Journal of Environmental Radioactivity 141 (2015) 1e7

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Environmental consequences of uranium atmospheric releases from fuel cycle facility: II. The atmospheric deposition of uranium and thorium on plants L. Pourcelot*, O. Masson, P. Renaud, X. Cagnat, B. Boulet, N. Cariou, A. De Vismes-Ott Institut de Radioprotection et de Sûret e Nucl eaire IRSN/PRP-ENV, CEN Cadarache BP3, 13115 St-Paul-lez-Durance Cedex, France

a r t i c l e i n f o

a b s t r a c t

Article history: Received 3 October 2014 Received in revised form 14 November 2014 Accepted 25 November 2014 Available online

Uranium and thorium isotopes were measured in cypress leaves, wheat grains and lettuce taken in the si (South of France). The comparison of activity surroundings of the uranium conversion facility of Malve levels and activity ratios (namely 238U/232Th and 230Th/232Th) in plants with those in aerosols taken at this site and plants taken far from it shows that aerosols emitted by the nuclear site (uranium releases in the atmosphere by stacks and 230Th-rich particles emitted from artificial ponds collecting radioactive waste mud) accounts for the high activities recorded in the plant samples close to the site. The atmospheric deposition process onto the plants appears to be the dominant process in plant contamination. Dry deposition velocities of airborne uranium and thorium were measured as 4.6  103 and 5.0  103 m s1, respectively. © 2014 Elsevier Ltd. All rights reserved.

Keywords: Uranium Plants Aerosols

1. Introduction Environmental monitoring of uranium fuel cycle facilities requires the determination of uranium in plants, whether as bioindicators or crops. Thus uranium is measured in crops such as wheat grains and leafy vegetables, allowing evaluation of the consequences of the releases in the terrestrial environment surrounding the facilities and to calculate doses due to their ingestion (Jeambrun, 2012). In addition, the isotopic ratio 235U/238U can be used as an indicator to characterize more precisely the contamination due to the releases in routine conditions (Pourcelot et al., 2011a, 2011b; Pettersson and Holm, 1992; Bellis et al., 2001a) or in accidental situations (Bellis et al., 2001b). Studies on thorium activity in the environment are scarce (Zararsiz et al., 1997) and U/ Th ratio in plants, although it has been rarely used up to now, can be an additional indicator of the contribution of uranium releases (Bellis et al., 2001c). Moreover, it remains difficult to estimate on the basis of activities measured in plants the activity level in the atmosphere because the lack of data available for uranium in the air. si facility located in south of France purifies natural The Malve uranium and converts “yellow cake” into U tetrafluoride (UF4)

* Corresponding author. E-mail address: [email protected] (L. Pourcelot). http://dx.doi.org/10.1016/j.jenvrad.2014.11.018 0265-931X/© 2014 Elsevier Ltd. All rights reserved.

(about 10,000 tones per year). The process leads to two types of releases: an atmospheric release of natural uranium and a liquid release of U-decay products, mainly 230Th into ponds where evaporation allows reducing waste volumes. The main atmospheric U release occurs from the stacks of ovens where UO3 is reduced to UO2 at 500  C. This process involves the release of uranium into the atmosphere (see Table 2 thereafter). The releases from the ponds are poorly known. They take place especially when strong winds blow resulting in spray emissions. Particles can also be released from the ponds (dust emissions) when the mud dries out and/or when earthworks are carried out at the ponds. Previous studies carried out in the environment of the si uranium facility detailed the industrial process and the Malve  main sources of radionuclides (Pourcelot et al., 2011a, 2011b; Giere et al., 2012). Up to now, there are a few available data on the transfer of uranium and thorium from air to plants. The aim of the present study was to compare the activity levels of uranium and thorium isotopes in the atmosphere with those in wheat, leafy-vegetables and cypress leaves (Chamaecyparis nootkatensis). Thus activity ratios (238U/232Th and 230Th/232Th) recorded in vegetation samples si were compared with those commonly reported in taken at Malve the same environmental matrixes taken at sites remote from this facility, allowing to evaluate the influence of the main sources of radionuclides involved in plants (discharges from the nuclear site and the soil). An attempt was also made to evaluate the main

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L. Pourcelot et al. / Journal of Environmental Radioactivity 141 (2015) 1e7

processes involved in plant contamination and to deduce the dry deposition velocities of airborne uranium and thorium. 2. Experimental Wheat, leafy-vegetables and cypress were sampled in fields si facility between 2007 and 2013, mainly in surrounding the Malve the zone potentially influenced by atmospheric releases, i.e. to the si site according to the wind rose and at eastern part of the Malve various distances from the fence (Fig. 1). Cypress barks were cut between one and two meters above the ground. Back in the laboratory 2e3 kg of leaves were separated from the barks by hand. Samples of wheat grains and lettuce were collected at maturity in fields and gardens, respectively. Wheat grains were sampled six times between 2007 and 2013 in the field located at the edge of the nuclear site (the field is called “Tauran”, about 50 m from the fence), where the highest activities were measured. Those data make it possible to evaluate the variation in activity concentrations occurring with time in a single foodstuff. Some additional wheat grains were taken in the neighboring fields in 2008 and 2009 to evaluate the spatial variability of the released atmospheric radionuclides. At each sampling site, ears of wheat were cut on the spot at maturity (June, in most cases), using a shear. Back in the laboratory, each sample was 2e3 kg of grain separated from ears. One wheat sample (namely 2010-181) was further separated into grain and husk by hand-picking after gentle crushing of ears. Four samples of lettuce were collected in two private gardens, located 800 and 1000 m from the site in 2008, 2011 and 2013. At both gardens lettuces were cut with a knife. Back in the laboratory 2e3 kg of leaves were washed in order to remove soil particles. The washing, carried out with tap water, intended to reflect typical household washing. Aerosol particles were sampled using a high-volume sampler at an average flow rate of 335 m3 h1. This sampler belongs to the OPERA-air network which is dedicated to the routine monitoring of radionuclides at trace levels in France. The flow rate was regulated

and thus kept constant during weekly sampling periods regardless of the ambient dust load. The sampler was located in the vicinity of si facility at about 1100 m downwind from the main the Malve release stack close to the Tauran field. Air was sampled at 1.8 m above ground level (Masson et al., 2014). Air filtration was performed using 4-layer polypropylene fiber filters (JPE 13160, Jonell Inc., USA). This filter has a minimum collection efficiency of about 95% for particles with aerodynamic diameter of 30 nm and can thus be considered as very efficient regarding the spectrum size of ambient aerosols. Wet-only deposition and bulk deposition were sampled using an automatic opening funnel and a bulk funnel, each of 1 m2. Samples were taken on a monthly basis. The dry deposition was estimated by the difference between bulk and wet-only deposition. This made it possible to assess the contribution of dry to total deposition, especially during the growing period of wheat (April to June). During the aerosol sampling period, the main meteorological parameters (precipitation, wind direction, wind speed and temperature) were acquired in order to evaluate the proportion of dry condition (i.e. precipitation

Environmental consequences of uranium atmospheric releases from fuel cycle facility: II. The atmospheric deposition of uranium and thorium on plants.

Uranium and thorium isotopes were measured in cypress leaves, wheat grains and lettuce taken in the surroundings of the uranium conversion facility of...
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