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Journal of Environmental Radioactivity 99 (2008) 1636–1643

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Journal of Environmental Radioactivity

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Organically bound tritium (OBT) for various plants in the vicinity of acontinuous atmospheric tritium release

L. Vichot*, C. Boyer, T. Boissieux, Y. Losset, D. PierratCommissariat a l’Energie Atomique, CVA/DSTA/SPR/LMSE, 21120 Is-sur-Tille, France

a r t i c l e i n f o

Article history:Received 16 October 2007Received in revised form 29 April 2008Accepted 22 May 2008Available online 31 July 2008

Keywords:TritiumOrganically bound tritiumVegetablesCoefficient rateTreeLichensLettuceChronic releases

* Corresponding author. Tel.: þ33 3 80 23 49 91.E-mail address: laurent.vichot@cea.fr (L. Vichot).

0265-931X/$ – see front matter � 2008 Elsevier Ltd.doi:10.1016/j.jenvrad.2008.05.004

a b s t r a c t

In order to quantify tritium impact on the environmental, we studied vegetation continuously exposed toa tritiated atmosphere. We chose lichens as bio-indicators, trees for determination of past tritium re-leases of the Valduc Centre, and lettuce as edible vegetables for dose calculation regarding neighbour-hood. The Pasquill and Doury models from the literature were tested to estimate tritium concentration inthe air around vegetable for distance from the release point less than 500 m. The results in tree ringsshow that organically bound tritium (OBT) concentration was strongly correlated with tritium releases.Using the GASCON model, the modelled variation of OBT concentration with distance was correlatedwith the measurements. Although lichens are recognized as bio-indicators, our experiments show thatthey were not convenient for environmental surveys because their age is not definitive. Thus, tritiumintegration time cannot be precisely determined. Furthermore, their biological metabolism is not wellknown and tritium concentration appears to be largely dependent on species. An average conversion rateof HTO to OBT was determined for lettuce of about 0.20–0.24% h�1. Nevertheless, even if it is equivalentto values already published in the literature for other vegetation, we have shown that this conversionrate, established by weekly samples, varies by a factor of 10 during the different stages of lettuce de-velopment, and that its variation is linked to the biomass derivative.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Large quantities of tritium (3H) were released by thermonuclearbomb tests between 1954 and 1963. This radionuclide is nowadaysinevitably produced either by accidental releases or by routineoperations of certain nuclear facilities. Its production by cosmicrays in the upper atmosphere was pointed out by Grosse et al.(1951), and subsequently numerous studies have been done on 3Hdistribution and its behaviour in the environment (Belot, 1986;Bogen and Welford, 1976; Murphy, 1993). The three major forms oftritium which are present in the environment (tritiated water va-pour-HTO, molecular tritium-HT and tritiated methane-CH3T) areeasily incorporated (Belot, 1996) as tissue free-water tritium(TFWT) into living organisms, and then, it may label the organicmatter as exchangeable or non-exchangeable organically boundtritium (OBT) through metabolic processes. Equilibrium is quicklyreached between TFWT and global activity in the environment ofthe biological system; therefore this fraction of free tritium isrepresentative of instantaneous tritium levels. Conversely, becauseOBT remains in the organisms for a long time, it is used for retro-spective measures, biological survey, evaluation of atmosphere

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pollution, or dose estimation for humans near nuclear plants(Gulden and Raskob, 2005; Kim and Han, 1999; McFarlane et al.,1979; Pointurier et al., 2004).

Usually, TFWT is removed from a sample by freeze-drying andrecovered in the form of tritiated water (HTO). Exchangeable OBT iscollected by isotopic exchange of the organic material with tritiumfree-water. On the contrary, the bonds between carbon and hy-drogen/tritium atoms that define non-exchangeable OBT are strongand can only be broken by decomposition of organic compounds,most of the time by oxidative combustion (Baglan et al., 2005).

OBT is three times more radiotoxic to humans than tissue free-water tritium (Diabate and Strack, 1993). Actually, even if the TFWTconcentration is higher than that of OBT, because TFWT concen-tration is reduced rapidly after exposure, OBT may be the dominantcontributor to the dose by ingestion. In this context, predictivemodels of doses received by the environment and by man throughthe food chain are essential and a good knowledge of tritiumtransfer mechanisms from air to plants is required.

Photosynthesis is the most important process for tritium in-tegration as OBT in green plants. Previous studies have shown thatOBT is essentially formed from HTO, and very little from HT (Belotet al., 1996). As precursor of OBT, the TFWT compartment could belinked to the OBT compartment by a conversion rate n (Atarashi-Andoh et al., 2002):

-2

2

6

10

14

18

days

Backg

ro

un

d activity (B

q/L

)

Stren

gth

w

in

d (m

/s)

Pressu

re (h

Pa)

26060

60280290310

200

200

220220

100220

180

60

100180180

5431 62 87

60

strength wind (m/s)

60 wind origin (degrees)

activity resulting in combustion of free tritium water (Bq/L)pressure (hPa)

Fig. 1. Contribution of wind direction and intensity, and atmospheric pressure on resulting background activity. Measurements were performed on samples of 30 mL of free tritiumwater during the preparation and combustion steps. Even though meteorological data have an influence on measured activity, it is included in the range of global measurementuncertainty.

