currentunderstanding oforganically bound tritium (obt) in

CW-121262-CONF-001UNRESTRICTED

Current understanding of organically boundtritium (OBT) in the environment

S.B. Kim l), N. Baglan2

), P.A. Davis3)

1) Environmental Technologies Branch, Nuclear Science Division, Chalk RiverLaboratories, AECL, Canada 2) CEA/DAM/DIF-F91297 Alpajon, France 3) Rad. SafeInc., Canada

Abstract

It has become increasingly recognized that organically bound tritium (OBT) is the moresignificant tritium fraction with respect to understanding tritium behaviour in theenvironment. There are many different terms associated with OBT; such as total OBT,exchangeable OBT, non-exchangeable OBT, soluble OBT, insoluble OBT, tritiatedorganics, and buried tritium, etc. A simple classification is required to clarifyunderstanding within the tritium research community. Unlike for tritiated water (HTO),the environmental quantification and behaviour of OBT are not well known. Tritiatedwater cannot bio-accumulate in the environment. However, it is not clear whether or notthis is the case for OBT. Even though OBT can be detected in terrestrial biologicalmaterials, aquatic biological materials and soil samples, its behaviour is still in question.In order to evaluate the radiation dose from OBT accurately, fUliher study will berequired to understand OBT measurements and determine OBT fate in the environment.The relationship between OBT speciation and the OBT/HTO ratio in environmentalsamples will be useful in this regard, providing information on the previous tritiumexposure conditions in the environment and the current tritium dynamics.

Key words: Organically Bound Tritium, Environmental behaviour and determination


1. Introduction

In the past years there has been increasing interest in the behaviour of tritium in theenvironment. Recent, conferences and workshops such as TRITIUM 2010 in Japan(TRITIUM 2010) and ICRER 2011 in Canada (ICRER 2011) have dealt with severalaspects of tritium in the environment. This tritium can be in the form of tritiated gas(HT), tritiated water (HTO), organically bound tritium (OBT), tritiated organic moleculesand other tritiated compounds (Atarashi et aZ., 1998; Hisamatsu et aZ., 1998; Pointurier etaZ., 2003; Takeda et aZ., 2005; Jean-Baptiste et aZ., 2007; Baumgartner et aZ, 2009).

Recently, more countries have become interested in studying or monitoring OBT inenvironmental samples as part of their radiological assessments. It has also becomeapparent that different people have different understanding of OBT, regardless of theirexpertise ((Baglan et aZ., 2013; Kim and Roche, 2013; Baglan et aZ., 2010). Differentdefinitions and classifications of OBT in the environment exist. Therefore, significantconfusion and misunderstandings are possible when discussing the behaviour ofenvironmental tritium. For this reason, it is important to establish clear definitions of thevarious forms of environmental tritium to avoid any miscommunication among thescientific colllll1unity. When talking about OBT, it must be clearly specified whether it isthe food chain, samples or organic molecules that have been labelled for researchpurposes that is being referred to.

Recent questions about the behaviour of tritium in the environment have focused onpotential accumulation of OBT in organisms from tritium released into the environmentand methods for assessing the biological impact of tritium in humans and theenvironment. Canadian Nuclear Safety Commission (CNSC) have published a series ofreport on the environmental and health effects of tritium and suggested a limit for tritiumin drinking water (CNSC, 2011). These reports convey the limited information on OBT,mostly resulting from investigations in Canada. Even though Davis et al. (2010), Davisand Galeriu (2012) and Galeriu and Melintescu (2010a) have reviewed tritium in theenvironment, there is insufficient information on the classification of OBT in theenvironment. The specific issues regarding OBT in the environment are 1) the potentialfor tritium accumulation up the food chain; 2) whether there is a need to reassess thehealth effects of tritium, and 3) consequences of the future increase in tritium discharges.Discharges of tritium into the environment are forecasted to increase due to changes inthe fuel management methods at nuclear power plants and the operating of new tritiumemitting facilities (Galeriu and Melintescu, 2010b; Nikolov et aZ., 2013).

The purpose of this article is to summarize the current understanding of OBT in theenvironment. OBT distribution and its determination are discussed in the context ofestimating radiation dose from OBT. In addition, a tentative discrimination between themacroscopic views focusing on OBT arising from metabolic processes and microscopicapproaches dealing with tritiated organic molecules (TOM) is proposed.

