Last update on 9 August 2012
As an isotope of hydrogen, tritium is intimately tied to the cycle of this element in the environment. It may be found in all hydrogenated molecules and associated both with water in tissue as with the organic material of plants and animals. In the form of tritiated water, HTO, this radionuclide is extremely mobile in the environment and all biological systems, and thus quickly integrated into numerous cycles of the geo- and biosphere. In equilibrium, it does not appear to accumulate preferentially in a particular environmental or biological component.
In assessing the impact of repeated discharges, radioecologists assume that tritium follows the water cycle. Models specific to tritium are simple and based on continuous discharge conditions, constant concentrations in the air and soil at any depth and anticipation of equilibrium.
If tritium is introduced into the environment in the form of tritiated organic molecules, the fate of tritium will be that of the organic molecule under consideration until it is degraded by biological or physicochemical processes. The behaviour of tritium will then be that of decay products (mainly HTO). Considering the variety of organic molecules that can be associated with tritium, it is impossible to make an inventory of the behaviour of these molecules in the environment.
The environmental toxicity of 3H is related only to radioactive emissions of the pure, low-energy beta type. It results then mainly from uptake processes.
Tritium is the radioactive isotope of hydrogen identified by the symbol 3H or T. Its chemical properties are identical to those of hydrogen. Whether of natural or man-made origin, tritium is extremely mobile in the environment and in all biological systems.
Tritium exists in three chemical forms:
Tritiated water (HTO): also known as "super heavy water" (11% heavier than H2O), this is the most abundant form of tritium in the natural environment and in living organisms. When tritium is not introduced into the environment in this form, then HTO most usually is the result of HT oxidation resulting from light or bacteria action. The speed with which it exchanges with the hydrogen in water promotes the homogenisation of HTO concentrations in living organisms.
In molecules in which hydrogen is replaced by tritium, the difference in atomic mass can be seen in natural transition processes such as evaporation, condensation or solidification. This leads to low-level tritium enrichment in the condensed phase compared with light hydrogen.
Gaseous tritium (HT): this chemical form, which concerns but a small fraction of tritium released into the air, could become more significant due to the development of nuclear fusion for the production energy. As a result of the oxidation process, tritiated hydrogen is transformed into tritiated water and re-enters the water cycle.
Organically Bound Tritium (OBT): this form, in which tritium is bound to organic matter, results from tritium being incorporated in various organic compounds during the synthesis process of living matter. Such organic compounds are distributed according to their specific chemical properties, which may explain the possible heterogeneous distribution of tritium among tissues. How stable tritium is within such compounds depends on the nature of the bond between the tritium and the organic molecule and on the organic molecule’s affinity with the different biological tissues. A distinction is made between:
- exchangeable tritium: there is an exchangeable fraction when hydrogen atoms bound to oxygen, sulphur or nitrogen are replaced by tritium and are readily accessible for new exchanges; this fraction of tritium bound in labile form to biomolecules is in equilibrium with the concentration of tritiated water in the intracellular environment.
- non-exchangeable tritium: tritium is covalently bound to a carbon. This is a permanent bond as long as the biomolecule itself is not transformed nor destroyed by an enzymatic reaction. The amount of time that tritium remains incorporated therefore depends on biomolecular turnover: fast in the case of molecules involved in the energy cycle, and slower in the case of structuring molecules or macromolecules such as DNA or energy reserve molecules.
These exchange mechanisms are common to all living organisms, plant and animal alike. The distribution between tritiated water, exchangeable and non-exchangeable tritium varies according to the respective intake of HTO or OBT, the nature of the organic bonds generating OBT and the metabolism of each individual species.
Tritium is heavier than hydrogen and, due to the isotope effect, enzymatic reactions implicating the radioactive isotope are slower than those that use the stable isotope (see Belot et al., 1996).
Hydrogen has six isotopes, with masses 1 to 6, the first two of which are stable. Tritium (3H) is the only radioactive isotope with a half-life of more than one year.