L. Vichot et al. / Journal of Environmental Radioactivity 99 (2008) 1636–1643 1637

dCOBT

dt¼ nCHTO (1)

where

� COBT is the OBT concentration in the plant leaves (Bq L�1 ofcombustion water),

� CHTO is the tissue free-water tritium concentration (Bq L�1),� n is the conversion rate from HTO to OBT (% h�1), and� t is the time (h).

Belot et al. (1996) have established an empirical formula to ex-press the links in equilibrium between TFWT concentration andtritium levels in air and soil:

TFWT ¼ 1:1� HR � Cair þ 1:17� ð1� HRÞ � Csoil (2)

where

� HR is the relative humidity (%),� Cair is the tritium level in air (Bq kg�1), and� Csoil is the tritium level in soil (Bq kg�1).

Lots of studies have been done on tritium incorporation intoplants such as trees (Brown, 1979; Fuma and Inoue, 1995; Kalinet al., 1995; Kozak et al., 1993; Murphy and Corey, 1979; Stewartet al., 1972; Sweet and Murphy, 1984; Yamada et al., 1989), radishes(Choi et al., 2005), tomatoes (Spencer, 1984), and wheat (Diabateand Strack, 1997). Nevertheless, some data are missing about thebehaviour of tritium in the environment and the associated trans-fers. Simulations of 3H impact have been computed, but only fewinvolved real experimental conditions. Moreover, most of the time,acute exposures were considered rather than long time and chronicreleases. This study deals with the first experimentation carried outin the surroundings of Valduc Nuclear Centre (Burgundy, 45 km tothe North-West from Dijon, France), for the determination of theuptake of tritium by vegetation from the atmosphere.

Experiments used lichens, oak trees and lettuce, in the vicinityof the centre. Investigations included effects of meteorological data,diffusion of tritium in air and soil, transfers in the biological ma-terials and dose estimation. The experimentation dealt with out-door conditions of wind, hygrometry and geological parameters.

Lichens are described as bio-indicators for environmental pol-lution. It has appeared in the last decades that they concentratesome pollutants; in particular, they are bioaccumulators for radio-nuclides (Daillant et al., 2004a,b; Ichimasa et al., 1989).

OBT measurements in annual tree rings have been conducted toestimate the historical tritium levels in the environment (Brown,1979; Stark et al., 2005; Yamada et al., 2004). Such analyses alloweda first investigation on tritium impact by the Valduc nuclear facil-ities on the nearby environment. Assays on quick-growth plantslike lettuce enable more accurate estimation of tritium in ediblevegetables exposed to chronic 3H releases for dose estimationthrough the food chain. First investigations about Pasquill andDoury models establishment have been done regarding experi-mental values and the absorption of tritium by plants for very shortdistances from the outlet tritium point (<500 m).

2. Materials and methods

2.1. Experimental sites

The biological samples were collected either in the vicinity or inside the ValducCentre. The Commissariat a l’Energie Atomique (CEA) is in charge of this establish-ment located in Northern Burgundy. Waste and emissions, particularly for tritium,are restricted and strictly controlled. Local concentrations are not high enough toconstitute a health hazard, but provide a convenient environmental tracer for thestudy of low-level tritium transfer to the ecosystem, in particular to plants. As muchas possible, sampling sites were chosen according to the prevailing winds.

2.2. Sampling methods

Special care was taken during the experiments to avoid cross-contaminationbetween samples. In particular, biological samples were immediately packaged inpolyethylene bags and put in a freezer after collection. They were weighed beforefreezing to obtain the fresh mass. At the end of each experiment, meticulouswashing of the apparatus was performed to prevent any contamination. Backgroundlevels were determined by analysing tritium in washing waters before next assay.

2.3. Biological species

For the tree samples, a 50-year old beech tree (Fagus silvatica L.) was taken at9 km to the East from the source of tritium releases. Annual rings were separatedmanually from a slice of 50 mm of wood for tritium analysis. In order to evaluate theimpact of distance on OBT integration, another tree, a centenary oak (Quercus pet-raea Liebl.) grown inside the Valduc site, was cut down at 1 km from the same 3Hsource. Packs of 30 g of dried matter were cut for analysis of OBT levels, corre-sponding to periods of 15 years for tritium accumulation.