2. Definitions and Molecular Configurations


Terms associated with OBT are widely used, with meanings depending on the field ofexpertise of the users, leading to possible misunderstandings. Indeed, analysts in chargeof environmental monitoring (Pointurier et al.) 2003; Baglan et aI., 2005) are usingdefinitions for exchangeable OBT (E-OBT) and non-exchangeable OBT (NE-OBT)suited to hydrogen isotope behaviour on a macroscopic level. At the same time, OBTfractions are also referred to as carbon-bound tritium (CBT) and X atom-bound tritium(XBT) (Baumgartner et al., 2009). The latter is divided between rinsed tritium (XBTrillsed)and buried tritium (XBTburied, or BT), where buried tritium corresponds to tritium in theexchangeable position that remains out of reach for the solvent during labile exchangewith a macromolecule. The procedure currently carried out to separate E-OBT from NEOBT consists of immersing the dehydrated sample in water which is initially free oftritium. If only hydrogen physico-chemical properties are considered, isotopic exchange(OBT + H20 ~ HTO + NE-OBT) might govern tritium distribution. However, at themolecular scale, several phenomena are taking place simultaneously, making it difficultto discriminate between tritium fractions. Solubilisation of several molecules occurs,which depends on their physico-chemical properties (Baglan and Alanic, 2011-a). Someexchangeable tritium could remain trapped in the macromolecule and non-exchangeabletritium could be found in labile exchange water when it is part of a solubilized molecule.Therefore, from an analytical point of view, an impOliant task is to determine the balancebetween each phenomenon. To improve our understanding of OBT behaviour in theenvironment and its transfer to man, it is impOliant to focus on the molecular level to seewhich molecules are involved in tritium migration.

In the IAEA (International Atomic Energy Agency) Environmental Modeling ofRadiological Safety (EMRAS) program, definitions related to OBT were debated and thefinal accepted definitions were incorporated in the final document (IAEA, 2010). Thedocument gives the definition of buried tritium and exchangeable and non-exchangeableOBT. Some tritiated organic molecules which do not originate through a metabolicpathway in a living organism are often referred to as OBT. Even though these moleculescan be velY helpful to study some specific metabolic mechanisms because of their nature(protein, amino acids, cellulose etc.), they should not be considered as OBT. With theaim of promoting a common nomenclature for OBT, several forms of tritium mentionedin the literature (see Table 1) will be explained and defined in this study. With the intentto be as exhaustive as possible, tritium interaction with some non-biological chemicalsare also included.

Table 1. Existing "OBT" terminology found in the literature.

Physical form Example References

Organically bound tritium -Diabate and Strack (1993)

Non-exchangeable OBT -Exchangeable OBT


Carbon-bound tritium - Baumgaliner and DonharlBuried tritium (2004)

-

Tritiated organic Labelled compounds McCubbin et al. (2001)molecules (thymidine, leucine... ) Williams et al. (2001)

Soluble tritiated organic Tritiated methane DOE (2004)Tritiated solvents

Tritiated UTritiated Pd

Insoluble tritiated organic Tritiated nylon DOE (2004)DustRust

Pump oil dropletsTritiated flyash

Special tritium compound - DOE (2004)

2.1. Organically Bound Tritium (OBT);

OBT is formed in living systems through natural or biological processes from HTO(Diabate and Strack, 1993; Kim et al., 2012b; Belot et al., 1996). OBT is the sum of theexchangeable and non-exchangeable forms (Figure 1) and it can be expressed as Bq/kgfresh weight, Bq/kg dry weight or Bq/L combustion water (Baglan et al., 2008).

Ii

C 0

NE.OBT E-OET

Cellulose

Figure 1. Example of OBT that contains both exchangeable and non-exchangeableforms.

This includes exchangeable OBT and non-exchangeable OBT as defined by the analyst,as well as carbon-bound, buried and rinsed tritium as proposed by Baumgartner andDonhar! (2004). However, it does not include tritiated organic molecules.


2.2. Exchangeable OBT (E-OBT):

Tritium bound to sulphur, nitrogen or oxygen atoms in organic molecules can be removedby washing with tritium free water and is called exchangeable OBT (Diabate and Strack,1993; Boyer et al., 2009; Pointurier et at., 2003). Exchangeable OBT is in equilibriumwith HTO in the plant or animal in question, and behaves the same as HTO in allrespects. This fraction of OBT depends strongly on the HTO concentration present at thetime of sampling and can exchange with water vapour during analysis. This fraction alsovaries as a function of the HTO exposure type (i.e. chronic or acute). Fraction of theexchangeable OBT would lead to measurements that are highly variable and difficult toquantify. In the washing process, exchangeable tritium is replaced by the hydrogen in thetritium free water. Measurements of OBT after the rinsing process reflect the specificactivity of the non-exchangeable tritium. This specific activity can be estimated bydividing the measured concentration by the fraction of hydrogen in the dry sample that isnon-exchangeable. Exchangeable OBT can undergo fast isotopic exchanges withsurrounding hydrogen. Table 2 shows calculated hydrogen contents of various foods.