(Nucleonica; EC, 2009)
The quantity of tritium present in the biosphere considerably increased due to thermonuclear weapons testing in the atmosphere and with the development of the nuclear power industry. At global scale, a return to concentration levels similar to those that preceded weapons testing is underway. Thanks to radioactive decay, almost 90% of the tritium introduced into the environment between 1945 and the late 1960s has disappeared.
The abundance of tritium in the environment is very low compared to hydrogen or deuterium. Tritium is produced naturally mainly due to nuclear reactions in the upper atmosphere between atmospheric nitrogen and oxygen atoms and high-energy cosmic rays.
Around 99% of the tritium produced in the upper atmosphere is oxidised to tritiated water (HTO) and dispersed into surface water. Annual natural production in the atmosphere is between 0.15 and 0.20 kg, i.e. 5.0 to 7.0 x 1016 Bq, presuming that production balances out physical decay (see Lebaron-Jacobs et al., 2009).
A small fraction of natural tritium is produced in the Earth’s crust during neutron capture from traces of lithium-6 contained in the rock. The neutrons implicated in this reaction are produced by the spontaneous fission of uranium-238 or by reactions (a, n) engendered by uranium and thorium alpha rays. Production in the Earth’s crust appears to be infinitesimal compared with production in the atmosphere.
Many radionuclides, including tritium, were released into the atmosphere during the atmospheric testing of nuclear weapons, which mainly took place between 1945 and 1963. These tests released approximately 650 kg (2.3 x 1020 Bq) of tritium (520 kg – 1.84x1020 Bq into the Northern hemisphere and 130 kg – 4.6x1019 Bq into the Southern hemisphere). Given the radioactive half-life of tritium, in 2007 there remained around 40 kg – 1.4x1019 Bq distributed between the atmosphere (1%), and the oceans (90%) and continental waters (10%). This quantity can be compared against the permanent inventory of natural tritium, which is 3.5 kg (1.2x1018 Bq; UNSCEAR, 2000; Guétat et al., 2008).
Nuclear facilities that release tritium into the environment include pressurized water reactors, irradiated fuel reprocessing and recycling plants and reactors dedicated to tritium production.
In reactions in heavy water nuclear reactors, the amount of tritium produced by means of neutron activation of the deuterium in heavy water (approximately 1.9 g – 6.8x1014 Bq per year for a 900 MWe reactor) is considerably larger than that produced in light water reactors, in which the tritium produced by the ternary fission of certain uranium and plutonium isotopes mainly remains in the actual fuel. In a pressurized water reactor, the tritium released is produced by neutron capture by 10B boron: approximately 0.03 g (1.1x1013 Bq per year) for a 900 MWe reactor and approximately 0.09 g (3.2x1013 Bq) for a 1300 MWe reactor.
Tritium in irradiated fuel is mainly retrieved during reprocessing, when the fuel is sheared. It is found in the form of tritiated water in liquid effluent most of which is released into the sea. At the La Hague plant, for example, approximately 30 g (1016 Bq) of tritium is released every year per 1,600 metric tons of nuclear material and at Sellafield (UK) approximately 8 g (2.8x1015 Bq) is released every year (see Lebaron-Jacobs et al., 2009).
Tritium is present in the nuclear waste buried at sea between 1949 and 1982, mostly in the Northeast Atlantic Ocean. The inventory gives approximately 112 g (4x1016 Bq) of tritium.
Tritium is used in certain industries: in military, medical and research applications, and in the production of luminous objects (clocks and watches). Worldwide, the quantity of tritium released is still low, but can result in high concentrations in localised areas. The chemical form of tritium can also cause significant levels of labelling in living species or sediment in localised areas (cf. chapters below).
Tritium concentration given in Bq/L is homogeneous for all the components of the two major systems i.e. the continental environment (atmosphere, surface water, terrestrial and freshwater flora and fauna) and the marine environment.
In the continental environment not influenced by nuclear facilities, concentration levels of tritium (“background”) currently range between 1 and 4 Bq/L of water. The difference between this and cosmogenic background levels (0.1 to 0.6 Bq/L) can be explained by the decreasing influence of fallout from nuclear weapons testing in the atmosphere which, in the case of tritium, generated maximum activity concentrations in 1963 of nearly 470 Bq/L in rainwater (Figure 1).
