March 2016

How can forest environments be managed after a nuclear accident?
Lessons learned following the Chernobyl and Fukushima accidents

In both Chernobyl and Fukushima, a large proportion of the highlycontaminated territories is covered by forest. Knowledge about the evolution of
the radionuclides deposited within these ecosystems following the accidents, aswell as differing management practices regarding these environments between
the two countries, enable valuable lessons for the post-accident management of
this type of environmental resource to be learned.
The main characteristics of the forest ecosystems impacted by radioactive fallout differ
between the two accidents. In particular, the appearance of the "red forest" is specific to theChernobyl accident and has not been observed at Fukushima. Despite the fact that the surface
areas of the terrestrial environments affected by the disaster are very different (e.g. soils withcontamination levels exceeding 600,000 Bq/m2 represent around 600 km2 for the Fukushima
accident compared to 13,000 km² for the Chernobyl accident), when these two accidents occurred alarge proportion of the highly-contaminated territories were forest ecosystems. In Japan, 75% of theterritories contaminated following the Fukushima accident are forest land, compared to 39% in themost contaminated areas following the Chernobyl accident in Belarus (Steinhauser et al., 2014). In
Chernobyl, the forest occupying 53% of the exclusion zone - covering a surface area of 2,600 km2 that was established after the accident, suffered irreversible damage. During the first weeks
following the accident, 90% of pines (Pinus sylvestris L.) died, thereby creating the "red forest"
within a 6 km2 zone (lethal absorbed dose of 60-100 Gy) and, in a 38 km2 "sub-lethal" area (absorbed
dose between 30-40 Gy), 40-75 % of the trees turned brown and the growth of 95% of them wasaffected (Geras'kin et al., 2008). As part of the cleaning operations, dead trees were felled andburied over an area of 4 km2. Following the progressive planting of new herbaceous species anddeciduous trees (such as birch), today the surface area of the exclusion zone covered by forest is
close to 90% (Yoshenko et al., 2011). In Fukushima, no mass acute effect of the "red forest" type has
been reported in literature, and the forest ecosystems in place have not undergone drastic changes
in terms of the structure of their component stands. Watanabe et al. (2015)1 recently highlighted for sites in the contaminated territories where the ambient dose rate in 2015 lies between 5 and 40µGy/h - a significant frequency of an increased loss in the apical dominance of the Japanese fir(Abies firma)2.
The processes governing the evolution of radionuclides in forest ecosystems imply a highremanence of the radioactive contamination of these environments. The longevity of forest
ecosystems combined with periods of high radioactive decay for some of the radionuclides
deposited following the accidents (e.g. 30.2 years for caesium-137) implies a high remanence of thecontamination of all of the components of these environments. However, the redistribution of theradionuclides in the different compartments of the forest ecosystem is very dynamic. It is the result
1This study was the subject of an IRSN information note published on the institute's website in 2015 (In French:
http://www.irsn.fr/FR/Actualites_presse/Actualites/Documents/IRSN-Note-Lecture-Fukushima-PinContamination_20151110.pdf)
2
For more information, the reader can consult the fact sheet on the ecological consequences of nuclear accidents.

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March 2016of different processes that are part of the radionuclides' cycle (foliar and root absorption, falling ofleaves/needles/branches towards the leaf litter, leaching of the canopy by rain and runoff on thetrunks with a return of the rain-leached water towards the soil, internal transfers within the trees,
immobilisation in trunks...) as illustrated in figure 1. Moreover, there is a strong correlation
between the biogeochemical cycle of radionuclides in the forest and that of organic matter within
this ecosystem, and the flow dynamics between compartments depends on the types of trees
considered, the soil type, climatic and anthropogenic factors (i.e. practices for the management of
the forest and its resources). This management also depends on the characteristics of theradionuclides deposited and their capacity to be immobilised or internalised within the foliage(penetration in gas or particulate form in the case of iodine, dissolved for caesiums andplutoniums...) and on their mobility within the vegetation (high for caesium and strontium, very low
for plutonium).
Rain
Dry or wet deposits
INTERCEPTION

Non Perenial Biomass(Leaves/needles)
IMMOBILISATION

TRANSLOCATION

Perenial
Biomass

Throughfall

Litterfall
Sap flux(trunk,branches)
MINERALISATION

Humus

Roots
UPTAKEMIGRATION
Available

Fixed

Leaching

Figure 1- Representation of the biogeochemical cycle of elements in forest ecosystems (adapted from IAEA, 2002).