L. Vichot et al. / Journal of Environmental Radioactivity 99 (2008) 1636–16431638

Lichens harvested in the surroundings of the Valduc area have already beenstudied (Daillant et al., 2004a,b). To compare the results, the species collected inMarch 2005 for the present work were chosen according to the same criteria asDaillant et al. (2004a,b). Gelatinous or other terricolous (living on or in the ground)species were excluded because of their poor availability and the doubt about thepossibility of tritium transfer from soil to thallus. In the aim of studying the atmo-spheric tritium contribution only, lichens with chlorophyceae and grown on treeswere harvested exclusively. As a matter of fact, because they have no contact withthe soil, the influence of the tritium in soil water can be excluded. Two species werefinally chosen for this campaign: Evernia prunastri and Parmelia sulcata, because theywere available in large quantities. A set of samples was collected at 800 m froma local tritium source, near a pond named ‘‘Grand Etang’’ (quoted as ‘‘GE samples’’).Other samples were taken inside the centre, at 360 m from the same emitting source(quoted as ‘‘Valduc samples’’).

Lettuce species sampled were Lactuca sativa L. ‘‘Rouge de Grenoble’’. Theirgrowth was studied during 8–10 weeks from the seeding to maturity, during summer2004 and summer 2006. The plants were cultivated in individual vinyl bags of loam,in order to have uniform soil and to study the accumulation of tritium in soil. Loam forculture was distributed by ‘‘Villaverde Society’’; it was composed of black peat moss(ref NFU 44 551), and contains 62% organic matter on a dry weight basis. Fertilizercomposition was NFU 42002/1 (NPK 13% 12% 17%) and NFU 42001 (21% of nitrogen).The loam pH was 6.3. Blank vinyl bags of loam without culture were exposed tothe tritiated atmosphere to study the tritium integration into the soil. Theculture bags were placed outdoors, and so were exposed to natural conditions ofwind, temperature and humidity; however, they were protected against pre-cipitation by a transparent plastic plate. Plants were irrigated with tritium free-water(activity< 2 Bq L�1). Two different places were dedicated to the cultures; the first one(location A) was a plot exposed to the prevailing winds at 360 m from a tritiumemitting source. The second place was chosen in front of a non-nuclear building,representative of tritium background of the centre (location B). Lettuce were har-vested at different steps of growth; the roots were removed and all the leaves wereanalysed. The collected lettuce were chosen so their average size (determined bymeasuring the crops) was representative of the global population. Usually one plantwas collected for analysis, except for very young lettuce when two plants wereharvested in order to get enough dry matter for the measurement. Correspondingsoils of culture were also sampled for analysis each week.

2.4. Analytical methods

In order to prevent any contamination when the laboratory is downwind oftritium release, preliminary experiments were done to check the impact of mete-orological conditions on background level during the different steps of samplescombustion. Fig. 1 gives the measured tritium activity for the combustion of tritiumfree-water as a function of pressure, wind direction and speed. Since the laboratoryis situated South-West from the main outlet, it can be noted a good agreementbetween meteorological data and measured activity. When the wind comes from0� to 100� , the laboratory is downwind: in this case the reference activity is up to

0

5

10

15

20

25

30

35

40

45

1960 1965 1970 1975 1980 1year

An

nu

al tritiu

m releases

activity (P

Bq

)

Tritium releases from the VaOBT in rings of a tree grownOBT in rings of a tree grown

Fig. 2. Evolution of tritium releases from the Valduc facilities and tritium activities in ringemissary; analysis ring by ring led to an annual history for OBT integration between 196measurements were performed for packs of rings representative of around 15 years of OBT

10 Bq L�1 and can reach 18 Bq L�1. Conversely, when wind is opposite the activity isless than 6 Bq L�1. As a conclusion to Fig. 1, since most of following results given inthis study are above 100 Bq L�1 and the laboratory background is less than 18 Bq L�1,it can be assumed that influence of centre activities on samples preparation is in-cluded in the uncertainty range. Nevertheless, we took care to undertake sampletreatment and analysis with correct and consistent background meteorologicalconditions.

2.4.1. Atmospheric tritiumA commercial bubbling system (MARC 7000-SDEC France) was used for moni-

toring of tritium atmospheric levels. The principle of such an apparatus is the cap-ture of tritium by water. Ambient air is sucked by a pump and sent successively totwo pots full of tritium free-water to trap the atmospheric water vapour containingtritium. Then air crosses a catalytic furnace at 450 �C where molecular tritium-HT isoxidised to tritiated water (HTO). The produced vapour is collected in the two lastpots filled with tritium free-water according to the same exchange process. Aftersome days (generally a week), the activities of the water in the pots were measured,summed and divided by the total volume of air which had crossed the system toobtain the specific activity in air (expressed in Bq m�3).

2.4.2. TFWTTFWT was extracted by freeze-drying methods done within 24–48 h after

samples were stored in a freezer at �20 �C. A specific apparatus (Lyolab 3000 ETO)was used for this operation. The total removal of TFWT was confirmed by weightingresidual samples. Special attention was given to the washing of the chambers forfreeze-drying and tests were performed to ensure no contamination.

2.4.3. Exchangeable OBTExchangeable tritium fraction was removed by isotopic exchange; the principle

of this operation is to mix the dehydrated samples with tritium free-water (Choiet al., 2005; Diabate and Strack, 1997; Guenot and Belot, 1984). In our case, thisoperation was carried out over 1–3 days, and the proportions used were 10 g of drymatter for 200 mL of water. Agitation was needed to obtain greater transfer and thusan efficient exchange. Although the laboratory atmosphere usually had higher tri-tium levels than the natural atmosphere, preliminary experiments showed therewas no significant contamination of the exchange water on condition that theexchange operation was achieved in less than two weeks.

