March 2016

Activity of caesium deposited on the territories and associatedambient dose rates following the Fukushima Daiichi accident

The evolution of the activity of radiocaesiums1 deposited on the territories (Bq/m2) and of theresulting dose rates in the air (µSv/h), are decisive elements in guiding post-accident strategies
regarding the management of populations (in particular their return) and decontamination. The
evolution of the radiological situation since 2011 has been characterised by multiple campaigns andassociated measurement actions, the results of which can be consulted on the Japanese Nuclear
Regulation Authority website2.

The different types of measurement
Airborne measurements
Ten airborne and/or helicopter-borne campaigns were carried out between April 2011 and
September 2015. These campaigns enabled the obtention, at close intervals, of mappings of theestimated dose rate at ground level (at 1 m) as well as an associated estimation of the residual
radiocaesium activity. These campaigns, the results of which are reproduced in figure 1 within an 80
km radius of the site, named A1 to A10, show an overall decrease in dose rates which, for the mostpart, is the result of the radioactive decay of caesium-134. These mappings are pertinent as they
provide an overall picture of contamination and ambient radiation across all of the territory
affected by radioactive fallout, including zones that are relatively difficult to access such as forests,
which are predominantly found in this region (covering almost 75% of the surface area) or hilly
areas. The estimations of dose rates and ground activity can nevertheless be accompanied by aquite significant degree of imprecision, due to the approximations required in order to interpret ameasurement carried out at altitude3. These estimations can also have shortcomings due to theirlack of spatial resolution, the flight paths being relatively spaced out (around 2 km for campaigns
A1 to A6, then < 1 km in the most contaminated areas from campaign A7 onwards), and the factthat a measurement produced at altitude has a tendency to smooth out any heterogeneity on theground (within a diameter of between 300 to 600 m for these campaigns). Airborne means still
remain a key element of the measurement programme to be deployed in a post-accident situation,and essential for the establishment of zoning of the radioprotection management of populations.
High-resolution airborne campaigns have been carried out in the vicinity of the damaged site since2013 with the help of a drone flying at low altitude. The use of this device made it possible to1

These radiocaesiums are caesium-134 and caesium-137.
http://radioactivity.nsr.go.jp/en/3
The conversion of the signal measured at altitude into dose rates and estimated activity on the ground
depends on simplifying hypotheses that are only rarely borne out in Fukushima: absence of aerial
contamination during the flight campaigns, absence of relief and dense plant cover (i.e. forest) in the regions
flown over, absence of snow cover, homogeneity of deposits and of their penetration depth into the soil, etc.
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obtain maps with a higher spatial resolution (80 m) revealing contamination lobes that thetraditional airborne device had been unable to detect (figure 2).

29 avril 2011 (A1)

26 mai 2011 (A2)2 juillet 2011 (A3)

5 novembre 2011 (A4)28 juin 2012 (A5)

16 novembre 2012 (A6)28 septembre 2013 (A7)

19 novembre 2013 (A8)

Dose rates in theDébitdemeter
dose ambiant
air onefromà 1mau -dessus
du soltheground(µSv/h)(mSv/h)20 septembre 2014 (A9)29 septembre 2015 (A10)

Figure 1: Mappings of dose rates in the air one meter from the ground (µSv/h) established from airborne
measurements (campaigns A1 to A10). The spatial resolution of the airborne measurements has been refinedin the most contaminated zones (dose rates > 0.2 µSv/h) since summer 2013 (campaigns A7 to A10).

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Doserates
in ambiant
the air
Débit dedoseone
from duthesolà 1mmeter
au-dessus
ground (mSv/h)(µSv/h)31 juillet 2013

7 janvier 201422 juillet 2014

Figure 2: Mappings of dose rates in the field close to the Fukushima site
carried out using a drone, between July 2013 and July 2014.