The interception of radioactive deposits by the canopy and transfers of radionuclides towards
the leaf litter and soil are the most important processes in the early phase and first months
following the accident. Little knowledge exists about this period with regard to the Chernobyl
exclusion zone, but the data collected at Fukushima since 2011 enables a better understanding of
the processes at stake. In the highly-contaminated zone (radiocaesium levels3 above 1,000 kBq/m2),
two sampling campaigns carried out in August and September 2011 have shown that theradiocaesium concentration in leaf litter varied between 24 and 319 kBq/kg, a contribution of
between 22 and 66% towards the total quantity of radioactive caesiums present in the forest
ecosystems, for a biomass representing barely a 3% mass of forest components (i.e. leaves/needles,
trunks/branches, leaf matter) of the plots studied (Hashimoto et al., 2012). These initial results led
the Japanese to consider the removal of leaf litter as an effective decontamination method, mainly
for forests with deciduous trees. Moreover, as the Fukushima accident occurred in March (end of thewinter period), a distinction must be made between, on the one hand, deciduous forest (e.g., oak,Quercus serrata) where the majority of the contamination was deposited directly on the leaf litter
3

These radiocaesiums are caesium-134 and caesium-137.

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on the ground (Yoshihara et al., 2013; Hashimoto et al., 2012) and, on the other hand, theparticularly dense conifer forests in Japan (with the Hinoki cypresses, Chamaecyparis obtusa, theJapanese cedar, or Sugi, Cryptomeria japonica) whose canopy was able to intercept up to 90% of
the deposits (Kato et al., 2012). Hashimoto et al. (2012) have shown that, following the radioactive
deposits, the contaminated branches, leaves and leaf litter in the highly-contaminated zone
represented a total volume of 33 million m3 for a mass estimated at 21 million tonnes (dry matter).
After initial interception by the canopy, the transfer of contamination towards the soil results fromtwo processes: leaching by the rain of the canopy and trunks, and the fall of biomass (leaves,needles, branches...) to form leaf litter. Various authors have shown that, in Chernobyl as well as
Fukushima, the natural decontamination of the canopy, via rain leaching and the fall of biomass toform leaf litter, took place following two kinetics: one quick and the other slower (Bunzl et al.,
1989; Kato et al., 2015). Although aerial biomass litterfall for conifers was more significant inFukushima than in Chernobyl, certain observations have shown that the median remanence time ofradiocaesiums in the conifer canopy during the first months following the accident was longer inFukushima (6 to 10 months) than that observed at Chernobyl (3 months) (Kato et al., 2015). Thisbehavioural difference can be attributed to the nature of the tree species (different canopy
renewal rates), and the type of deposits (dry or wet) that collectively influence the interception ofradionuclides by the canopy, but also their residence time (Gonze et al., 2015). Figure 2 shows the
temporal evolution since the accident of the relative contribution of the processes of leaching by
the rain, runoff on the trunks, and the fall of biomass towards the leaf litter for a plot of mature(31 years) or young (15 years) Japanese firs, and for a mixed plot comprising three deciduous
species (oak - Quercus serrata, viburnum - Viburnum furcatum and pine- Pinus densiflora).
Regardless of the plot type, Kato et al. (2015) quantify a major contribution during the first monthsof the rain leaching process of caesium flows towards the soil, and a very low contribution of runoff
along the tree trunks. According to Bird et Little (2013), a campaign by the Japanese Forestry
Agency, published in December 2011, showed that around half of the radiocaesium deposits were
present at the soil surface and in the leaf litter, with the other half in the leaves, trunks andbranches (with a greater balance in forest soils in deciduous forests). Other trials, also commented
on by Bird and Little (2013), have made it possible to quantify that by cutting one tree out of three
the ambient dose rate was reduced by up to 23%, and that this remediation measure, combined withthe removal of leaf litter, enabled a 50% reduction in the ambient dose rate. However, given theimportance of leaching by the rain and the formation of contaminated leaf litter over time, thefelling of trees becomes less effective because the soil's contribution to the ambient dose rate
becomes preponderant.

Figure 2 - Temporal evolution since the accident of the relative contribution of the processes of leaching by the rain of aerial
biomass, runoff on trunks and the fall of biomass towards the leaf litter for a plot of mature Japanese cedars (31 years) and amixed plot comprising three species of deciduous trees - Quercus serrata, Viburnum furcatum and Pinus densifolora(from Kato et al., 2015).