Figure 1: changes in tritium concentrations in rainwater in the Northern Hemisphere between 1945 and 2008 (values given in red are for natural background levels in 1945 before nuclear testing and for current levels in 2008)
In the marine environment, tritium emitted during nuclear tests has been totally "diluted" in cosmogenic tritium and concentration levels at the surface have remained around 0.1 to 0.2 Bq/L. These concentrations are not as high at the sea bottom.
In the atmosphere, the concentration level of 1 to 2 Bq/L measured in water vapour corresponds to an activity concentration of 0.01 to 0.02 Bq/m of air.
In biological matrices, total tritium concentration is 1.5 to 2.5 Bq/kg wet in the continental environment, with a variable proportion of free and bound forms (Figure 2). In the marine environment, it is generally less than 2 Bq/kg wet. For example, it is less than 0.5 Bq/kg wet in Mediterranean mussels.
Figure 2: orders of magnitude of current tritium concentration levels for some foodstuffs in France, in areas not influenced by nuclear facilities releases
The influence that a nuclear facility has on tritium levels in the environment is always highly localised. Thus, activity levels added to the terrestrial environment due to atmospheric releases from facilities which are below 5x1011 Bq/year (for example, the least powerful NPPs) cannot normally be detected since they are blocked out by current background activity. The influence of tritium releases from such facilities is potentially only visible in the aquatic environment.
In the environment surrounding facilities that release more than 1 to 2x1013 Bq/year of gaseous tritium (mainly implying the CEA in Valduc, the Marcoule site, AREVA-NC La Hague plant, the CEA centres at Bruyères-le-Châtel and Saclay), higher activity levels, ranging from a few tens to a few hundred Bq/L (Valduc and Marcoule), have been observed in zones extending over a few square kilometres to over a hundred square kilometres (Marcoule) in the terrestrial environment. At these sites, levels in surface water and groundwater are also consistent with these values.
In the case of annual atmospheric releases ranging between 5x1011 and 1013 Bq (for example, the most powerful NPPs), activity levels higher than background activity (5 to 10 Bq/L) have occasionally been measured in the environment without it having been possible to conclude, in statistical terms, that contamination had occurred. This situation could eventually change due to a) an increase in tritium releases due to new fuel management methods at NPPs and b) a drop in the contribution originating from former atmospheric weapons testing. The influence of atmospheric releases of tritium from NPPs will be easier to detect once the background level has dropped.
In rivers, and especially in the downstream part of the Rhone river, tritium activity ranges from 1 to 10 Bq/L, and even 20 to 50 Bq/L immediately downstream from the facilities (Antonelli, 2007; Antonelli, 2008). In the Channel, the influence of tritiated releases from electricity-generating nuclear power plants is blocked out by the influence of the releases from the AREVA La Hague plant where releases result in activity concentrations of between 3 and 10 Bq/L on the Cotentin Peninsula, and 1 Bq/L at the Gravelines NPP; the latter value is 5 to 10 times higher than background activity excluding the influence of nuclear facilities (Masson et al., 2005). In the waters here, average activity concentrations of HTO and OBT in different marine species (algae, molluscs, crustacea) are around 10 Bq/L in combustion water. Over 85% of the tritium in these organisms is free tritium (IRSN, 2009).
In general, whether without (1 to 3 Bq/kg wet or Bq/L), or with (5 to over 100 Bq/kg wet or Bq/L) the influence of nuclear facilities, tritium activity levels in the environment are the highest of all artificial radionuclides, after the levels measured for carbon-14 and krypton-85 in the vicinity of the La Hague plant.
Metrology, analytical techniques and detection limits
3H in an environmental sample may be quantified by activity measurement or by counting the 3He atoms produced by the decay of tritium.
Tritium contained in the test portion is extracted in the form of tritiated water measured by liquid scintillation (b- emissions from 3H). This method applies to free tritium and to organically bound tritium. The analysis protocol then depends on the nature and the form of the sample (Cossonet and Gurrarian, 2009; Figure 3).
Figure 3: sample analysis protocol for measuring tritium (LS: liquid scintillation)
Water samples can be measured directly or following distillation (AFNOR, 2000a and 2000b ISO, 2009). Other matrices undergo a transformation process (bubbling for gases, freeze-drying, combustion, distillation).