For tree ring analysis, the exchangeable tritium level was low compared to OBTandwe decided to not consider it anymore. As a consequence, the whole OBT was measuredby combustion, not discriminating exchangeable and non-exchangeable fractions.

2.4.4. Non-exchangeable OBTConventional OBT extraction methods involve a commercial combustion appa-

ratus (Bogen et al., 1973; Brudenell et al., 1997; Garland and Ameen, 1979; Guenotand Belot, 1984; Kim and Baumgartner, 1994). The principle of our device providedby Eraly has been described in the literature (Baglan et al., 2005; Pointurier et al.,2003, 2004). The dehydrated samples were placed in a furnace swept with a pure

985 1990 1995 2000 20050

0,5

1

1,5

2

2,5

3

3,5

4

OB

T activity in

tree rin

gs

(K

Bq

/L

co

mb

ustio

n w

ater)

lduc Centre at 1km from tritium emitting point at 9km from tritium emitting point

s of trees grown in the vicinity. One tree was collected at 9 km from the main tritium0 and 1991. Another tree was cut inside the centre, at 1 km from the same source;accumulation.

y = 0,2449xR2 = 0,91

y = 0,0653x - 0,1802R2 = 0,65

0

1

2

3

4

5

0 5 10 15 20 25 30 35 40 45Tritium releases activity (PBq)

OB

T activity in

tree rin

gs

(K

Bq

.L

-1 co

mb

ustio

n w

ater)

OBT in rings of a tree grown at 1km from tritium emitting pointOBT in rings of a tree grown at 9km from tritium emitting point

Fig. 3. Comparison between OBT formation in trees grown at 1 km and 9 km froma tritium emitting source according to releases’ activities of the centre, expressed oncombustion water activity (Bq L�1), and on corrected activity for background and ra-dioactive decay. OBT concentration for tree grown at 1 km represents the average of 15years accumulation, while OBT measured for tree grown at 9 km is done for each year.

Table 1Comparison between the OBT in rings (Bq L�1 combustion water) of trees grown at1 km and 9 km from Valduc Centre, respectively, on average for two periods of 14and 12 years

Period 1 km(Bq L�1)

9 km(Bq L�1)

Ratio1 km/9 km

1961–1975 3654 288 12.71975–1987 2325 150 15.5

L. Vichot et al. / Journal of Environmental Radioactivity 99 (2008) 1636–1643 1639

and dry oxygen flow (60 L h�1), and composed of two burning zones. Pyrolysiscombustion took place in the first one, which is movable. The sample was initiallyplaced in a quartz boat between the two heating regions. Then, the first part movedgradually forward the sample to ensure that the combustion of the organic matterwas slow and complete. Catalytic oxidation of the combustion gases was performedin the second one by using CoO. The temperatures of the two parts were in-dependent and adjustable. The temperature of the first one (pyrolysis) was about600 �C� 10 �C, while the temperature of the second one (catalytic oxidation) wasabout 800 �C� 20 �C. Approximately 30 g of dried material were combusted,excepted for young lettuce for which several plants were harvested in order toobtain 4–5 g of dry matter.

Water vapour was condensed in a U-tube immersed in a cold trap (ethanolcooled at �20 �C). Desiccant was placed after the cold trap in order to rescue theescaped water vapour (only few percents). This water was purified by distillation;because of the impurities like salts, the combustion water was not suitable for directanalysis by liquid scintillation counting. The pH was also adjusted (pH¼ 6–8) withsodium peroxide Na2O2. Before each biological analysis, 20 mL of tritium free-waterwere burnt to check there was no contamination of the furnace and to determine theyield of combustion. On average, this yield was about 99%.

2.4.5. Liquid scintillation countingTri-Carb (Tri-Carb 2900 TR and Tri-Carb 2750 TR/LL, Packard) scintillation

counters were used for 3H liquid scintillation counting (LSC). Samples were mixedwith Packard ‘Ultima Gold LLT’ cocktail in the proportions of 10 mL of water and10 mL of cocktail. The detection limit was 5.4 Bq L�1 for a counting time of 200 minand with a background of 3 cpm. It is a little bit higher than detection limits usuallymentioned in literature (Pointurier et al., 2003), but nevertheless appropriate takinginto account the local tritium levels in the centre. Quench correction was appliedaccording to quenching curves established by the laboratory and tritium standardsource.

The low amount of fresh mass during the first growth stage of lettuce led to littlecombustion of water. In this case, combustion water was mixed with tritium free-water to perform analysis by LSC. Sample activity was then calculated by taking intoaccount of the dilution factor and associated uncertainty.

The activity A of the water samples in Bq L�1 is calculated as

A ¼ N � No

hVt(3)

where

� N and No are the number of counts measured for the sample and for thebackground, respectively,

� h is the counting efficiency (%),� V is the sample volume (L), and� t is the time for counting (s).