Table 2. Calculated hydrogen contents of various vegetables and meats (based on Geigy,1981)

% of total dry weight Mean (%)

Sample Protein Fat Carbo Total Non-ex. Ex.hydrogen hydrogen hydrogen

Beef (round) 61 39 0 9.0 7.8 1.2

Pork (ribs) 30 69 0 10.4 9.7 0.7

Chicken (breast) 90 10 0 7.5 5.9 1.6

Fish (trout) 86 10 0 7.2 5.7 1.5

Clam (fresh) 62 8 18 6.4 5.0 1.4

Milk (cow) 28 32 40 8.3 7.0 1.3

Bean (string) 19 2 72 6.1 4.4 1.7

Beet (root) 13 1 78 5.9 4.2 1.7

Cabbage (white) 18 3 72 6.1 4.4 1.7

Carrot 10 2 80 5.9 4.2 1.7

Cucumber 18 2 68 5.7 4.1 1.6


Lettuce (head) 27 4 51 5.5 4.1 1.4

Peas (pod) 19 1 76 6.2 4.4 1.8

Potato 10 1 88 6.3 4.5 1.8

Tomato 17 3 72 6.0 4.4 1.6

2.3. Non-exchangeable OBT (NE-OBT):

Tritium can bind to the carbon chain of organic molecules and is referred to as nonexchangeable OBT. Such molecules are strong and can't be dissolved easily. Themolecules can only be biodegraded through metabolic reactions. Therefore, nonexchangeable OBT has a longer retention time in biota than exchangeable OBT (Diabateand Strack, 1993; Boyer et al., 2009; Pointurier et al., 2004). From an analyticalperspective, non-exchangeable OBT is the activity in dry bio-matter that is notexchangeable with water. In order to measure non-exchangeable OBT, exchangeableOBT should be removed by moderately drying the sample without decomposing theorganic molecules, washing the residue repeatedly with tritium free water and then dryingthe material again. The non-exchangeable OBT concentration can then be determined asthe tritium activity in the dry sample by combusting the sample or by analyzing thesample using 3He mass spectrometry (Kim and Roche, 2013; Jean-Baptiste et al., 2010,Kakiuchi et al., 2011). Mainly, OBT is integrated in plants by photosynthesis and has alonger residence time (Pointurier et al., 2003, Baglan et al., 20ll-b) and allowsretrospective studies (Bourlat et al., 2010).

2.4. Buried tritium (BT):

Some tritium can occupy exchangeable positions in large bio-molecules in dry matter butcannot be removed by rinsing with tritium-free water. Such tritium is not carbon-bound,but is simply folded into macromolecules by a biochemical mechanism (i.e. protein,cellulose, etc.) and is not accessible for exchange with water. Buried tritium contributesto the OBT concentration in the traditional determination of OBT. It is analogous toburied hydrogen in biochemistry. For example, the rapid increase in plant OBTconcentrations observed in the first few hours following acute HTO exposure in previousexperiments may be evidence of buried tritium forming by exchange through nonmetabolic processes as well as exchangeable OBT (Strack et al., 2005; Kim et al., 2008).

2.5. Carbon-bound tritium (CBT):

CBT is the tritium bound to the carbon chain of organic molecules as a biological productresulting from photosynthesis. It is of the same structure as non-exchangeable OBT orsome tritiated organics (Baumgartner and Donharl, 2004).

2.6. Tritiated organic molecules (TOM):


Different types of organic tritium such as tritiated methane, tritiated amino acids, tritiatedpump oil, radiochemicals, etc. should be defined as tritiated organics (Takeda et al.,2005). They can exist in any chemical or physical form. Tritiated organics can beproduced by physical means in the nuclear industry, leading to tritiated organics wherethe tritium is bound to carbon (Figure 2). Croudace et aI., (2012) described tritiatedorganic to biogenic, non-biogenic and technogenic.

Figure 2. Formula of tritiated Leucine and Lycine

2.7. Soluble tritiated organic molecule (STOM):

Solvents such as octane, cyclohexane, or acetone have been used in pump cleaning andmaterial dispersion processes (DOE, 2004). They can become tritium-labelled fromextended exposure to tritiated materials. Since these solvents are volatile, they areconsidered soluble. Individual intake of tritiated solvents can occur through inhalation,ingestion or by diffusion through the skin.

2.8. Insoluble tritiated organic molecule (ITOMt

This material is formed mainly by incidental contamination of environmental dust and isfound in many tritium contaminated areas. Large tritiated organic molecules (originalmolecules of oil labelled by tritium) of tritiated pump oil are considered to be insoluble.Tritiated pump oils are considered to be a mixture of three components includinginsoluble large tritiated organic molecule, soluble small tritiated organic molecule andHTO in an approximately 8:1:1 ratio. Tritium fi'om the insoluble large oil molecules doesnot dissolve in the body due to the stability of the carbon-tritium bond in oils (DOE,2004).

2.9. Special tritium compound (STC)

A STC is any compound (except for water) that contains tritium either intentionally (e.g.,by synthesis) or inadvevertently (e.g., by contamination mechanisms). They are alsoreferred to as tritiated organics. They can be classified as a soluble STC or an insolubleSTC (DOE, 2004).

