Assessment Study of Ambient Dose Rates Dynamics in the Fukushima Terrestrial Region

Transcription

1 1/13 Assessment Study of Ambient Dose Rates Dynamics in the Fukushima Terrestrial Region M.A. Gonze 1, C. Mourlon 1, P. Calmon 1, E. Manach 1, C. Debayle 1, R. Gurriaran 1, J. Baccou 2 1) Institute of Radioprotection and Nuclear Safety (Environmental division, France) 2) Institute of Radioprotection and Nuclear Safety (Nuclear Safety Research division, France) Summary The Fukushima nuclear accident led to high atmospheric releases and deposition of volatile fission products in northeastern Japan. The main gamma emitters that were deposited onto the ground surfaces were radioisotopes of Iodine, Tellurium and Caesium (e.g., 134 Cs, 136 Cs and 137 Cs). Due to its relatively long half-life (30 years), 137 Cs is likely to dominate and maintain ambient gamma radiations at relatively high levels in the next decades. Thus, a longterm and scientifically-based management of contaminated terrestrial environments is of great environmental, economic and social concerns for Japanese authorities. In this context, the Institute of Radioprotection and Nuclear Safety (IRSN) launched in late 2013 a research and development project, called EDOFU, which aims at better characterizing, understanding and predicting the radiation impact of gamma-emitters in a complex terrestrial environment contaminated by accidental atmospheric fallouts. A special focus is put on the key environmental processes that are likely to control the short and long term dynamics of ambient dose rates, that is: decontamination of urban environments by natural processes, radioactivity interception and recycling in forest systems, migration of radiocaesium in arable soils and deliberate remediation actions in inhabited areas and agricultural lands. In this paper, we briefly present the approach that has been developed to model ambient radiations dynamics and discuss preliminary results obtained in Fukushima region with IRSN and EDF s Radiological Risk Assessment platform, called SYMBIOSE. 1 Introduction Since the Fukushima accident occured, the Japanese Nuclear Regulatory Authority (NRA) and the Japanese Atomic Energy Agency (JAEA) coordinated multiple monitoring campaigns in order to characterize the extent and dynamics of ambient radiations. A special effort was made on the most contaminated areas in eastern Fukushima Prefecture, located within 80 km from Fukushima Dai-ichi Nuclear Power Plant (FDNPP). These surveys allowed to produce maps of either dose rates (in Gy/h), equivalent dose rates (in Sv/h) or radiocaesium inventories (in Bq/m 2 ) in terrestrial environments, at different dates since April Measurements of dose rates were made through either in-situ gamma spectrometry at 1m height above soil or gamma detectors embedded in mobile devices such as cars (carborne monitoring along road networks) or helicopters/airplanes (airborne monitoring). Detailed information regarding technologies, monitoring procedure and data analysis can be found in recent publications (Refs. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] ) and on NRA website (Refs. [18] [19] ). Both carborne and airborne surveys revealed that ambient radiations have been decreasing at rates significantly greater than the rate predcited from the physical half-lives of radiocaesium isotopes (up to a factor of 2), although variably in space. As demonstrated by Gonze et al. [20], this decrease was especially marked in dense urban regions. Carborne measurements further revealed that, in contrast to what was observed in inhabited or cultivated areas, ambient radiation levels in semi-natural terrestrial media, such as forests, could be significantly higher than in surrounding areas and decreased quite slowly ([10] [11] ). In-situ measurements of ambient radiations in bare soils of inhabited zones (e.g., gardens, schoolyards ) tend to show that the decrease was also very limited in this type of environment, at least in places which remained undisturbed since the accident.