The corresponding uncertainty U is given by the following formula:

UðAÞA¼ 2

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiN þ No

ðN � NoÞ2þ�

sh

h

�2

þ�sL

L

�2þ�si

i

�2

vuut (4)

where

� si is the uncertainty related to the incriminated parameter i.

Terms corresponding to uncertainties linked to mass measurements and tocounting efficiency are very near to zero.

The detection limit in Bq L�1 is calculated as

DL ¼ 22�1þ

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2No þ 1p �hVt

(5)

where

� No is the number of counts measured for the background,� h is the counting efficiency (%),� V is the volume (L), and� t is the time for counting (s).

3. Results and discussion

3.1. Tree rings

The history of Valduc tritium releases and OBT concentrationsmeasured in combustion water of tree dried tree wood collected at1 km and 9 km from the main tritium source is shown in Fig. 2. Thelong-term trend for the tree located at 9 km from the chief source of

tritium shows temporal variation of the tree ring concentrationswas strongly linked to the centre releases, and OBT concentrationsappear like suitable values for retrospective evaluation of tritiumfallout. Since the Valduc Centre started tritium activities in 1969,OBT in tree rings has increased as a function of tritium releases untilreaching a concentration of 3500 Bq L�1 in 1975. At this time, a lotof extraction procedures have been developed by the CEA in orderto diminish tritium releases, and from 1975 to 1980 the reduction ofatmospheric tritium led to the simultaneous and significance de-crease of OBT.

Nevertheless, OBT concentrations of about 200–1000 Bq L�1

were observed in tree rings before 1969, i.e. before the beginning oftritium releases by the centre. This phenomenon is probably due tothe first nuclear bomb tests performed at this time; it was alreadyreported in the literature (Brown, 1979; Stark et al., 2005; Yamadaet al., 2004).

Parallel time series of OBT activities in tree rings and activities oftritium releases of the centre led us to suppose the existence ofa conversion rate from atmospheric tritium to OBT. In order to as-sess this coefficient, OBT analysis of a more exposed tree wasneeded. A slice of an oak tree from inside Valduc site, at 1 km fromthe prevailing tritium source, was cut in packs of rings corre-sponding to 15 years of tritium accumulation, from 1900 to 2002.As there was only natural tritium in the area before 1950s, therough data dealing with 1900–1945 reveal a wood contamination,probably due to HTO diffusion from exterior young rings to heartolder rings. The same phenomena have already been mentioned inliterature (Brown, 1979). For that reason, we decided to deduct thetritium level of these years as a background for all values. Resultsexpressed in Bq L�1 of combustion water were compared to thoseobtained for the first tree in Fig. 3. All the data were corrected forradioactive decay between the date of OBT measurement and theone of tritium releases. It is noticeable that good results wereobtained for all these OBT measurements on wood matrix, withuncertainties between 12.1% and 13.7%.

Table 2OBT results expressed on dry matter (Bq kg�1) and on combustion water (Bq L�1) and TFWT (tissue free-water tritium) results expressed on Bq L�1 and on fresh matter(Bq kg�1) for lichens samples collected in March 2005

Sample characteristics Non-exchangeable OBT TFWT

Samplename

Distance fromsource (m)

Species Bq L�1

combustion waterBq kg�1

dry matterBq L�1

TFWTBq kg�1

fresh matter

Valduc 1 360 Parmelia sulcata 10 100� 710 3500� 245 3720� 190 2050� 110Valduc 2 360 Parmelia sulcata 8270� 580 2580� 180 4220� 211 2760� 140Valduc 3 360 Parmelia sulcata 10 390� 730 4460� 320 3717� 190 2190� 110Valduc 4 360 Evernia prunastri 12 610� 890 5520� 390 2070� 105 730� 40Valduc 5 360 Evernia prunastri 8880� 620 4480� 320 2800� 140 1170� 60Valduc 6 360 Evernia prunastri 13 630� 960 6020� 430 970� 50 495� 25Grand Etang 1 800 Evernia prunastri 3220� 230 1680� 120 275� 15 77� 5Grand Etang 2 800 Evernia prunastri 4085� 290 1840� 130 60� 5 14� 1Grand Etang 3 800 Evernia prunastri 4175� 290 2650� 185 60� 5 34� 2Grand Etang 4 800 Evernia prunastri 3330� 240 1390� 100 280� 15 60� 5

L. Vichot et al. / Journal of Environmental Radioactivity 99 (2008) 1636–16431640

The OBT concentrations in rings appear as strongly correlatedwith tritium releases. However, considering the slope of regressionlinear curves would correspond to the conversion rate, HTO to OBTtransfer is clearly more important in the tree, the nearest of tritiumsource.