3. OBT distribution in the environment


3.1. Enviromnental behaviour in terrestrial systems

3.1.1. OBTinplants

A fraction of tritium that enters plants as HTO can be incorporated into organiccompounds to form OBT (Diabate and Strack, 1993). Non-exchangeable OBT formed byphotosynthesis appears initially in carbohydrates. Subsequently, metabolic reactionsresult in the incorporation of OBT in complex molecules such as polysaccharides,proteins, lipids and nucleic acids. The amount of OBT produced depends on a largenumber of enviromnental factors and plant parameters including light levels, oxygen andcarbon dioxide concentrations, temperature, air circulation and water supply, all of whichshow considerable diurnal and seasonal variations (Korolevych and Kim, 2013; Kim etal., 2012b). OBT concentrations are reduced by conversion back to HTO throughmaintenance respiration, but the process is slow. In the enviromnent, OBT makes up afew percent of the total tritium activity in most plants but up to 90% in grains and haycrops, which have a high organic content.

OBT can be produced in the dark by non-photosynthetic assimilation but it is not wellunderstood. The processes involve the metabolic turnover and synthesis of organiccompounds such as proteins, oils and vitamins using the energy of respiration. Planttissue is dynamic, breaking down and rebuilding bio-molecules continuously, and tritiumatoms from HTO can be incorporated into non-exchangeable sites during the rebuildingphase. The night-time the rates are one-third to one-tenth of those resulting from daytimephotosynthesis (Atarashi et al., 1998; Strack et al., 2005; Galeriu et al., 2013). However,the night-time HTO concentration in air is much higher than the daytime one in the realconditions. Therefore, the night-time OBT production can be close to the day-timeproduction.

OBT is produced in the green patis of plants in the daytime, but can be translocated toroots, sterns and edible fruits. The products formed by photosynthesis at the beginning ofthe vegetative period are used primarily to build the structural material of the plantorgans. When leaf formation is complete, subsequent assimilates are transported in theform of sucrose, fructose and glucose to aid growth of roots and sterns after flowering.Later in the growing season, most of the assimilates are used in the development of fruitsand tubers (Kim et al., 2012b; Galeriu et al., 2013). Diabate and Strack (1997) observedthat the major factor influencing translocation is the growth stage of the plant, and lightconditions do not playa significant role. For OBT formation, translocation index (TLI)represents the ratio between the concentration of the combustion water at harvest and theconcentration of HTO in leaves at the end of exposure.

OBT concentrations reflect the time-integrated HTO concentrations in the plant over themonths prior to sampling. Measured OBT concentrations in the different vegetablesvaried from 5.4 to 23.8 Bq/L in Kingston, 6.2 to 13.9 Bq/L in Killaloe and 7.8 to 22.4Bq/L in Lakefield (Kim et al., 2009). The study demonstrated that OBT concentrationsvaried by the type of vegetable during the same period and in the same areas. Also, the


importance of the soil microbial community can influence the fluctuation of the OBTconcentration. Litter composition is dependent upon a variety of biotic and abioticfactors, including litter chemical composition, plant species and soil properties (Silveiraet al., 2011). Measured NE-OBT concentrations in oak leaves in France were below 1Bq/kg of fresh material in the vicinity of Arcachon, and varied from 1.6 ± 0.3 to 66 ± 11Bq/kg of fresh material in the vicinity of Bruyeres Ie Chatel (Baglan et al., 20ll-b,Baglan et aI., 2013-b).

3.1.2. OBT in animals

Tritium concentrations in animals fluctuate over time, but do not follow the daily changesof the concentrations in their environment. Non-exchangeable OBT in animals arisesprincipally from assimilation of ingested OBT in food (NCAS, 2001; Van den Hoek,1986). As much as 50% of the OBT in food fed to animals appears as OBT in the animalitself, attached to biochemical compounds such as amino acids, sugars, protein, lipids andcell structural materials. A small fraction «5%) of HTO taken in by the animals isirreversibly incorporated into organic molecules through the enzymatic reactionsinvolved in carbohydrate metabolism (Kirchmann et al., 1977). Thus, the transfer to nonexchangeable organic compounds in animal products is much more important when thetritium is ingested in its organic form rather than as tritiated water (Kim et al., 2013;JawscWce and Bradshaw, 2013).

OBT has a longer retention time than tritiated water in animals. OBT is lost slowly fromanimals as a result ofmetabolic oxidation to HTO, which is then excreted. The biologicalhalf-life of OBT varies with the type of molecules, since different molecules havesignificantly different metabolic turnover rates (Melintescu et al., 2011). The half-livesfor animals reflect protein and lipid metabolism, and are longer than half-lives for plants,which are composed primarily of carbohydrates (Rudran, 1988; CERRIE, 2004).