2 2/13 Despite this abundant litterature and tentative analysis of Fukushima data, we must however recognize that the processes effectively involved in the decrease of ambient dose rates and the factors responsible for such a contrasted situation among terrestrial media have not been clearly identified yet. The purpose of our study is to bring new insights into the understanding of the behaviour of ambient radiations in a terrestrial environment contaminated by atmospheric fallouts. We will more specifically focus on the simulation of external dose rates in Fukushima region, through the use of a dynamic, spatially-distributed and process-based modelling approach. 2 Modelling approach In order to simulate the dynamics of ambient dose rates induced by the presence of radiocaesium in an environment contaminated by atmospheric fallouts, the following processes have to be modelled: (i) atmospheric deposition of airborne aerosols onto the various ground surfaces, (ii) migration and dissemination of deposited radionuclides within each constitutive medium, and (iii) propagation and interaction of the emitted gamma rays within the environment, including the atmospheric layer. The simulation chain is depicted in figure 1. ATMOSPHERIC DEPOSIT (Bq/m 2 /d) Onto forest, agricultural and inhabited lands FOREST LAND (Bq/m 2 ) Evergreen coniferous forests Deciduous broadleaf forests AGRICULTURAL LAND (Bq/m 2 ) Grassland fields Cropland fields (Paddy fields) INHABITED LAND (Bq/m 2 ) Urban surfaces Bare soil areas DOSE RATE (ngy/h, nsv/h) At varying altitude In each medium (land) SPATIAL INTEGRATION (ngy/h, nsv/h) At varying altitudes In a multimedia environment Figure 1: Architecture of the SYMBIOSE simulator dedicated to the numerical simulation of ambient dose rates dynamics in the Fukushima terrestrial environment. The most seriously contaminated areas in Fukushima are located within 80 km of the damaged FDNPP site. This 9,000 km 2 territory comprises forests (70%), agricultural lands (20%) and inhabited areas (10%). Fukushima forests are mainly represented by either evergreen coniferous or deciduous broadleaf species, or a mixture of them. Cultivated lands mainly consist of paddy fields, annual crops and meadows, while inhabited lands are represented by both rural and dense urban environments, including some bare soil areas. All these systems have been considered in the assessment study. Aquatic systems such as rivers and lakes were not considered because they occupy a too small proportion of the territory (<1%) and are not likely to impact ambient radiation levels in terrestrial environments (except in areas close to the river banks). Based on a spatio-temporal prediction of radiocaesium inventories in these different media (expressed in Becquerels per unit surface area, i.e. Bq/m 2 ), the expected dose rates (in Gy/h) and equivalent dose rates (in Sv/h) have then been estimated, considering each medium independently from the others. To mimic the variety of gamma ray monitoring techniques, different altitudes of the detector have been simulated, ranging from 1m height up to several hundreds of meters above ground (airborne surveys were carried out at flying altitudes ranging between 150 and 300 m height, typically). Because the terrestrial landscape in Fukushima region can be very heterogeneous and fragmented,