The analysis results are thus expressed in Bq/L of natural or distilled water, or water from freeze-drying or combustion. In the case of biological matrices, they are often converted into Bq/kg wet weight.
The detection limits of standard or ultra-low background liquid scintillation counters are around 5 and 1 Bq/L respectively for non-enriched water samples (Belot et al., 1996). To lower these limits, tritiated water can be artificially enriched with the tritium it contains, using one of two methods (Mook, 2000). The most widely-used method consists in increasing the water’s conductivity by adding Na2O or NaOH and then decomposing it by electrolysis. Isotopic fractionation of the hydrogen is high, 90% of the 3H in the sample remains in the water. So, if the volume of water is reduced by a factor of 10, it is enriched by a factor of around 9. At the end of the electrolysis process, the sample is distilled to remove the electrolyte before measuring activity. The second method (thermal enrichment) is relatively complicated and few laboratories are in a position to apply it.
Measuring free tritium (HTO) is a delicate operation, and one that can easily go wrong. Because of the rapid and constant exchanges with water vapour in the air, the concentration of free tritium in a sample only takes account of ambient environment activity in the hours preceding sample collection. As a general rule, the sample taken must be carefully confined throughout each stage of the process to avoid, as far as possible, “polluting” the sample by the air which is more or less tritiated than it is (cf. section on Sampling).
Analysing free tritium in biological matrices entails freeze-drying to extract the water in the sample, and measuring this by scintillation to obtain a result expressed in Bq/L of water with the same detection limits applicable as above. Conversion into Bq/kg wet weight is based on the water content of the fresh product, which is extremely variable depending on the matrix (0.9 L/kg for milk, 0.05 L/kg for cereals).
Organically-bound tritium (OBT) is also analysed by liquid scintillation using the water obtained from combustion of this organic matter (dry matter, in practice), which makes it possible to extract the bound tritium by catalytic oxidation of the combustion gases. According to the current state-of-the-art, bound tritium is measured by taking the sum of the exchangeable and non-exchangeable parts without differentiation.
The results, initially expressed in Bq/L of water from combustion, can be converted into Bq/kg wet weight by applying multiplicative factors that are less variable than the water content of wet products (1 kg of dry organic matter produces approximately 0.6 kg of combustion water).
The quality of the bound tritium measurement is largely dependent on the characteristics of the combustion process and the test portion processed, which are related to the combustion equipment, the most widely-used being oxidizers, pyroxidizers and Parr bombs.
Combustion of the sample at 900°C using catalysers produces tritiated water vapour, collected by condensation in the form of water from combustion. The test portion is around 0.2 to 0.5 g of the dehydrated sample.
Given the small quantity of combustion water obtained, this method entails detection limits that can be very high: from a few tens to a few hundred Bq/L of water from combustion. There can be a great deal of uncertainty using this method and measurements can be difficult to reproduce because the test portion may not be representative enough, especially with heterogeneous samples.
To bring down the detection limit, reduce uncertainty and provide a more representative aliquot of the sample, larger combustion chambers (pyroxidizer tube chamber) can be used. It is then possible to obtain detection limits similar to those obtained for free tritium (1.5 to 3 Bq/L) and with less uncertainty.
Working on the same principle as an oxidizer, a pyroxidizer is used to process samples weighing just a few tens of grams. Combustion occurs in two stages, pyrolysis in argon followed by oxidation in oxygen.
The water produced collects in a cold trap and is then distilled to limit "quenching" during liquid scintillation. 3H activity is given in Bq/L of combustion water collected. By measuring the percentage of hydrogen in the dry sample it is possible to obtain the activity per kg of dry matter.
Depending on its nature, 15 to 40 g of dry sample are processed to obtain at least 10 g of water, the optimal weight for measuring by liquid scintillation. Under such conditions and for a count time of 1,000 minutes, the detection limit obtained is around 1 Bq/L of combustion water.
Flash combustion is obtained by applying a very high voltage to the sample in the presence of oxygen. Depending on the matrix, the test portions can weigh several grams.