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As a result of leaching of the canopies and fall of aerial biomass, the soil progressively
becomes the preponderant reservoir where radiocaesiums can be found. Even if a greaterremanence of radiocaesiums in the Fukushima conifer canopies was observed during the first months
compared to those contaminated by the Chernobyl accident, five years after the Fukushima
accident almost all of the radiocaesium deposited on the forest ecosystems (between 80 and 90%)
was entrained into the top layers of the soil or humus - which is comparable to the rates observed in
Chernobyl. Twenty years after the Chernobyl accident, the aerial biomass from trees and trunks
contains less than 20% of the radionuclides present in the forests (Ministry of Ukraine of
Emergencies and Affairs of Population Protection from the Consequences of Chernobyl Catastrophe,2006). This progressive decontamination of the canopies and its return to the soil also explain a fallin dose rates measured by the Japanese authorities during the airborne campaigns (airborne
detectors at an altitude of between approximately 100 and 300 m, between April 2011 andDecember 2012): digital simulations carried out by the IRSN (Gonze et al., 2015) have demonstrated
that in a forest environment the dose rates at considered flying altitudes have decreased by around
40% per year (consistent with the measures), with half of this decrease being induced by the
progressive decontamination of the canopies that themselves increase the mitigation by plant coverof the radiation measured by airborne detectors. In contrast, simulations demonstrate that the dose
rates close to the soil in a forest environment rose slightly during the first months following theaccident, the soil contamination having logically increased due to the effect of leaching (Gonze etal., 2015).
During this phase, organic input to the soil (fall of biomass to form the leaf litter) decomposes, anda fraction of the contaminants that were immobilised are released into the soil thus becoming
available for removal by roots and deep drainage. Radiocaesiums, at a rate of 99% of the soil stock,
are mainly found in the top 10 centimetres (Figure 3), where the percentage of organic matter isover 10%, suggesting that an essential part is played by the cycle of organic matter in the
distribution and therefore mobility of caesium in the soil (Teramage et al., 2014a et b).

Depth (cm)Litterfall

Figure 3- Distribution of the caesium-137 stock (Bq/m2) in undisturbed soil in conifer forest in
Fukushima in October 2012 (adapted from Teramage et al., 2016).

The migration velocity of 137Cs in forest soils appears to be quicker in Fukushima than Chernobyl(Teramage et al., 2014b). This difference can be explained by differences in the prevailingenvironmental conditions at each site. Firstly, rainfall is higher in Fukushima (≈ 1200 mm/year) thanin Chernobyl (≈ 500 mm/an), which can entrain the radiocaesiums available downwards more easily.
On the other hand, over a large section of forest soils in Fukushima, humus is thicker than for the
majority of the Chernobyl soils, which generates greater organic matter content in the top layers of
the soil. This phenomenon, combined with a lower mica clay mineral content in the forest soils of
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Fukushima (Andosols/Cambisols), is a factor that favours caesium mobility (e.g. Valcke, 1993;
Vandebroek et al., 2012). Over the coming years we can therefore expect a higher recycling of 137Cs
via root removal in the Fukushima forests than at Chernobyl. Thiry et al. (2009) had estimated that
the Chernobyl pines could annually incorporate 0.004% of the caesium-137 present in the soil
through root transfer. The AMORAD project, coordinated by IRSN4 will make it possible to confirm or
refute if this incorporation rate is also valid for Fukushima.
The long-term studies carried out at Chernobyl have shown that the migration velocity of 137Cs
decreased over time. For example, Rosén et al. (1999) demonstrated that between 1987 and 1995,
the migration velocity of 137Cs had been reduced by half for Swedish soils contaminated following
the Chernobyl accident. With a fall in migration velocity, the majority of the 137Cs deposited in theforests close to Chernobyl are still currently found in the top centimetres of soil and in the organic
layer (humus), even 30 years after the accident. This stabilisation phenomenon will also certainly
occur at Fukushima, as suggested by Teramage et al. (2014b).