The extent of OBT incorporation depends on the length of exposure and the metabolicactivity, which differs from organ to organ. Thus, OBT can be localized in animals in arelatively small number of cells and at relatively high concentrations. However, the OBTingested with food is metabolically oxidized to HTO during digestion and assimilation,and enters the compartment of free body water. Fats and proteins retain more of theirOBT during digestion than do carbohydrates. The constant synthesis of organiccompounds in dairy cows during milk production results in the continuous formation ofOBT, which is excreted in milk, urine and feces at a rate only slightly slower than that ofHTO. There is no OBT results on farm animals or wild animals, and birds, while a fewmodeling results of animals have been published (Galeriu et aI., 2007; Melintescu andGaleriu, 2010; Galeriu and Melintescu, 2011)

3.1.3. OBT in soil

Even though soil is considered one of the important ecological compartments by tritiummodellers, OBT concentration in soil is not well understood. This is because soil contains


organic and inorganic materials, and only limited methods are available for analysing soil(Clark et al., 2010; Hisamatsu et al., 1998). Kim et al. (2012a) reported that OBTconcentrations at a historical HT release site were considerably higher than HTO activityconcentrations in the same soil samples. The OBT activity concentration in soil can bereflective of historical tritium releases into the environment. Also, substantial year to yearvariations in soil OBT have been reported, which are larger than any potential trophicaccumulation of OBT. At present there is no information about OBT decomposition insoil, potential root uptake of soluble OBT and OBT recycling in environment. Therelationship between the OBT concentration in soil and the tritium released to theenvironment will be useful information for evaluating environmental tritium effects andthe fate of tritium in the terrestrial ecosystem.

3.2. Environmental behaviour in aquatic systems

Most aquatic organisms are totally immersed in water and have high water exchangerates. Uptake of HTO is very quick and concentrations in tissues become equal to waterconcentrations within minutes or hours (Rodgers, 1986; Kim and Korolevych, 2013;Yankovich et aI., 2011).

The process of OBT formation in aquatic plants is much the same as it is in terrestrialplants. Total OBT is formed through photosynthesis during the day and through a varietyof metabolic processes in the dark. The OBT can be translocated from its place offormation to storage organs in the plant, and is degraded to HTO through respiration.

A small fraction of the HTO taken up by aquatic animals is convelied to OBT in anabolicprocesses, but most OBT in animals arises from the direct incorporation of OBT iningested food. Metabolic processes in the animal depend on water temperature andseasonal effects should be taken into account in estimating OBT formation (Galeriu et al.,2005). Fish metabolism slows down appreciably when the fish growth rate becomes lessthan 10% of the optimum growth rate, which occurs for water temperatures below 6-8°C.

Kim et al. (2013) repOlied that the OBT formation rate was slower when fish wereexposed to HTO compared to when fish were ingesting OBT. Moreover, OBT can bioaccumulate in fish tissues following ingestion of OBT-spiked food. There is no clearevidence between OBT residence time and the nature of the molecules used to spike thefood. Further investigations are required.

Jean-Baptiste et al. (2007) found elevated tritium concentrations in fish, plants andsediments both upstream and downstream of a nuclear generating station in France. HTOconcentrations in the organisms were in equilibrium with the river water, but nonexchangeable OBT levels in fish, plants and sediments exceeded the levels in water byfactors of 3, 100 and 10,000, respectively. These results suggest that aquatic plants andanimals can accumulate OBT after chronic tritium exposure. Indeed, tritium turnover ineach compmiment in the living organisms are different and turnover rates might play animpOliant role in apparent bio-accumulation of OBT. Taking into account radioactivedecay, OBT levels present in sediment are equivalent to the HTO levels in precipitation


40 years ago and this time lag is similar to the average residence time of organic matter insoils (Siclet, F.., 2012; Baglan et al., 2013-a)

Dissolved tritiated organics released directly into water can get taken up by aquaticanimals. This situation was observed in Wales, where a variety of tritiatedpharmaceuticals were released to Cardiff Bay (Williams et al., 2001; McCubbin et al.,2001; Environmental Agency, 2001). OBT concentrations in mussels and benthic fishwere 1000 times higher than the HTO concentration in the water. This case representsnon-equilibrium conditions resulting from an accidental release.

Melintescu and Galeriu (2011) have modelled the biological half-life of OBT in aquaticorganisms using bioenergetics. The model addressed OBT in sediment, molluscs, preyand predatory fish and explained the Cardiff Bay case (Williams et ai., 2001; McCubbinet ai., 2001) clearly.

4. Determination of OBT concentration in the environment

4.1. Biological samples

The determination of the OBT concentration in samples from plants, animals andfoodstuffs is more complicated and difficult than the measurement of the concentration ofHTO or tissue free water tritium (TFWT). Also, uncertainties can arise at various stepsthroughout the procedure. All samples must first be dehydrated completely using bothfreeze-drying and possibly oven-drying. Then the tritium must be combusted using one ofseveral methods (e.g., Parr vessel, Tube furnace, PyrolyseI', Carbolite, Hyperbaricoxidizer, Oxidizer). Finally, the quantity of tritium present must be measured using oneof several possible analysis techniques (e.g., Liquid scintillation counter, 3He massspectrometer, Accelerated mass spectrometer, Static noble gas mass spectrometer).