Road network measurements
The Japan Atomic Energy Agency (JAEA) has carried out measurement campaigns along the roadnetworks on a regular basis since June 2011. As with the previous campaigns, these enable theobtention of mappings of the dose rate measured at 1 m from the ground using a detector loaded
onto the vehicles, covering large areas in a short time. They also therefore provide essential
information for the spatial characterisation and monitoring over time of the contaminatedterritories, which complements airborne means. Due to the contribution of the road surface in theradiation measured, these measurements may however not reflect the radiological atmosphereencountered in environments next to the road network. In theory, this skew is even more significant
when measurements are carried out late after the accident since, as a general rule, thedecontamination of roads takes place more quickly than that of natural ground surfaces (as a resultof leaching by the rain in particular). The conversion of the dose rates measured into surface
radiocaesium activity in surrounding environments is therefore somewhat risky. Moreover, theJapanese have not undertaken this exercise. During the road campaigns the JAEA also proceeded tocarry out, but only on an occasional basis, measurements of dose rates on neighbouring soil plots
left unoccupied since the accident. These measurements clearly demonstrated that irradiationlevels increased the further they were from the road.

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Measurements in situ
Dose rate measurements carried out with the help of static or mobile detectors, placed 1 m fromthe ground, certainly provide the most pertinent results on a small spatial scale, typically that of aplot of land. The Japanese authorities have organised three major campaigns of this kind, focussing
on inhabited zones within an 80 km radius of the damaged site. In order to improve therepresentativeness of the measurements, they were carried out solely on large soil plots, with littleslope, and with little vegetation (public parks, school yards, etc.). The knowledge about dose rates
acquired during the first campaign in June 2011 was considerably refined during the third campaign
in December 2012, where the number of dose rate measurements was increased to 6,100 sites
compared to 2,200 for the first (figure 3a). These dose rate measurements enable a reasonably
reliable estimation of radiocaesium activity in the soils, thanks to knowledge about thecharacteristic vertical migration depth of these radionuclides. Specific analyses were carried out forthis purpose during winter 2012 through soil samples on 85 sites. They showed that theradiocaesiums had migrated little depth-wise and remained on average confined to the top 3
centimetres of soil. The mapping of the resulting radiocaesium activity is reproduced in figure 3b.
The main limitation of the maps obtained in this way is that they do not necessarily reflect thesituation encountered in types of environment other than those investigated, such as agriculturaland forest land or man-made surfaces.
For agricultural areas, the relation between the dose rate at 1 m and the activity measured in soil
samples was the subject of a specific study conducted between October 2011 and January 2012, andwhich was published in 2014. This study, which was carried out on 3,500 agricultural plots, showedthat the estimation of radiocaesium activity using ambient dose rates could be refined if the soil
and crop types encountered were taken into account (paddy fields, pasture, orchards and othercrops). The utilisation of these results, together with the map of dose rates from the fourth
airborne campaign (November 2011), enabled the obtention of a first mapping of radiocaesium
deposits on agricultural land.

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Massic activity measurements in soils
During the first campaign (June 2011), five soil samples were taken from a square of 9 m2 for eachsite, with a view to measuring massic activity (Bq/kg). These samples revealed a significant
variability between samples, linked to the natural heterogeneity of the initial deposits and potential
horizontal redistribution via micro-runoff, hence the difficulty in reconstituting surface activity(Bq/m2) at plot level. Moreover, the correlation between the dose rate measured and estimatedmean massic activity for each site proved to be fairly average. The Japanese have not carried out
any more campaigns of this type, except for the campaign on agricultural plots presented earlier.

(a)

(b)

Figure 3: Mapping: (a) dose rates in the air (µSv/h) and (b) surface activity (Bq/m2) in caesium-137 ininhabited zones on undisturbed soil plots (December 2012 campaign).
Activities are distinguished according to whether they were estimated from dose rate measurements via
gamma spectrometry in situ (400 stations) or portable NaI probe (6,100 stations).