Tritium in a sample decays, producing 3He atoms which can be counted (Jean-Baptiste et al., 2010). This is the preferred method for accurately analysing the tritium in a large number of samples, provided the fact that it takes several months is acceptable.
- Measuring 3He by mass spectrometry
The degassed sample is stored to promote 3He ingrowth for a period that depends on the desired detection limit. The concentration of this noble gas is then counted by mass spectrometry.
Detection limits of around 0.01 to 0.1 Bq/L can be obtained (0.1 Bq/L for routine analysis of samples weighing a few tens of grams and a storage period of around 3 months).
The technique, which does requires neither freeze-drying nor combustion, is nondestructive to the sample and does not require the use of an enrichment technique.
To measure tritium levels, particular care must be taken with sampling to ensure equilibrium between the tritium and the ambient environment (Belot et al., 1996).
In air, free tritium can be sampled by drawing air though a known volume of water with low tritium content, or through columns of a solid desiccant (e.g., silica gel). As the total volume of air sampled is known, the concentration of free tritium in the air can be deduced. Another method, developed by IRSN, consists in directly sampling the water vapour in the air by condensing into a cold trap. This system, called PREVAIR, can be used to collect in just a few minutes an adequate quantity for measuring, without to dilute the water vapour in the water in the collector and without a desorption stage.
In continental and marine waters, samples are usually collected by hand in perfectly leaktight containers, made of glass if possible. Precipitation must be separated from the ambient water vapour by a film of mineral oil or octane, added in the precipitation collectors.
Samples can be taken from loose soil (e.g. cultivated land), by coring, the sample collector tubes must then be sealed and kept in a freezer until they are analysed. In the case of plants, samples are, where possible, inserted directly into the container in which they will later be processed. If interim storage is required, it is recommended that plant samples be frozen in sealed containers.
Mobility and bioavailibility in terrestrial environments
The most abundant forms of tritium in the atmosphere are, in descending order: tritiated water (HTO), tritiated hydrogen (HT) and tritiated methane (CH3T). In the atmosphere, the latter forms are converted into HTO very slowly, or even hardly at all in the case of HT.
At the soil-atmosphere interface, tritium exchanges occur in both directions, from the air to the soil (deposition) and from the soil to the air (re-emission by evaporation). Figure 4 shows how tritium is transferred in terrestrial environments.
Figure 4: tritium transfer in terrestrial environments at the air-soil-plant interfaces and in animals, including transfer to foodstuffs
Tritium in the air is deposited in the soil by dry deposition or wet deposition (rain). Nonetheless, there is little transfer of tritiated hydrogen and methane via rainwater since they are not very soluble.
On contact with and in the first few centimetres of soil, tritiated hydrogen is rapidly transformed into tritiated water, the conversion rate increases in line with soil temperature and water content, within the limits of 46°C and 25% humidity. This phenomenon is attributed to the presence of microorganisms which can oxidise HT. Tritiated water vapour in the air is deposited between 10 and 100 times more rapidly than HT, by means of a fast process of exchange with the water vapour present in the pores of the surface layers of the soil, between around 2 and 8 cm (Nogushi and Yokayam, 2003; Yokoyama et al, 2004). These exchanges slow down the dryer the soil is and in the presence of vegetation.
Most of the tritium deposited in soil only remains there for a brief period. A large proportion of the tritiated water is thus re-emitted into the atmosphere by evaporation, at an estimated velocity ranging from 5 to 10% per minute to 1% per second depending on conditions. Very little is known about the tritium fraction thus released back into the atmosphere. Part of it is absorbed by plant roots and a small part of it quickly migrates into the underlying soil horizons (Choi et al., 2000; Choi et al, 2007). Little is known about this last fraction too, especially as it is subject to significant variation depending on the hydraulic characteristics of the soil and on climatic conditions.
Little is understood about the behaviour of organically-bound tritium in soil. Isotope discrimination in favour of tritium has been observed in humic and fulvic acids (Wierczinski et al., 2005). It is thought that the processes relative to this tritium fraction are slow (Balesdents and Recous, 1997; Momoshima et al., 1999). In France, organically bound tritium is generally considered to be negligible in cultivated soil with relatively low humus content.