To measure the non-exchangeable OBT, an additional stage in which the exchangeableOBT is completely removed from the dehydrated sample must be added prior tocombusting the sample, which introduces a new source of uncertainties. Kim and Roche(2013) and Baglan et al. (2010) evaluated different methods of determining OBT as pmiof inter-laboratory comparison exercises. Each method has some advantages, but areliable method should be chosen based on the availability, sample type and detectionlevels.

4.2. Non-biological samples

Tritium is routinely encountered in various non-biological matrices such as sediment/soiland concrete decommissioning at nuclear facilities and remediation of contaminated sites.In order to measure the tritium activity concentration accurately, it is impOliant todevelop an appropriate sampling, storage and analytical strategy. The fraction of tritiumin the sample must be well understood and it will depend on the origin of the tritium, thechemical form of the tritium within the sample and the sample composition (Baglan eta1., 2013).


For non-biological samples, a Parr vessel apparatus is not suitable for tritium extraction.A tube furnace is considered suitable. Temperatures in excess of 800°C are required toliberate tritium from graphite, metals and irradiated materials. However, 600°C issufficient for extraction of tritiated molecules in all environmental sample types(Warwick et at., 2012).

5. Dose contribution from OBT in the environment

For assessment of occupational or public dose from tritium, the source type is importantin determining the OBT contribution. Under routine tritium release to the atmosphere,public dose depends on exclusion zone, cultivated crops and local food consumptions aswell as the reactor operation performance. Table 3 shows the published estimates of thecontribution of OBT to the total tritium dose to members of the public living near nuclearfacilities.

Table 3. Contribution of OBT to the total tritium dose for chronic atmospheric tritiumreleases.

Investigated site

Eastern OntarioBackground areaNear a nuclear facilityVery near a nuclear facility

Savannah River

Deep RiverOttawa

Lawrence Livermore NationalLaboratory

Wolsong (Korea)

USA

Germany

France

OBT dose Referencecontribution (%)

1719 Osborne (2002)14

8 ATSDR (2002)

50 Kotzer and Trivedi (2001)26

20 LLNL (2001)

25 Kim and Han (1999)

10 DOE (1994)

20 Gulden and Raskob (1992)

< 20 ASN (2010)

The variability is large, but in most cases, OBT makes up less than 25% of the totaltritium dose, with a mean of 21 %. The variability in the estimates arises from differences

CW-121262-CONF-00 IUNRESTRICTED

in assumptions concerning the type and amount of each food item in the diet, and thefraction of food that is contaminated. The value of 50% repOlied for Deep River isalmost twice as high as any of the other values. This result was obtained from a modelthat predicts HTO and OBT doses from the ratio of OBT/HTO in urine (Trivedi et al.,2000). This approach should provide a reliable estimate of the OBT contribution since itis based on the measurement of HTO and OBT in the body rather than on assumptionsconcerning intakes. However, the value in question was obtained from a single,cumulative urine sample collected from a single individual. Moreover, the OBTconcentration in the sample was very low and therefore subject to considerableuncertainty. When most vegetables are ingested, the dose from HTO is greater than thedose from OBT because the dry matter fraction is quite small. OBT becomesincreasingly impOliant to tritium dose as the fraction of dry matter increases (LLNL,2001). It is only in the case of grains and telTestrial animal products that OBT makes upa significant percentage of the total tritium dose, up to over 80%.

For routine aquatic tritium release, the only exposure pathway for aquatic organisms, fishbeing the primary organism affected, is typically by the ingestion of drinking water.Ciffroy et al. (2006) reported that dose contribution from drinking water is about 30times higher than that from fish ingestion. Using the current ICRP dose conversionfactors of HTO and OBT, the OBT dose contributes only about 1% to the total tritiumdose for most aquatic releases. This contribution increases if the contaminated water isused to irrigate agricultural crops, but even in this case, it reaches only about 10%.

A recent study ofOBT in the Severn Estumy (Croudace et al., 2012), the Severn Estumyprovided clear evidence of systematic preservation of OBT in a specific sedimentary sinkthrough strong sorption of some classes of tritiated organic molecules on sediments. It isprobably mediated in part by the presence of anthropogenic carbonaceous sorbents. Also,an estimation of the OBT depositional inventOly shows it represents only a velY smallfraction of the total discharge. This area was contaminated by a pharmaceutical companythrough a release of tritiated organics. Tritiated organics contribute less than 10% of theoccupational tritium dose received by most nuclear workers.

Under the accidental condition, OBT intake can be higher. Dose estimates stronglydepend on the human dosimetic model used. Soluble OBT, pmiicularly exchangeableOBT, migrates through the skin or lungs into the bloodstream by the physical processesof dissolution and diffusion, and is readily absorbed through the GI tract followingingestion. The two processes are inseparably linked and often simply called"adsorption". Since HTO and soluble OBT are rapidly assimilated physiologically andexcreted via urine, these dose components are readily evaluated via urine bioassay.Volatile OBT or tritiated organic molecules (TOM) produces vapours that can becomedispersed in air, resulting in a second potential intake pathway of vapour inhalation andskin absorption.