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Spatial comparison of the different dose rate mappings
The dose rate mappings established at the end of 2012 are reproduced in figure 4 for each of themethods deployed by the Japanese. This comparison makes it possible to confirm that, despite
metrological specificities and uncertainties, there is a relatively good overall coherence between
the predictions, at least in the scale range over 10 kilometres. These maps essentially differ by wayof the extent of their spatial cover. The road campaigns and, to a lesser extent, the groundcampaigns in inhabited zones, only provide a small amount of information on the Abukuma forest
massif, located to the west of the damaged site, as well as the volcanic range located to theextreme west of the area studied. Nevertheless, a more careful comparison makes it possible toobserve that the dose rates estimated along the roads are, in general, lower than those estimatedby the other methods, notably on bare soils in inhabited zones where the difference is around 50%.
In addition, an IRSN study showed very significant local differences between airborne measurements
and measurements on bare soils (up to factor 3). At the level of highly-urbanised regions, such as
the Abukuma valley that shelters the towns of Fukushima and Koriyama, dose rates estimated byairborne methods from October 2011 onwards proved much lower than those measured in situ on
soil plots. This most likely results from an accelerated decontamination of urban surfaces comparedto soils, due to the combined action of natural leaching by rain and a progressive rehabilitation of
the urban environment during the weeks or months following the accident. Conversely, higher levelsof activity than on bare soils were revealed to the south of the Abukuma massif, predominantly
covered by conifer forests. These anomalies were interpreted as being dry deposit zones since, in
the absence of rainfall, forest canopies capture airborne contamination more effectively than aneighbouring plot of bare soil.

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doseAirDébit
Dosederates(µSv/h)dansl’air-( mSv / h )(Novembre20122012)
November(a)

(b)

(c)

(d)

Figure 4: Mappings of dose rates (µSv/h) measured or estimated at 1 m from the ground: according to (a) thecampaign on soil plots in inhabited zones in December 2012, (b) the November 2012 airborne campaign, (c,d)
the road campaigns of September and November 2012 respectively.

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Evolution over time of dose rates and activity of deposited caesium
As figure 5 illustrates, all of the measurement campaigns - irrespective of the method used indicate that the dose rate in the air decreased by an average of factor 2 to 3 between mid-2011
and end-2013 in the 80 km around the damaged plant. This decrease was relatively sustained during
the first two years, and took place at a quicker pace than expected through the radioactive decayof caesium-134 (period of two years) - the radioactive decay of caesium-137 (30 years) being almost
insignificant over this period. This decay has slowed since 2013, more or less complying with theexpected theoretical evolution.
In the case of airborne measurements, the decrease was in the region of 40% between November
2011 and November 2012, only half of which is explained by radioactive decay. Numerous natural or
anthropogenic mechanisms have been put forward in order to explain this decrease, such as theeffect of leaching by rain, migration into the soil, the effect of ploughing and decontamination
measures. The contribution made by these processes is most certainly significant in urban andagricultural environments. Nonetheless, none of these mechanisms make it possible to explain thefall in dose rates at forest surface level, which do however cover almost 70% of the territory. AnIRSN study has demonstrated that the progressive decontamination of coniferous canopies could
noticeably modify the signal measured at altitude, notably due to the screening effect applied by
the vegetation to radiation emitted at ground level. To find out more about the forest environment,
see the specific information sheet.

1

A2

I1

C1

DécroissancephysiquePhysical decay

I2
de doseDébit
rate
dosed'équivalent
Equivalentvalue initiiale
initialby itspar
normalizedsa valeurnormalisé

0.8A30.6

Aéroporté(campagnes
A2 à A10)
Airborne (A2to A10 campaigns)
situ (campagnes(I1 to I3a,b campaigns)
InInsitu
I1 à I3a,b)A4Road network
to C7 campaigns)
Réseauroutier(C1(campagnes
C1 à C7)

I3aI3bA5C2A6C3A7 A8

C40.4

A9C5C6C7A10

0.2

003/11

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03/14

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Figure 5 – Evolution over time of dose rates measured by all of the methods deployed and expected
evolution through the physical decay of the caesium. The values represented are average values within an
80 m radius from campaigns A2 to A10, standardised by the initial level observed in June/July 2011
(campaign A1 in figure 1). The negative anomaly observed for the airborne campaign A3 is the result ofdetector calibration problems.
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The accelerated decrease in dose rates observed along the road network very probably results fromincreased decontamination of asphalt surfaces through leaching by rain and road traffic. With thefirst road campaign only having been conducted in June 2011, there is every reason to believe that
this decrease had already begun during the first three months following the accident.

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