3 3/13 down to sub-hectometer scales, dose estimations must integrate the complexity and the specificity of the terrain which is embraced at each detector position. As depicted in figure 1, this spatial integration of ambient dose rates constitutes the last calculation step. Models have been implemented in a dedicated simulator within the SYMBIOSE software environment (Refs. [21] [22] ). Key modelling features and assumptions are now presented in more details. 2.1 Atmospheric deposits The characteristics of the Fukushima releases and the conditions under which they dispersed have been assessed by numerous atmospheric studies. The duration and complexity of this accident were such that numerical simulations have still difficulties to accurately reproduce the spatial pattern of the radioactive deposits and even more difficulties to estimate the dry and wet contributions to these deposits (Refs. [23] [24] [25] ). Knowledge of these respective contributions remains an important element of the post-accidental analysis because it determines the fraction of airborne particles which is initially intercepted by plant canopies, and consequently, their pathways and dosimetric impact within soil-plant systems. Thus, we prefered to estimate atmospheric deposits following the method proposed by Gonze et al. [20]. This method relies on the comparison between measurements of radiocaesium deposits in large bare soil areas and airborne measurements which integrate radiation from the underlying land surfaces. By combining both spatial estimations and introducing some physical assumptions about dry deposition mechanisms, it was demonstrated that reasonable estimates of both dry and wet contributions could be produced, in each of the terrestrial medium considered here. Some refinement has been brought to the original method, by reconsidering the spatial interpolation method that was originally used for estimating deposits from (point-scale) field observations. The spatial treatment is now performed in the geostatistical framework (Ref. [26] ). Observations are assumed to be realizations of a random process which, under stationarity assumptions, can be characterized by the so-called semi-variogram. This quantity provides the spatial structure of the process by estimating the empirical similarity between pairs of observations with respect to their distance. It is then integrated in an interpolation procedure called Kriging. Since the measurements exhibit a skewed histogram, a preprocessing (or transform) of the data was necessary. There exists several ways to perform this step (Ref. [27] ). Among them, one can mention parametric strategy such as the MultiGaussian approach or non-parametric one such as the indicator approach. In this work, we focused on the first type which is based on a normal score transform of the data. 2.2 Radionuclide transfer The approach used for forest ecosystems was originally developed in the course of international research programmes launched by the European Commission and the International Atomic Energy Agency after Chernobyl accident ([28] [29] [30] ). This approach is currently being improved and tested in the frame of a dedicated research programme (AMORAD project) supported by the Investissements d avenir grant ANR-11- RSNR Following this approach, forest biotope is decomposed into a hierarchy of components - from top level compartments (e.g., tree canopy, soil layer ) down to more elementary pools of contamination (e.g., leaf internal tissues, available pool of the organic soil layer, ) - which exchange contamination through mass transfer processes. The two processes which are likely to determine the evolution of ambient radiations shortly after an accident (i.e., days to months after the accident) are: the interception of dry and wet fallouts by the above-ground tree components and tree decontamination by either rainfall or biomass senescence (e.g., throughfall, stemflow, leaffall, branchfall). Processes such as migration of radiocaesium into the soil profile, radiocaesium uptake by tree roots and biomass growth can also play a role, but on a longer term (i.e., months to years after the accident). It must be noticed that neither normal management actions, like forest thinning, nor decontamination actions have been accounted for in this study.