The management of forest ecosystems contaminated following the Fukushima accident differsfrom that applied to the forest ecosystem in the Chernobyl exclusion zone. The Chernobyl
exclusion zone is one of the realisations of the strategy adopted by the Soviet (then Ukrainian)authorities for the management of the health impact of the accident. Emptied of residents and withall practices prohibited (apart from those linked to urgent cleaning operations and work to secure
the facilities), the trees and other plants making up the ecosystem in this zone stabilisecontamination (e.g. in Sr90, Cs137 and plutonium isotopes) that essentially remains in the forest soils
for many years more. The ban on all human activity (residence, farming, logging, hunting andharvesting) contributes to preventing the dissemination of the radionuclides outside the zone andmakes it possible to limit doses to surrounding populations. The forest is therefore, as a strategic
choice, left to grow naturally. On the contrary, Japan has chosen a strategy based on extensive
decontamination. For the forest ecosystems, which are an important resource in Japanese farming,
after a year of conducting exploratory trials of a variety of exposed decontamination techniques(collection of leaf litter, felling of trees...), the authorities ultimately decided that it was
unrealistic to decontaminate the forests in their entirety, due to the volumes of waste generated.
Moreover, the dramatic ecological consequences that could result from mass actions to remove leaf
litter or fell trees, boosting soil erosion which is naturally low thanks to the density of the forest
cover, have been underlined. Modifying this stand density would inevitably lead to significant
erosion and landslides, due to the steep slopes on which the forests are established.
Without such measures, a small percentage of radionuclides currently present in the forests migrate
via water or air (Bird and Little, 2013). The IAEA (2011) recommended the implementation of
restrictions on uses of the forests and consumption of forest products such as wild and cultivatedmushrooms, game, etc. Some of these products still regularly exceed the marketable limits, withfor example concentrations over 10,000 Bq/kg fresh weight still observed in 2015 in wild boar meatfrom the most affected areas in the Fukushima prefecture5. Japan has integrated, in an adaptedand progressive way, the management of forest environments into the plan to recover territories by
distinguishing three types of forest surfaces: those located around the residential areas with theremoval and disposal of contaminated leaf litter and humus within a 20 m radius of the houses, ormore if the ambient dose rate is too high; those visited by workers on a daily basis, particularly formushroom (shiitake) farming, where decontamination measures are in place and, finally, those of
the so-called "deep" forest where measures aim to restrict the dispersion of radionuclides mainly by
preventing soil erosion, the major risk being landslides. Research and development actions in orderto prevent this dispersal are under way (Ministry of the Environment, Japan, 2016). On this basis,the monitoring of contaminated Japanese forest sites as part of the AMORAD project carried out by
4 The AMORAD project is co-funded by the ANR RSNR ("Improvement of models of dispersal prediction and evaluation of the
impact of radionuclides within the environment").
5

For more information, the reader can consult the fact sheet on foodstuffs.

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the IRSN (Figure 4) will enable an enrichment of existing databases and contribute towards theunderstanding of contamination recycling processes and kinetics within forest environments anddissemination flows via the air or water. Moreover, this will enable the obtention of expertise
adapted to France and its varied forest land, in order to prepare elements of post-accident
management of these environments.

Finally, there is a high risk of fire in the Chernobyl exclusion zone where the forest, left toevolve naturally, can be exposed to periods of drought - the probability of this occurring
increases with climate change. This risk is comparatively lower in Japan because it is limited by
the short dry season in the spring. Nonetheless, such fires would constitute catastrophic events,
leading to the mass dissemination of radionuclides on a local, and even regional, scale (Zibtev etal., 2015; Bird et Little, 2013). The fires of summer 1992 that affected the Chernobyl exclusionzone led to a significant increase in radionuclide concentrations in the air, from 0.017 to 1.5 Bq/m3
in the town of Chernobyl, close to where the fires broke out. Combining this information withknowledge about the radionuclide stock of forest biomass, Zibtev et al. (2015) estimated that the
total caesium-137 activity released into the air at this time was between 28 and 130 TBq6. By
extrapolation, a fire affecting 1 km2 of forest with soils contaminated to a level of 40MBq/m2 (or atotal stock of 40 TBq of caesium-137) represents potential releases into the air of around 40 to 70%of the Cs137 contained in combustible matter (trees and leaf litter; Amori et al., 1996), i.e. around 6
TBq.