Generally speaking, soil is not an accumulation compartment for tritium.
Tritium in the form of tritiated hydrogen is not absorbed by vegetation (Figure 4). Tritium in the form of tritiated water however is very easily absorbed by plants, through exchanges with tritiated water vapour in the atmosphere in the case of foliar transfer and through tritiated water in the soil in the case of root transfer.
Foliar absorption occurs via the stomas and can therefore be linked to the foliar surface index, which in turn is correlated with photosynthesis intensity (Melintescu and Galeriu, 2005). Tritium activity in leaves is still lower than that in water vapour in the atmosphere, because of dilution phenomena related to water exchanges between the plant and the soil. These exchanges are very fast. Following a point contamination, the tritiated water content of leaves decreases by several orders of magnitude in the space of a few hours (Choi et al., 2002; Keum et al., 2006).
Following transfer via the root system, the residence time in plants is longer (by a few days) and depends on that of the tritium in the soil (Belot et al., 1996).
Part of the tritium absorbed is incorporated into the organic matter of the plant through photosynthesis, with isotope discrimination slightly to the detriment of tritium. Between around 10 and 30% of this organic fraction is considered to be exchangeable (Belot et al., 1996; Pointurier et al., 2004). If there is no change in ambient tritium activity, the tritium in plants is in isotopic equilibrium and the OBT/HTO ratio is 0.9. However, the mechanisms by which tritium is incorporated in plant organic matter can lead to tritium persisting in certain organs for varying lengths of time. Our understanding of these mechanisms remains fragmented.
In animals, tritium incorporation mainly results from ingestion or from absorbing water vapour via inhalation or via the skin (Figure 4). Since it is an inert gas, tritiated hydrogen (HT) is not very soluble in water nor in body fluids. Its rate of assimilation is around 10,000 times lower than tritiated water. Tritium in the form of HTO is very easily absorbed by inhalation: 99% is retained by animals in a few seconds. Inside an animal’s body, tritiated water diffuses quickly and freely and equilibrates with body fluids within minutes. Percutaneous absorption is practically the same as absorption by inhalation. Tritiated water is also absorbed through ingestion of the animal’s drinking water or ingestion of the water in animal feed. Within minutes, tritiated water ingested in one of these ways appears in the bloodstream, as well as in the animal’s various organs, fluids and tissues. Tritium can also be absorbed in the form of food that contains organically-bound tritium. The majority of this (97% according to models) is transformed in the animal into tritiated water, while the remainder is directly incorporated in the organically-bound fraction in the body (see Belot et al., 1996). This proportion may vary significantly depending on the organic molecule to which the tritium is bound in the food and on its specific concentration within that molecule; for the IAEA (2010) for instance, this fraction is 50%.
Tritium is usually eliminated in the form of tritiated water, with a half-life of a few days in the majority of cases and of a few tens or hundreds of days in the case of the remaining few percent. Eliminating this fraction, i.e. the organically-bound fraction, depends on the metabolism of the organic molecules involved (see Belot et al., 1996).
Mobility and bioavailability in fresh water ecosystems
In river systems, tritium is found almost entirely as HTO (99%) and its behaviour follows that of water flow (Figure 5). Since exchanges between river systems and the atmosphere are governed by the natural processes of phase change (evaporation, condensation, etc.) and precipitation, tritium transfer depends on its concentration in air and water, with the condensed phase slightly enriched due to isotope discrimination. Exchanges of tritium between water and sediment are also basically governed by water flow, with tritium having only low attraction to solid particles.
Figure 5: principal mechanisms for dispersion and transfer of free tritium in river systems
Knowledge relating to the behaviour of tritium in aquatic organisms is limited and relatively dated. In addition, there is no reason to distinguish aquatic from terrestrial organisms with regard to tritium transfer by one of the numerous physiological processes that they share.
In the form of HTO, tritium transfers very easily to plants, as well as animals, which also intake it by consuming food. The exchangeable fraction of organic tritium returns to equilibrium with the environment after several hours. Its non-exchangeable fraction behaves differently due to the nature of the marked organic molecules which have slower kinetics that can lead to tritium remaining in the organisms for a fairly long period.