4 4/13 The conceptual approach adopted for agrosystems meadows, crops and arable soils is quite analog to that adopted for forest systems. The processes involved in radionuclide transfer are basically the same, except that in case of cropping systems, agricultural practices such as irrigation, soil ploughing or crop harvesting can dramatically modify radiocaesium distribution and ambient radiation levels. The effect of ploughing on the vertical distribution of contaminant has been explicitely accounted for in the model, except for meadows where the soil layer remains undisturbed. Although cultivation of rice is very specific, no particular model has been implemented for paddy fields in this assessment study; it was assumed that radiocaesium dynamics in this medium obeyed to the same physical model than that for cereals. Another important simplification in the context of Fukushima application is that no decontamination actions has been accounted for in the agricultural environment (e.g., soil scrapping, grass cutting, covering with clean soil ), because knowledge of what has been precisely done, where and when, is not available yet. After the Chernobyl accident, international research programmes were organised in order to improve the understanding and modelling of the processes that affect the spread of contamination and radiation exposure in urbanised areas (Refs. [31] [32] [33] ). Quite complex models have been developed and tested against post-chernobyl data, which all require a detailed knowledge and description of the urban environment considered (e.g., variety of buildings, building materials and pavements, spatial organisation, sewage system, meteorological conditions, ). Due to the lack of specific knowledge of Fukushima urban environments, these approaches could hardly be used here. It was rather prefered to adopt a simple compartmental model based on the use of empirically derived transfer rates (similar approaches can be found in Refs. [34] [35] [36] [37] ). Urban canopy is modeled as a uniform equivalent medium, without any distinction between the various objects it usually consists of (e.g., houses, buildings, roads, gardens, trees, ). The model, however, can account for the important mechanisms governing early-phase urban radioecology: deposition, run-off and retention of radionuclides on urban surfaces. Longer term decontamination of urban surfaces through either traffic erosion, water erosion or ageing of materials was also introduced (as an agregated process). We know that Japanese authorities have organized the decontamination of inhabited environments shortly after the accident, in large cities (e.g., Fukushima, Koriyama and the coastal cities). Unfortunately, no precise information could be found in the open litterature, so that the effect of remediation action was implicitely accounted for in our simulations through the calibration of decontamination kinetic rates against carborne monitoring data. 2.3 Ambient dose rates Ambient dose rates were computed in SYMBIOSE simulations through the use of dose coefficients that were precalculated with MCNP code (Ref. [39] ). These dose coefficients (expressed in ngy/h per Bq/m 2 ) quantify, for each caesium isotope, the contribution to the dose rate at detector position of a unit source (1Bq) per unit surface area of the emitting medium. These coefficients basically depend on detector position, radioactive source position and characteristics of the medium (e.g., geometry, density and elemental composition, ). The calculation of dose coefficients was based on the assumption that media were infinite and homogeous in the horizontal plane. As depicted in figure 2a, it was further assumed that soilvegetation systems could be modelled as an equivalent medium consisting of three superposed layers, each layer having a specific depth, density and elemental composition. Radionuclide concentration was supposed to be homogeneous in the horizontal plane but could vary with height z. Dose coefficients were calculated for different altitudes of the detector, different source heights and varying characteristics of the layers (about 50,000 simulations). The contributions of both primary and scattered photons were integrated in the calculation of the photons fluence and its energy spectrum at detector position. Fluences were transformed into Kerma rates, dose rates and equivalent dose rates through the use of conversion coefficients tabulated in Refs. [40] [41]. An example of results is displayed in figure 3 for a tall and dense vegetation layer (50 m height, 6 kg/m 3 ), with detectors situated either inside or above vegetation. In the present case, the Becquerel of radiocaesium per unit surface area was assumed to be homogeneously mixed within the vegetation or the topsoil