Concerning the tree grown inside the centre, linear regression ofthe data led to a straight line with a correlation coefficient ofr2¼ 0.91. The regression was not as good for data concerning thesecond tree collected at 9 km from the centre (r2¼ 0.65). The dif-ference in periods considered for measurements may explain thisdiscrepancy. As a matter of fact, variability in meteorological dataand in biological behaviour from year to year is more noticeable forannual measurements than for an average on several years.Moreover, tritium deposition, linked to atmospheric dispersion, isvery more variable at 9 km than at 1 km from emitting point.Taking into account of these facts, the dispersion of the results isreally acceptable, and data for the two trees can be compared.

The slopes of the two series are clearly different (0.245 for thefirst tree vs. 0.057 for the second). The ratio of about 4.3 betweenthe two values reveals that OBT integration in the tree grown insidethe Valduc Centre is more important than one in the tree grown at9 km from the tritium source. To assess this difference, averages ofOBT concentration integrated for the whole period for the two treeswere considered. For each period, the ratio between OBT concen-trations from the two different trees was the same. The increase ofthe distance from the tritium source was thus responsible for a dropof OBT concentration in tree rings with a ratio of 12.7–15.5 for 8 km(Table 1).

The GASCON model was used to validate these results. Thismodel, dedicated to chronic atmospheric releases and dose esti-mation, has been developed by CEA and is used for facility safetyassessments (Iooss et al., 2006). It is based on Doury’s equations(Doury, 1972), i.e. a Gaussian puff model. GASCON includes

Table 3Tritium release in the atmosphere and tritium activity around potted plants (sum-mer 2004)

Spendtime (h)

Tritiumrelease (Bq)

Average of activity near the potted lettucesbetween considered time and the previous one

HTO (Bq m�3) HT (Bq m�3) HTO/HT

192 3.56Eþ12 12.8 1.7 7.4360 2.75Eþ12 22.9 2.4 9.6528 2.54Eþ12 17.2 2.0 8.4696 3.01Eþ12 36.8 3.8 9.7864 3.03Eþ12 5.4 0.7 7.81032 3.57Eþ12 21.2 1.6 12.91200 2.31Eþ12 24.1 2.6 9.31228 2.60Eþ12 19.5 2.6 7.4Total 23.37Eþ12

a specific model for tritium, and thus allows estimating HTO and HTactivities in the atmosphere annually. Because of the lack of realmeteorological data (especially wind direction and speed) for theyears before 1990, estimation of atmospheric concentration wasnot possible for the periods 1961–1975 and 1975–1987 using theGASCON model. However, the establishment of a model was pos-sible for years up to 1990; it gave ratios of tritium atmosphericlevels between the two points of interest about 15. Supposing thatannual wind speed and direction are constant over time, this isconsistent with ratios mentioned between OBT in the two trees inTable 1.

3.2. Lichens

Table 2 gives results of TWFT and OBT measured in P. sulcata andE. prunastri for the two different distances considered. Valuesranged from 62 Bq L�1 to 4218 Bq L�1 for TFWT and from3219 Bq L�1 to 13 628 Bq L�1 on combustion water for OBT. Theseresults have been reproduced and are quite close to those given byDaillant et al. (2004a,b).

Results are different according to the distance and to the speciesconsidered. At the same collecting point (360 m from the emittingpoint), the two species studied show different OBT and TFWTconcentrations, that seems to indicate different mechanisms oftritium integration: an average concentration of 9500 Bq L�1 forOBT and 3000 Bq L�1 for HTO was measured for P. sulcata, a muchhigher OBT concentration (11700 Bq L�1) and an lower HTO con-centration (2000 Bq L�1) was found for E. prunastri.

We have tried to use these kinds of materials as suitable bio-monitors for atmospheric emission, but it would not be possibleassuming that, firstly, the age of lichens could not be determinedprecisely. The lichens growing on trees are obviously younger thanthese trees, which could be dated by their number of annual rings.It is also reasonable to think that lichens collected on the branchesare younger than those on the trunk, but this is not sufficient toknow their age exactly. As a result, it is very hard to correlate thehistoric emissions of the Valduc site and OBT measurements in li-chens collected around the site. Secondly, lichens are well known tohave a very high and specific metabolism. Such organisms are not

Table 4Measured and estimated values of atmospheric tritium activities near lettuces usingPasquill’s and Doury’s models

HTO activity(Bq m�3)

HT activity(Bq m�3)

Average of activity measurements 20.0� 1.0 2.2 � 0.5Pasquill model 21.6 2.4Doury model 11.0 1.2

Table 5Results for lettuces sampling carried out in summer 2006 for locations A and B

Spend time (h) HTO activity(Bq m�3 air)

HT activity(Bq m�3 air)

Lettuce masses (g) TFWT(Bq L�1)