Solid pmiiculate OBT, corresponding to tritiated dust or rust, is considered primarilyinsoluble and intake by skin absorption is not expected. However, inhalation is a possibleintake mechanism (DOE, 2004). Following ingestion or inhalation, some fraction of


insoluble OBT will likely be digested in the stomach and converted to HTO or solubleOBT. In addition, insoluble OBT may reside in the lung for a period of time, deliveringdose to the lung. There has been a lengthy, intense debate on adequacy of the comment ofICRP model and recommendations. Recently, Galeriu and Melintescu (2010b) havetested OBT model with human data. The model demonstrates that ICRP model is underpredicting by a factor of 2-5 and refutes some claims that dose coefficients should behigher. The model distinguishes between the low dose rate during normal reactoroperation and the high dose rate during abnormal operation. For occupational Dosimetry,the bioavailability of OBT-containing compounds should be considered.

No published data could be found for the contribution of OBT to the total tritium dose tonon-human biota. On that specific point, independently of the tritium's form, the Ericatool (2012) can be used to provide an estimation of the impact tluough extrapolation ofthe transfer coefficient.

6. Effect of food processing on OBT

Tritium concentrations in foods are affected by standard food preparation techniques suchas washing, boiling, roasting or removing parts of the raw food. However, limitedquantitative information is available (Watterson and Nicholson, 1996) in terms of thefood processing retention factor. This factor is the total amount of tritium in processedfood divided by the total amount in the original raw food. OBT contents are affected lesssignificantly, but are still reduced by 25-60%. These values assume that the water used incooking is uncontaminated, which may not always be the case. Moreover, it is the customin some cultures to consume the cooking water, in which case any tritium lost to thewater would still be ingested.

7. Terminology with theory and observations

It is well established that tritium as HTO can't accumulate in the natural environment(Murphy, 1993). However, it is not clear whether or not tritium can be accumulated in theform of OBT, which exists in the environment. Moreover, tritium speciation in theenvironment is quite complicated due to the wide variety of molecules, reaction schemesand metabolic pathways in which hydrogen isotopes are involved. Consequently, tritiumis studied and used by scientists in various fields of interest. This has lead to varyingterminology for the same concept, e.g., tritium bound to other atoms of a sample, orsimilar terms for concepts which could differ between groups, e.g., bio-concentration,bioaccumulation, bio-amplification or persistence. Typically, bio-concentration is usedfor HTO concentrations in aquatic biota with the bio-concentration factor being definedas. Investigations have been shown that OBT in marine fauna varied be a factor of 1000with respect to the HTO concentration in the sea (McCubbin et aI., 2001; EnvironmentalAgency, 2001). Bio-accumulation is related to the discharge of tritiated water. Maximumtritium values in molluscs and flat fish show a 1-2 year time lag with respect to maximumdischarge values (ASN, 2010). In contrast, tritium in the environment does not show anybio-accumulation around La Hague (ANS, 2010). From the 1970's and 1980's, somepublications suggest that tritium could bio-amplify in some aquatic trophic chains


through the nutritional pathway (ANS, 2010). In order to validate bio-amplification, moredata are required. If the living organism was exposed to higher environmentalconcentrations in the past than in the present, past contamination may remain in some ofits tissue. This is one of the reasons for the high OBT/HTO ratios in nature. Currently,many publications are available to explain the high OBT/HTO ratios.

Thus, the authors proposed a definition of OBT species. Practically, several differentOBT species were described previously, but a new classification of OBT is suggestedusing available knowledge and findings from the tritium community (Table 4).

Table 4. Suggested OBT classification based on the sample characteristics.

Type Subclass I Subclass II ExampleOBT Exchangeable

Living organism Plants, animals, OBT E-OBT or XBT(*)Tritium integrated human, soils orthrough biological litter (organic Non

processes part through Exchangeable NE-OBT or CBT(*)microbial OBTactivity)Tritiated Labelled moleculesorgamc TOM radiopharmaceutical

moleculesNon-living organism Tritiated solvents

(only physico-chemical Liquid tritiated methaneprocesses) tritiated oil

Tritiated Tritiated nylon, tritiatedorgamc Solid concrete, tritiated fly

compound ash, tritiated metal (D,Pd)

Air Tritiated methane

(*) With such a macroscopic view where only hydrogen properties are considered without evaluating eachindividual molecule's behaviour, E-OBT and XBT on the one hand and NE-OBT and CBT on the otherhand could be considered as identical. However, OBT stOly is not as simple and its distribution within theenvironment is possibly impacted by several factors related to the molecule on which OBT is supported andtheir own physico-chemical propelties.