Research on algae provides basic information. Uptake of tritium in the organic material of these plants results from the use of tritiated water during photosynthesis (simplified reaction: CO2 + HTO -> CHTO + O2). The plants are also able to use some tritiated organic compounds whose evolution closely depends on their nature (Blaylock et al., 1986; Strack et al., 1980; Diabaté and Strack, 1993).
Rare data available about tritium transfer in higher plants demonstrates disequilibrium between the organism and water for semi-aquatic plants is likely related to exchange between emerged foliage and atmospheric tritium (Harrison and Korranda, 1971).
Equilibrium is quickly achieved between surrounding tritiated water and water constituting the organisms due to regulation of their water balance by the respiration/osmoregulation process. Associated biological periods are less than a day (Elwood, 1971). As for OBT, biological periods are more on the order of around ten days.
While it can be assimilated directly, OBT is primarily incorporated through digestion. The metabolism of this organic fraction mostly leads to the production of tritiated water according to a kinetics that depends on the nature of the tritiated organic molecules. All physiological processes related to hydromineral metabolism of aquatic organisms may modify the biological period of OBT, especially reproduction. Thus, mussel spawning contributes to eliminating OBT incorporated by the animal (Yankovich et al., 2006).
In sum, the problem of tritium is basically the lack of knowledge about the behaviour of organic forms. Tritium in organic compounds from liquid effluent from nuclear facilities or other industries may include a special bioavailability demonstrating specific assimilation and excretion kinetics by organisms that are difficult to define even theoretically in the absence of additional knowledge about the nature of the compounds and physicochemical speciation after release into the aquatic environment.
Mobility and bioavailability in marine ecosystems
It is now accepted that the transfer of HTO to aquatic organisms quickly leads to equilibrium between HTO in the exposure environment and free tritium in biological tissue; exchange involving organic forms of tritium is still unknown. More particularly for salt and fresh water fish which constitute the compartments for which there are most in situ data, as well as marine invertebrates, concentration factors [OBT]organism/[HTO]water greater than 1 have been found in various situations; it appears that such observations are in rapport with the origin and physicochemical form of incorporated tritium (ingestion of tritiated organic molecules). In these conditions, the observation of concentration factors greater than 1 for the water in the surrounding environment cannot be interpreted as representative of a bioaccumulation phenomenon and the concentration factor of OBT in the animal organism should be determined using the measurement of tritium contained in products consumed by the animals.
To date, no phenomenon of tritium bioaccumulation has been observed in marine organisms on the French Channel coast. This observation leads to the conclusion that discharge from nuclear industry, led by the spent fuel processing plant in La Hague, are overwhelmingly in the form of HTO.
In light of the results from the Bristol Channel (McCubbin et al., 2001), it appears that small quantities of tritium discharged in the form of organic molecules with a high T/H isotope ratio may modify the equilibrium of tritium concentrations between surrounding seawater and the marine animals that live there, particularly for those that live in constant contact with fine sediment loaded with organic matter rich in tritium. Knowledge of the chemical forms of tritium found in aquatic environments is then essential.
Plants (brown algae)
Since transfers between seawater and organic matter are not instantaneous, concentration variations in algae do not immediately match those in saltwater. This is explained by a slower kinetics for tritium exchange between water and the organic matter of algae (in part controlled by photosynthesis). This phenomenon has been observed for other radionuclides, such as 137Cs (Fiévet and Plet, 2003) and 14C (Fiévet et al., 2006). The average ratio [OBT]algae/[HTO]seawater varies from 0.8 to 1.3. Individual disparity may be explained by disequilibrium caused by kinetic differences.
Results obtained in the Channel for crustaceans, molluscs and fish do not differ significantly from those for algae, whether it be in terms of concentration or the [OBT]species/[HTO]seawater ratio: 0.7 – 1.9. The results show no bioaccumulation effect and reflect the chemical form HTO that indicates industrial discharge. As soon as the number of available measurements is sufficient to allow robust use together with dispersion models, calculation of transfer kinetics between seawater and living species will be possible.