OBT (Bq kg�1

dry matter)Fresh matter Dry matter

A B A B A B A B A B A B

168 18.4� 0.9 0.4� 0.1 2.2� 0.2 0.3� 0.1 – – – – – – – –336 16.766� 0.8 4.6� 0.8 2.3 � 0.2 3.4� 0.1 – – – – – – – –500 1.0� 0.2 8.1� 1.5 0.4� 0.1 2.0� 0.8 29.9 28.1 2.9 1.5 146� 9 329� 16 126� 37 360� 83668 12.7� 0.8 16.3� 1.6 1.4 � 0.2 4.4� 0.7 83.2 56.8 6.0 3.2 826� 41 309� 19 427� 51 298� 51836 23.7� 1.5 8.1� 1.7 3.1 � 0.3 0.9� 0.2 79.8 75.5 7.3 6.5 776� 54 183� 15 669� 67 361� 511006 4.0� 0.4 9.2� 1.8 0.7 � 0.2 1.6� 0.2 147.6 105.7 13.2 10.1 174� 10 615� 31 376� 34 290� 351171 26.1� 1.7 5.8� 1.3 2.8 � 0.3 1.8� 0.7 – 103.8 – 9.0 – 101� 8 – 240� 221364 1.6� 0.3 7.5� 1.5 0.4 � 0.2 1.4� 0.2 78.8 87.0 10.7 10.1 81� 7 263� 18 492� 44 297� 271506 – 8.6� 1.7 – 0.9� 0.3 – 67.9 – 8.9 – 214� 13 – 433� 43

L. Vichot et al. / Journal of Environmental Radioactivity 99 (2008) 1636–1643 1641

representative of the whole vegetation; consequently, any conclu-sion on environment contamination in regard of tritium levels inlichens seems to be very hazardous. Lastly, meteorological data inmodelling tritium diffusion (such as wind, atmosphere humidity,and temperature) have to be known throughout the lichens lifetimeto understand the OBT levels in these organisms, that is difficult forsuch organisms grown in wild environment.

To sum up, OBT measurements in lichens collected aroundValduc site are not suitable for the evaluation of local effect of tri-tium. Because additional research on their metabolism is needed,lichens cannot be chosen as bio-indicators representative for thisecosystem. Moreover, since lichens are not identified as edibleproducts, we cannot get efficient information about what would bethe dose effect by ingestion on the population.

3.3. Lettuce

Table 3 shows the tritium releases and the tritium activityaround the potted plants for summer 2004. Table 4 comparesvalues obtained using Pasquill’s and Doury’s models (Doury, 1972;Laurent, 2003), considering a distance between tritium emissionand crops of 360 m, C type coefficient diffusion (for Pasquill’smodel) or normal diffusion (for Doury’s model), an angle betweenmain wind and direction of the crops of 4� and an average of 10%wind in a such direction, determined in relation with wind rose.

Table 4 shows that measured values and calculated ones for bothmethods are in good agreement, considering only an average ofmeasured parameters. A factor 2 is given using Doury’s model, butsince it has been performed to estimate atmospheric tritium con-centration for acute releases up to 500 m, it would be less conve-nient for these experimental conditions. On the contrary, for veryshort distances and chronic tritium release, Pasquill led to morerealistic data.

Average atmospheric concentration was measured each week inthe two locations and reached levels as given in Table 5. As tritium

Table 6Meteorological conditions (temperature, humidity, wind) during the cultivation oflettuces in the summer 2006

Weeks after seeding Averagetemperature (�C)

Average relativehumidity (%)

Averagewind speed(m s�1)

1 11.0 85.4 2.72 9.3 76.5 2.83 17.6 50.2 2.44 19.9 73.2 1.95 18.2 72.7 1.96 21.3 63.7 1.97 20.2 73.3 1.68 24.0 52.2 2.39 25.9 47.6 2.0

concentrations in location B were lowest, lettuce grown in thisplace were considered as representative of the background of thecentre. Obviously, cultures were performed in the same conditionsin location A. Meteorological data (temperature, relative humidity,wind speed) for the two locations were obtained by the specificstation of the Valduc Centre. They are detailed in Table 6.

We have plotted the growth of lettuce vs. time in Fig. 4. Asexpected, plants grew slowly during the first 500 h. A generativeperiod was then observed which corresponds to the formation offresh leaves, until the lettuce reached 12 g dry matter. Then, drymatter reached a constant level but can also decrease because of thedrying of old leaves without any new ones formed. This curve can bedescribed as a sigmoid and can be fitted as following (cf. Eq. (6)):

Mt ¼K

1þ ½ðK �M0Þ=M0� � e�rt (6)

where

� t is the elapsed time (day),� M0 is the dry mass of lettuce at the time t ¼ 0 (g),� Mt is the dry mass of lettuce at the time t (g),� r is the growth rate (g/(g day)), and� K is the maximal mass of lettuce (g).

Determination of the coefficient for the modelling curve by theleast square method leads to

� M0¼ 0.023 for both locations,� K¼ 11.95 and r¼�2.18� 10�1 for location A, and� K¼ 10.1 and r¼�1.94�10�1 for location B.

Regarding Eq. (1), the average value of the conversion coefficientaccounting for both daytime and night-time rate is 0.20% h�1 forlocation A and 0.24% h�1 for location B, that is consistent in regards

0

2

4

6

8

10

12

14

0 10 20 30 40 50 60

Time (days)

Dry

m

as

s (g

)

Location A : Measures

Location A : ModelLocation B : Measures

Location B : Model

Fig. 4. Lettuce growth vs. time (summer 2006 experiments).