Figure 4 - Sampling campaigns in the contaminated forests of the Fukushima prefecture(AMORAD programme. IRSN, 2013)

References
AIEA (2002). Modelling migration and accumulation of radionuclides in forest ecosystems. IAEA-BIOMASS-1. AEABIOMASS. Vienna, IAEA: 1,300
AIEA (2011). Final report of the international mission on Remediation of large contaminated areas off-site the
Fukushima-Daiichi NPP, 7-15 october, Japan. NE/NEFW/2011. 09/12/14 AIEA éd., Vienne, Autriche.
Amiro et al. (1996). Burning radionuclide question: What happens to iodine, cesium and chlorine in biomass
fires? The Science of the Total environment 187 : 8-9:
Bird,

Little (2013). A tale of two forests: Addressing postnuclear
Fukushima. Environmental Health Perspectives, 121 (3), pp. A78-A85.

radiationatChernobyl

andBunzl et al. (1989). "Interception and retention of chernobyl-derived 134Cs, 137Cs and 106Ru in a spruce stand."
Science of the Total Environment, 78(C): 8-9:
Geras’kin et al. (2008) Effects of non-human species irradiation after the Chernobyl NPP accident. An
international environment 8-9:
Gayle et al. Modelling the dynamics of ambient dose rates induced by radiocaesium in the Fukushima terrestrial
environment, Journal of Environmental Radioactivity, in press (2015) 1-13
Hashimoto et al. (2012). Radioactive contamination of natural ecosystems: Seeing the wood despite the trees.
Environmental Science and Technology 46(22): 8-9:

6

1TBq=1012 Bq

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March 2016
Kato et al. (2012). Interception of the Fukushima reactor accident-derived 137Cs, 134Cs and 131I by coniferous
forest canopies. Geophysical Research Letters 39(20).
Kato et al. (2015). Temporal changes in radiocesium deposition in various forest stands following the
Fukushima Dai-ichi Nuclear Power Plant accident. Journal of Environmental Radioactivity (in press).
Loffredo et al. (2015). Equation to predict the 137Cs leaching dynamic from evergreen canopies after a radiocesium deposit. Journal of Environmental Radioactivity 147: 8-9:
Ministry of the Environment, Japan (2016). Progress on Off-site Cleanup and Interim Storage in Japan. January
2016. https://josen.env.go.jp/en/pdf/progressseet_progress_on_cleanup_efforts.pdf?150918
Ministry of Ukraine of Emergencies and Affairs of population protection from the consequences of Chornobyl
Catastrophe, Kyiv (Ukraine); All-Ukrainian Research Institute of Population and Territories Civil
Defense from Technogenic and Natural Emergencies, Chernobyl (Ukraine). 20 years after Chernobyl
Accident. Future outlook. National Report of Ukraine, Baloga, V.I. (ed.); 225 p.
Rosén et al. (1999). Migration of radiocaesium in Swedish soil profiles after the Chernobyl accident, 1987-1995.
Journal of Environmental Radioactivity 46(1): 8-9:
Steinhauser et al. (2014). Comparison of the Chernobyl and Fukushima nuclear accidents: A review of theenvironmental impacts. Science of the Total Environment 470–471: 8-9:
Teramage et al. (2014a). The role of litterfall in transferring Fukushima-derived radiocesium to a coniferous
forest floor. Science of the Total Environment 490: 8-9:
Teramage et al. (2014b). Vertical distribution of radiocesium in coniferous forest soil after the Fukushima
nuclear power plant accident. Journal of Environmental Radioactivity 137: 1.45pm
Teramage et al. (2016). Small scale temporal distribution of radiocesium in undisturbed coniferous forest soil:
Radiocesium depth distribution profiles. Journal of Environmental Management, 170, pp. 97-104.
Thiry et al. (2009). Impact of Scots pine (Pinus sylvestris L.) plantings on long term 137Cs and 90Sr recyclingfrom a waste burial site in the Chernobyl Red Forest. Journal of Environmental Radioactivity 100(12):8-9:Valcke (1993). The behaviour dynamics of radiocesium and radiostrontium in soils rich in organic matter.
AGRICULTURE Louvain, Université catholique de Louvain.
Vandebroek L. et al. (2012). Relevance of Radiocaesium Interception Potential (RIP) on a worldwide scale toassess soil vulnerability to 137Cs contamination. Journal of Environmental Radioactivity 104(1): 8-9:
Watanabe Y. et al. (2015). Morphological defects in native Japanese fir trees around the Fukushima Daiichi
Nuclear Power Plant. SCM: Rep., 5, 13232; doi: + 33 (0)4 92 38 71 67
Yoshihara et al. (2013). Radiocesium contaminations of 20 wood species and the corresponding gamma-ray dose
rates around the canopies at 5 months after the Fukushima nuclear power plant accident. Journal ofEnvironmental Radioactivity 115(0): 8-9:
Zibtsev et al. (2015). Fires in nuclear forests: Silent threats to the environment and human security. Unasylva,
66 (243-244), pp. 40-51.

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