Results obtained in Cardiff Bay (Mc Cubbin et al., 2001) from discharge of molecules traceable to the Amersham plant show that for certain organic compounds, the [OBT]fish/[HTO]seawater ratio may exceed 3,000; in this case, the nature of the marked molecules, how they are introduced in the environment and transfer paths involved should be taken into account.
Tritium transfer between water and the atmosphere
Tritium discharged into the sea by nuclear facilities may be transferred to the earth through the evaporation of seawater. Maro et al., (2005) quantified tritium flow in a gaseous form from the sea to the atmosphere between the discharge point in a marine environment of the La Hague spent fuel processing plant and the Seine estuary. The flow of tritium from water to the atmosphere due to discharge in the marine environment of the La Hague processing plant was estimated at 2.7×107 Bq/km2/day for a surface area corresponding the Seine Bay (4,400 km2). The quantity of tritium transferred to the atmosphere in 2002 may be estimated at 3.9x1013 Bq, or 0.3% of the amount of tritium discharge in the sea by the La Hague plant this year.
Mobility and bioavailability in semi-natural ecosystems
This section is based on the international review of the literature as part of the revision of the guide relating to parameter values for predicting radionuclide transfer in temperate continental terrestrial and aquatic environments undertaken at the initiative of the IAEA (IAEA, 2010).
There is no specific information on the mobility and bioavailability of tritium in forest ecosystems.
There is no specific information on the mobility and bioavailability of tritium in arctic ecosystems.
There is no specific information on the mobility and bioavailability of tritium in alpine ecosystems.
The effects of exposure to ionising radiation depend on the quantity of energy absorbed by the target organism, expressed as a dose rate (µGy/h). This dose rate is evaluated by applying dose conversion coefficients (DCCs, µGy/h per Bq/unit of mass or volume) to radionuclide concentrations in exposure environments or in organisms (Bq/unit of mass or volume).
Characteristic DCCs for 3H were determined without considering decay products and without weighting type of radiation (relative biological effectiveness, RBE) using Eden software V2.3 (Beaugelin-Seiller et al., 2006) while taking into account shape, dimension and chemical composition of the organisms and their environments, as well as their geometrical relations. Model species considered were chosen as examples.
Except in the particular case of the fescue (10-4 µGy/h per Bq/kg wet), internal exposure is generally characterised by DCCs of around 10-6 µGy/h per Bq/kg wet.
External exposure is characterised by lower DCCs that vary by organism, within a range of 10-16 and 10-7 µGy/h per Bq/kg.
For more details on how to calculate DCC, see the Environmental Dosimetry Sheet.
Radiotoxicity of radioactive isotope 3H
Tritium is a low energy beta emitter with low penetrating power that causes radiation stress primarily due to internal irradiation if the radionuclide is incorporated. This low energy however leads to a local concentration where it is deposited that may increase the biological consequences of tritium uptake in living matter. Ionising radiation emitted may cause various DNA lesions that appear in the exposed organism as physiological effects (behaviour, reproduction, genetic damage, etc.) (Straume and Carsen, 1993). For tritium these lesions are basically DNA ruptures on the two strands of the molecule, termed double-strand breaks (Moiseenko et al., 2001), and constitutes a source of increased risk of inducing and transmitting genetic mutations between generations.
Effects following exposure of living matter to ionising radiation depend on the energy of the latter and its nature (alpha, beta or gamma), which is taken into account with the weighting factor known as RBE. For non-human species, only six in vivo studies on reproduction in vertebrates (mammals and fish) provide RBE values for tritium for deterministic effects of between 1 and 3.5 (IRSN, 2009). The lack of diversity in species and their stage of life, as well as the deterministic effect criteria examined until now, underscores the lack of knowledge about the subject. During calculation of doses received by non-human species exposed to tritium in the form of HTO, the variability of RBE, supposing that this factor varies between 1 and 3, affects the result by less than an order of magnitude, which is little considering the greater uncertainty found in the whole dosimetry calculation chain (especially transfer factors) and in determining resulting biological and ecological effects.
In parallel with the RBE data as such, knowledge about the mechanisms of tritium’s action is also incomplete.