Table 7Average of the conversion rates given in the literature and ones calculated in thiswork

Transfercoefficient n (% h�1)

References

Spinach leaves 0.20–0.32 Atarashi et al. (2002)Radish leaves 0.21 Atarashi et al. (2002)Radish roots 0.07 Atarashi et al. (2002)Cherry tomato leaf 0.13–0.19 Atarashi et al. (2002)Cherry tomato fruit 0.05 Atarashi et al. (2002)Lettuce leaves (location A) 0.20 This workLettuce leaves (location B) 0.24 This work

L. Vichot et al. / Journal of Environmental Radioactivity 99 (2008) 1636–16431642

to data given in the literature (Atarashi-Andoh et al., 2002) fordifferent vegetables leaves (Table 7). In the neighbourhood of nu-clear facilities, people grow 74% of their food locally and a dailyconsumption of 50 g of lettuce is assumed. Based on these data, theresults obtained on lettuce would lead in the more pessimistic caseto an ingestion dose of 0.2 mSv in a year (considering a dose in-gestion coefficient of 4.2�10�11 Sv Bq�1 for OBT and of1.8� 10�11 Sv Bq�1 for HTO (Directive96/29/Euratom, 1996)). Nev-ertheless this conversion coefficient depends of the growth of thecrops.

Fig. 5 represents the variation of the conversion rate and the drymatter difference of lettuce as a function of time. The difference ofdry matter is representative of the additional biomass between twoharvest times and so of the growth rate. Growth of lettuce and theconversion rate showed the same pattern. For both places, theconversion rate was calculated taking into account both measureddry matter at harvest and that calculated from Eq. (6). From the first

-4%

-3%

-2%

-1%

0%

1%

2%

3%

4%

5%

0 200 400 600 800 1000 1200 1400

Time (h)

Co

nversio

n co

efficien

t (%

.h

-1)

Co

nversio

n co

efficien

t (%

.h

-1)

0

1

2

3

4

Weig

ht d

ifferen

ce (g

)W

eig

ht d

ifferen

ce (g

)

0

-3%

-2%

-1%

0%

1%

2%

3%

4%

5%

0 500 1000 1500

Time (h) 0

1

2

3

4

5

Conversion rate/modeled growthConversion rate/measured growth

Dry matter difference

Conversion rate/modeled growthConversion rate/measured growth

Dry matter difference

A

B

Fig. 5. Conversion coefficient determined by taking into account either estimated ormeasured weight of lettuce, variation of dry matter vs. time for locations A and B.

step of growth until maturity of lettuce (i.e. a time of growth of836 h), the conversion coefficient was quite similar for each expe-rience and both determination methods. Conversely, after 800 h ofgrowth, some differences were noticed. This is ascribable to: firstlythe uncertainties attributed to the dry matter variation, secondly, tothe discrepancy between measured mass and modelled mass(Fig. 4) at the end of the generative period. Actually, at this timesome old leaves were dried up, leading to an important variation ofweight between measured mass and modelled. This induces diffi-culties for the realization of a model and the determination of thelettuce weight to be representative of the global population.Moreover, as soon as no new leaves appear, OBT concentration doesnot change anymore; so, by taking into account little variation ofOBT but great dispersion of mass variation between lettuce, somevery large uncertainties on the conversion rate determination re-main. For both locations A and B, the conversion rate was in therange of 0.07–2.40% h�1. The two experiments led to the samerange of conversion rate for a given dry matter mass. This impliesthat n is independent from TWFT concentration and from atmo-spheric HTO concentration near leaves of lettuce. That is to say,lettuce appears as a good reference material for the determinationof atmospheric activity and so ingestion dose calculation.

4. Conclusions

In order to gather information on tritium labelling in the vicinityof Valduc Centre, three different plants were studied. The history oftritium releases in the area was investigated through tree ringanalysis of two trees grown inside and around the centre, at 1 kmand 9 km from a tritium emitting point, respectively. The 8 kmdistance between the two trees was shown to be responsible for anOBT integration decrease of a factor 12–15.

Due to their specific metabolism, lichens cannot be simply cor-related to emissions from Valduc Centre and seem not to be repre-sentative of OBT assimilation by vegetables in the global ecosystem.

At the end of lettuce cultivation continuously exposed to at-mospheric tritium, conversion rates were estimated to average ofabout 0.20% h�1 for crops grown at 360 m from a tritium source,and 0.24% h�1 for crops grown in a location representative of thebackground level in the centre. It was shown that the conversionrate from TFWT to OBT varies with the growth of plants. Becausethis transfer coefficient seems to be linked to the variation of plantmass, the maturation of plants should be taken into account toevaluate the dose impact. Moreover, for a good evaluation of thedose, instantaneous coefficient rate, and not its average on time,has to be considered to be in the more unfavourable conditions interms of impact.

Acknowledgement

The authors wish to express their thanks to Philippe Guetat forhis advice and remark concerning this work.

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