5 Height of detector (m) 5/13 z ATMOSPHERE detector VEGETATION source 0 SOIL Figure 2a: Soil-Vegetation-Atmosphere (SVA) equivalent medium consisting of three horizontally infinite and homogeneous layers. Propagation of gamma rays emitted by a thin plane source, located either in S or V layers, is calculated for a point detector of varying height in V or A layers. Figure 2b: Ambient dose rate predicted at a detector located in a complex landscape is calculated by integrating gamma rays emitted from each contaminated plots in the field of view of the detector Canopy contribution Soil contribution h=3cm Soil contribution h=10cm MEXT airborne data 0 1E-14 1E-13 1E-12 1E-11 (Gy/h)/(Bq/m2) Figure 3: Contribution to the dose rate at various detector heights of a unit source of 134 Cs, homogeneously distributed in either a tall forest canopy (50 m height) or the topsoil layer (3 and 10 cm depth), estimated by MCNP calculations. Bulk soil and vegetation densities are 1000 kg/m 3 and 6 kg/m 3, respectively. Vertical profiles of airborne measurements carried out by the Japanese MEXT ministry in Fukushima prefecture are also displayed (see airborne campaigns reported in [18] ).

6 6/13 layer (i.e., volumic source), and dose coefficient were derived by integrating values obtained for plane sources along the z-direction, within each layer. These calculations confirm that dose rates decrease exponentially with altitude, due to photons absorption by the atmosphere, and that the predicted attenuation agrees satisfactorily with airborne measurements by Japanese. These results also demonstrate that the contribution of the topsoil layer (whatever its depth is) drastically decreases as the detector is raised from the forest floor to the top of the canopy. In other words, one Becquerel located in the canopy contributes much more to the dose rate measured above vegetation than one Becquerel located in the litter layer (by an order of magnitude, here). As depicted in figure 2b, extrapolation of this approach to a site-specific multimedia landscape should ideally be performed by integrating, within the field of view of the detector, dose coefficients obtained for land plots of finite length. In the presence of a marked orography, dose coefficients should also account for the inclination of land plots with respect to detector direction. Such extrapolation technique would be time consuming and would require a detailed elevation model and soil occupancy map. In this preliminary study, we prefered to adopt a simple approach where landscape level dose rates were estimated as a weighted average of dose rate contributions of each medium (as described above with assumptions that media are infinite and horizontal) with weighting factors being the percentage of the area covered by each medium. 3 Numerical Study The integrated modelling approach presented above was applied to the Fukushima terrestrial region, with the objective to better identify and quantify environmental processes responsible for the decrease of ambient radiations and factors involved in their spatial variability. Preliminary results obtained on the short-term evolution, from March 2011 to October 2013, are discussed in this section. Longer term pronostic simulations will not be presented here, due to the unknowns in the remediation strategies that will be adopted by Japanese authorities in the future. 3.1 Scenario description The spatial domain of interest is displayed in figure 4a. It covers the 80 km region from the FDNPP site which is located to the East on the Pacific coast. The agricultural Hama-Dori plain facing the ocean is separated from the densely inhabited Abukuma valley situated to the West (e.g., Fukushima and Koriyama cities) by the forested Abukuma mountains of moderate elevation. This montaineous range is dominated by evergreen coniferous forests in its southern and eastern areas. The landuse cover map which has been downloaded from the Advanced Land Observation Satellite website (ALOS, [47] ) provides information at m 2 spatial resolution. Calculations have been performed on a regular grid with a 2 2 km 2 mesh size (about 2,500 meshes). Time evolution of radioactive inventories, fluxes and dose rates have been computed for both 134 Cs and 137 Cs isotopes and landscape level dose rates were estimated by averaging medium-specific dose rates in each mesh; the output values are thus representative of the whole mesh area. As indicated in section B.1, radiocaesium deposits were estimated through normal-score kriging of field observations, then projected onto the calculation grid through block-kriging. Maps of atmospheric 137 Cs deposits are displayed in figures 4b,c. The spatial estimation of the wet deposit fraction by the atmospheric deposition is displayed in figure 4d. As discussed in Gonze et al. [20], this map confirms that intense wet deposition might have been predominant in the Abukuma valley (e.g., Fukushima and Koriyama cities) and to the northwest of the nuclear site. On the contrary, a noticeable part of contamination might have been produced by dry deposition in the high-land forested areas located to the south-west of FDNPP, where evergreen coniferous species are abundant.

7 7/13 (a) (b) (c) (d) A B(are) soil Figure 4: Maps of: (a) landuse (ALOS, [47] ), (b) 137 Cs deposit estimated from airborne measurements (fourth survey, [18] ) through spatial preprocessing (see main text), (c) 137 Cs deposit in bareland soil areas estimated from in-situ observations at stations indicated by blue dots (first survey, [19] ) through spatial preprocessing (see main text), (d) wet deposit fraction estimated on the basis of maps b and c by the atmospheric deposition module (see main text). Is also displayed the calculation grid comprising about 2,500 squared meshes of 2x2 km 2 area.