At the sample level, the easiest way to differentiate between OBT and TOM or tritiatedorganic compounds is with respect to their method of formation. For the first categOly,tritium is incorporated into the molecules either by metabolic or other biochemicalprocesses in living organisms, whereas for the second, only physico-chemical processesare involved.

CW-12l262-CONF-001UNRESTRICTED

For environmental monitoring purposes, these simple definitions might be sufficient toclassify each tritium compound to assess their concentration and to determine theirpotential impact. To get better insight into the behaviour of the fractions of tritium in theenvironment, investigations down to the molecular scale are important to study thevarious properties which could play a role. Indeed, independent of the tritium bonding,each molecule will behave in accordance with its physical and chemical properties.

An OBT measurement at a point in time reflects the integrated HTO effects of theprevious months, but the HTO concentration in the plant reflects the environmentalconditions over the previous few hours only (Kim and Korolevych, 2013). Under steadystate conditions, plant OBT concentrations are lower than HTO concentrations becausethe large difference in mass between hydrogen and tritium gives rise to significantisotopic discrimination effects in OBT formation (Baumgartner and Donhar!, 2004; Kimet aI., 2009). Exact theoretical estimates of the OBT/HTO ratio (R) are not availablebecause OBT fractionation effects are very difficult to quantify in plant or animalmetabolic processes. Nevertheless, OBT concentrations are expected to be slightly lowerthan HTO concentrations. The ratio can be indicative of whether or not environmentalcompartments are in equilibrium with respect to tritium concentrations under the specificactivity theory. Published results show that 77% of OBT/HTO ratios measured interrestrial plants and food items are greater than one, with a mean value of 1.92.However, for aquatic samples, 81 % of the published ratios are less than 1 (Jean-Baptisteet at., 2011).

A greater diversity in OBT/HTO ratios than expected has been observed in severalenvironmental samples from background locations (CNSC, 2011), but the bias is towardhigh values. In addition, the results for these background area samples suggest that plantsmay sequester tritium not only from water, but also from soil organic matter. Todetermine the mechanism involved will require more sophisticated characterization of thetritium forms that exist in the environment.

In order to have a better understanding of those points, several studies are ongoing todetermine which are the predominant molecules explaining tritium behaviour in theenvironment and its transfer to man. It is well known that environmental samples arecomposed of at least lipids, proteins, and carbohydrates. Their assessment remains achallenge due to the wide variety of individual types within each group, each having theirown chemical behaviour in terms of solubility, red-ox properties, complication andsorption capabilities, etc. Some examples focusing only on the hydrophilic orhydrophobic characters of some molecules are given in Table below.

Table 5. A possible classification based on tritiated molecule characteristics.

Category 1 Category 2 Category 3 Catel.Wry 4 Example

Protein soluble Albumin


insoluble Casein

solubleGlucose

Tritiated Saccharosemolecules Carbohydrate

insoluble Cellulose

Organicsoluble Butyric acid

LipidStearic acid

insolubleOleic acid

solubleAlanineLeucine

Amino-acidinsoluble L-Tryptophan

soluble Ascorbic acidVitamins

insoluble riboflavin

Nucleic acid insoluble DNA

- soluble Tritiated metals

InorganicTritiated fly ash

insoluble- Tritiated nylon

This tentative classification demonstrates that going down to the molecular level could beuseful for investigating tritium behaviour more precisely. For living organisms, theamount of molecules and reactions involved is very large.

7. Conclusions

OBT is produced through photosynthesis in plants and metabolic processes in animalsand can be detected in most compartments of organic materials such as plants, animalproducts and soils. It is not evenly distributed in natural ecosystems, even within thesame organism. In addition, OBT can be found in non-biological samples. Unlike forHTO, OBT behaviour is not well understood in the environment. Tritium as HTO can'tbio-accumulate in the environment. However, it is not well known whether or not OBTcan accumulate in the environment. OBT can be divided into soluble and insolublecomponents. Therefore, a new and simple classification for OBT in the environment isproposed based on molecular characteristics.

OBT measurement is a complicated, expensive and time consuming process. In order toevaluate OBT effects in the environment, better understandings on OBT formation andtranslocation in living organisms are important and a few well designed experiments are


required. Further study is required to understand OBT recycling and the fate of OBT inthe environment. OBT contribution to the total tritium dose to human and non-humanbiota varies as a function of exposure type and environmental pathways. Generally OBTintake will be less than 30% of HTO intake but OBT intake can be increased undertransient conditions. OBT contribution from food stuffs can be reduced through foodprocessmg.

Methodological clarification is required to compare environmental tritium measurements.In order to improve the OBT standardized measurement, sampling methods andanalytical protocols, the work should be carried out within an international framework.The OBT/HTO ratio from environmental samples will provide useful information on theprevious tritium exposure conditions in the environment and the current tritium dynamicsin ecosystems.

Aclrnowledgement

The funding for this study was provided through AECL PAl.5. The authors would liketo acknowledge Jennifer Olfert and Nick Scheier for improving the manuscript andproviding critical comments.


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