate question of the vertical distribution ofplumes and the impact of Japan's complex orography on their evolution. Only modelling tools can help
to answer this question.

Events of 20-21 March
From 20 to 21 March, several radioactive plumes were detected in the Tokyo metropolitan area, northnorthwest of the power plant and in the Nakadori Valley.
Tokyo Metropolitan Area Contamination

On 20 March at 2:00 am, a plume coming from the Pacific Ocean was first measured at Tokai (110 kmsouth of the facility) and then over the entire Tokyo metropolitan area, propagating westward. Amaximum of 40 Bq/m3 was observed at the southern end of Ibaraki prefecture (Katori). The plume wasvery spread out and was possibly released earlier (modelling suggests that the plume could have beenreleased on the 19th of March) and transported over Honshu Island after passing over the Pacific Ocean.
It was continuously observed (a few Bq/m 3) over a large area of the Kanto plain up until 19:00 whenanother more concentrated and much finer plume propagated. This second plume probably lightly
touched the coast south of the power plant as from 16:00. At 19:00, an activity concentration of 420
Bq/m3 of 137Cs was measured in Tokai. Further south, the activity concentrations also increased.
Rainfall observed as from 21:00 leached the plume along the coast towards Tokai and more to thesouthwest. The dose rate measurements suggest that the bulk of the plume was located south of Tokai
when the first rains began. The maps of the total deposits measured perfectly match the location of theplume at the time of the rain.
Contamination of the region North-West of the power plant and of the Nakadori Valley

Along the coast, to the North and then the North-West, a plume was measured as from 1:00 on 20March, until 3:00 the next day. A maximum of 360 Bq/m3 was measured at Soma (Point I) on 20 Marchat 11:00 am. The plume was probably split and followed two different directions. Part appears to havebeen transported towards the North-East, where it would be leached between the north of the Miyagi
Prefecture and the south of the Iwate Prefecture, leading to deposits of around 10-30 kBq/m². The
trajectory of this plume is not confirmed by measurements. The other part of the plume wastransported towards the Fukushima Basin (Point A) where 45 Bq/m 3 of 137Cs were measured on 20 Marchat 6:00. This represents almost twice the maximum contamination observed during the episode of 14-16
March. The plume then spread to the south of the valley where it was blocked and stagnated. Itcontinued to be measured up until 23:00. Scattered showers were observed, but they were not followedby an increase in the measured dose rate, suggesting that the deposition by rain was negligible
compared to that of the episode of 14-16 March.

4March 2016
It is noteworthy that the detection of the plume was not accompanied by an increase in the dose rate
at all of the stations that had been impacted by wet deposition during the episode of 14-16 March. The
measurements by radionuclides performed at Chiba and Tokai show that the proportion of iodine,tellurium and of rare gases was lower for this episode than during previous events. This partly explains
the fact that the dose rate levels did not increase.
Finally, over the entire territory, the importance of the dry deposition should not be forgotten either.
For example, during the event of 20-21 March, the light winds over the island of Honshu were able tocause the stagnation of the plumes responsible for a non-negligible dry deposition. The order ofmagnitude estimated from the activity concentration measurements was of the order of a few kBq/m²of dry deposit.

Hososhima, M., Kaneyasu, N., 2015. Altitude-dependent distribution of ambient gamma dose rates in a
mountainous area of Japan caused by the Fukushima nuclear accident. Environ. Sci. Technol.
49, 3341–3348. doi:10.1021/es504838w
Thakur, P., Ballard, S. et Nelson, R. (2013). "An overview of Fukushima radionuclides measured in thenorthern hemisphere." Science of The Total Environment 458–460: 577-613
Tsuruta, H., Oura, Y., Ebihara, M., Ohara, T. et Nakajima, T. (2014). "First retrieval of hourly
atmospheric radionuclides just after the Fukushima accident by analyzing filter-tapes ofoperational air pollution monitoring stations." Sci. Rep. 4.

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