8 8/13 Most of the radioecological parameters involved in the transfer modules were assigned generic values, in agreement with those proposed in the SYMBIOSE database for Caesium species. A few parameters could be estimated thanks to site-specific information, that is: seasonal precipitation heights, forest transfer parameters involved in the early-phase processes and decontamination kinetic rates in urban environments (derived from model calibration against carborne data). Parameterization of the short term processes in contaminated forests interception of dry and wet atmospheric fallouts by canopy and tree decontamination by rainfall or senescence could be tested against monitoring data acquired in Japanese coniferous forests after Fukushima accident ([42] [43] [44] [45] ), in the frame of the above-mentioned AMORAD project. A detailed discussion about model testing, calibration and sensitivity analysis can be found in Ref. [46]. This study indicated that coniferous forest canopies might have intercepted up to 90% of the total deposit in these regions that were contaminated by either dry deposits or moderate rainfall/snowfall in March This study also indicated that radiocesium transfer to soil occured quite rapidly, as nearly 70% of the total initial deposit was recovered at the forest floor after six months (September 2011) and 90% after 2 years (March 2013). 3.2 Spatio-temporal predictions Typical evolutions of equivalent dose rates are shown in figure 5a,b. These results were obtained in Kawamata town (see geographical location in figure 4a) where predicted wet deposits amounted to about 100 kbq/m 2 and dry deposits ranged from 50 kbq/m 2 for bareland areas to more than 300 kbq/m 2 in evergreen forests. As expected in the areas that were predominantly contaminated by dry deposits, ambient radiations are more pronounced in evergreen forests than in agricultural zones, inhabited lands or even deciduous forests (not shown). Radiation levels were shown to be much less contrasted in regions dominated by wet deposits, because wet deposition mechanism does not depend on landuse. The characteristic rate of decrease of ambient radiations strongly depends on medium. Decontamination is rapid in urban environments due to rainfall wash-off and (presumably) cleaning actions, with a halflife of about 4-6 months. Decontamination is also significant in crop fields due to both downward migration of radiocaesium in the soil layer and ploughing that occurs twice a year in this scenario (october and december). In meadows, the decrease is slower due to the absence of ploughing. In evergreen forest systems, dose rates slightly increase at 1m height above soil while they rapidly decrease at 200 m, at least until October This is caused by the transfer of radiocaesium from canopy to soil during the first few months, which in turn increased ambient dose rates at forest floor and decreased dose rates above forest (as shown in figure 3). The migration of radiocaesium into the forest soil profile also participate to this decrease, but its effect is quite limited because top forest soil layers (e.g., litter layer and organic layer) have a relatively low density (i.e., a few hundreds kg/m 3 ). Numerical results at 1m height can be compared with carborne measurements. Based on a soil occupancy map, the hundreds of thousands of carborne data have been splitted into subsets, depending on the type of medium encountered along the road, and each subset has then been averaged in space to provide a single medium-specific value for each survey (see Refs. [10] et [11] for details). These medium-specific values have been reported in figure 5a. Agreement between simulations and observations is quite satisfactory, except for forest systems, and meadows. We defend the idea that carborne surveys actually overestimate dose rate decrease in surrounding media, because of the contribution of the road pavement that may artificially decrease the signal measured by the detector (e.g., due to wash-off and traffic erosion). The representativity of carborne measurements in urban environments is probably satisfactory because urban surfaces and pavements are likely to behave in a similar way. The representativity of these medium-specific data may also be questionable in presence of a heterogeneous landscape, because multiple media contribute to the signal and these contributions may have not been properly discriminated here. This kind of smoothing effect

9 Dose rate (nsv/h) Dose rate (nsv/h) 9/13 might explain why carborne observations are much less contrasted than in the numercial simulation (a) detector height = 1 m 100 (b) detector height = 200 m MODEL Grass MODEL Crop MODEL Evergreen forest MODEL Urban DATA Grass DATA Crop DATA Evergreen forest DATA Urban 03/11 09/11 03/12 09/12 03/13 1 MODEL Grass MODEL Crop MODEL Evergreen forest MODEL Urban 03/11 09/11 03/12 09/12 03/13 Figure 5: Calculated evolution of equivalent dose rates induced by 137 Cs in Kawamata town, for a detector located at: (a) 1 m and (b) 200 m above ground. Carborne observations averaged over the 80-km region are also displayed (first, second and third surveys, [10] [11] ). Observations have been here ormalized to fit the predicted values at date of the first survey (June 2011). Spatial estimations of the relative decrease of landscape-level dose rates are displayed in figures 6a-d, for a detector located at 1 m height and 200 m altitude. Two periods have been distinguished: an early-phase from March 11 to October 31 (2011) and a mid-term phase from November 1 (2011) to October 31 (2013). Relative decrease is expressed in % per year. The maps produced at 200 m altitude mimic airborne measurements. Numerical simulation at this flying altitude indicate that ambient radiation levels have decreased, over the first 7 months (figure 6a), by more than 35% per year over urban environments (e.g., Fukushima, Koriyama, Iwaki and other coastal cities), between 25 and 35% per year in the very forested Abukuma mountain range and between 10 and 25% in the very agricultural regions (e.g., Abukuma valley, Hama-Dori plain). These results are rather consistent with those displayed in figure 5b in Kawamata town. As can be shown in figure 6b, the situation is totally inverse at 1m height in very forested areas, where ambient radiations have slightly increased (i.e., negative decrease), by nearly 10% in some areas, due to canopy-to-soil transfers. In 2012 and 2013, the simulation indicates that the annual decrease has slowed down. Annual decrease rates typically range from 10 and 25% per year in dense urban environments, from 5 to 20% in very cultivated zones and fall down to less than 5% in very forested regions (see figures 6c,d). There, ambient radiations are maintained at relatively high levels because of the efficiency of forest systems to retain and recycle radiocaesium. Decreasing rates tend to be higher at ground than at 200 m height. This might be due to the attenuation of gamma-rays by the atmospheric layer (this must be further explored).

10 10/13 (a) Mar.-Oct. 2011, 200 m height (b) Mar.-Oct. 2011, 1 m height (c) Nov Oct. 2013, 200 m height (d) Nov Oct. 2013, 1 m height Figure 6: Predicted annual decrease of Kerma rates at 1 m and 200 m heights, from: (a-b) March to October 2011 and (c-d) November 2011 to October Urban areas are indicated in black color, in the foreground.

11 4 Conclusions 11/13 A preliminary assessment of ambient dose rates in the Fukushima terrestrial region has been performed through the use of a dynamic, spatially-distributed and process-based model, implemented in the SYMBIOSE platform. The main processes controlling ambient radiation in urban, agricultural and forest environments have likely been accounted for. Despite the lack of site-specific data, our numerical study predicts dose rate decreases which are globally consistent with field observations and strongly dependent on the medium considered. Disagreement with carborne data in forested regions has been attributed to the parasitic effect of road pavement on gamma measurements. Our calculations in forest systems further demonstrate that ambient dose rates are very sensitive to the detector height. It is demonstrated that airborne measurements of ambient dose rates above forested regions may not be representative of what is actually measured at ground surface, due to the combined effect of canopy-to-soil transfers of radioactivity and the shielding of gamma rays by vegetation. Acknowledgements This study has been performed in the frame of the EDOFU project ( assessment study of External Dosimetry in Fukushima region ) co-funded by IRSN and Electricité de France (EDF). References [1] Blumenthal D.J., Introduction to the special issue on the U.S. Response to the Fukushima Accident, Health Phys. 102, (2012). [2] Lyons C. and Colton D., Aerial measuring system in Japan, Health Phys. 102, (2012) [3] Musolino S.V., Clark H., McCullough T., Pemberton W., Environmental Measurements in an Emergency: This is not a Drill. Health Phys. 102, [4] Sanada Y., Kondo A., Sugita T. and Torii T., Distribution of radioactive caesium measured by aerial radiation monitoring, Houshasen 38, , (2012) [5] Sanada Y., Sugita T., Nishizawa Y., Kondo A. and Torii T., The aerial radiation monitoring in Japan after the Fukushima Daiichi nuclear power plant accident. Progress in Nuclear Science and Technology, 4, 76-80, (2014). [6] Sanada Y. and Torii T., Aerial radiation monitoring around the Fukushima Dai-ichi nuclear power plant using an unmanned helicopter. Journal of Environmental Radioactivity, (sous presse) (2014). [7] Tsuda S., Tsutsumi M., Calculation and verification of the spectrum to dose conversion operator of various CsI(Tl) scintillation counters for gamma-ray. Hoken Butsuri 47 (4), [8] Tsuda S., Yoshida T., Tsutsumi M., Saito K., Characteristics and verification of a carborne survey system for dose rates in air: KURAMA-II. Journal of Environmental Radioactivity (sous presse). [9] Tanigaki M., Okumura R., Takamiya K., Sato N., Yoshino H., Yamana H., Development of a car-borne gamma-ray survey system, KURAMA. Nucl. Instrum.Methods Phys. Res. A 726, , (2013). [10] Kinase S., Takahashi T., Sato S., Sakamoto R. and Saito K., Development of Prediction Models for Radioactive Caesium Distribution Within the 80-km Radius of the Fukushima Daiichi Nuclear Power Point. Radiation Protection Dosimetry, 1 4 (2014). [11] Andoh M., Nakahara Y., Tsuda S., Yoshida T., Matsuda N., Takahashi F., Mikami S., Kinouchi N., Sato T., Tanigaki M., Takamiya K., Sato N., Okumura R., Uchihori Y., Saito K., Measurement of air dose rates over a wide area around the Fukushima Dai-ichi Nuclear Power Plant through a series of car-borne surveys. Journal of Environmental Radioactivity (sous presse) (2014).

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