Quantification of exchangeable and non-exchangeable organically bound tritium (OBT) in vegetation

Evaluating production rates of particulate organic hydrogen by marine phytoplankton for estimating phytoplanktonic productivity of organically bound tritium

To estimate the production potential of organically bound tritium (OBT) by phytoplankton from tritiated water in coastal areas adjacent to the Fukushima Daiichi Nuclear Power Plant (FDNPP), phytoplanktonic production rates of particulate organic hydrogen (POH) were evaluated in laboratory and field experiments using stable isotope tracers (2H and 13C). In the laboratory experiment, the production rate of POH was evaluated for five types of phytoplankton cultivated cultures (two diatoms, Haptophyceae, Chlorophyceae, and Cryptophyceae) at two temperatures (15 °C and 25 °C) and two 2H concentrations in the medium (1 and 5%). Additionally, the production rate of POH was especially focused on non-exchangeable POH (NE-POH) which is the chemical form of hydrogen connected tightly to organic matter. The production rates of NE-POH in the laboratory experiment varied (0.10 to 36 μmol L-1 d-1 μg-Chl a-1) with the productivity of particulate organic carbon, phytoplankton species, and temperature, with negligible influence of 2H concentrations. In the field experiment, in situ incubation of coastal seawater at water depths of 1 and 20 m with isotope tracers under light and dark conditions, respectively, was performed thrice (November 2021, May 2022, and October 2022) on the Pacific coastal ocean approximately 2 km from the land of northeast Japan. We observed variation in the production rate of POH (0.21 to 3.1 μmol L-1 d-1 μg-Chl a-1), which was theoretically explained by the data in the laboratory experiment. Using the phytoplanktonic production rate of POH obtained in this study, OBT production by phytoplankton and the subsequent accumulation potential of OBT in sediments in the coastal area adjacent to FDNPP were tentatively estimated, results of which suggested this potential to be small.

Assessing the ecological risk of tritium and Carbon-14 discharge on cyanobacteria through metabolic profiling

The ecological risk posed by tritium (T) and carbon-14 (C-14) discharge from nuclear accidents has gained attention. This study evaluated the toxic impact of T and C-14 (at a concentration of 37 kBq/L for 15 days) on the cyanobacteria (Synechococcus elongatus). The results showed that the assimilation efficiency of cyanobacteria was significantly higher for C-14 than T, and the intracellular C-14 activity reached 30.62-40.58 kBq/kg. T and C-14 exposure had no significant effect on cell proliferation but impacted photosynthesis and respiration. T exposure increased the content of Ca, Mg, Na, P, K, and Mn, while C-14 exposure primarily affected trace element absorption in cyanobacteria. 31, 27, and 58 different metabolites (DEMs) were identified under T, C-14, and combined exposure conditions. These DEMs were enriched in the amino acid biosynthesis pathway, and nitrogen assimilation was one of the crucial pathways affected by T and C-14 exposure. The absorption of mineral elements by cyanobacteria was influenced by the variation in metabolites in the ABC transporter pathway caused by T and C-14 exposure. Our findings provide insights into the metabolic response of cyanobacteria to T and C-14 exposure and will help to guide the ecological risk evaluation of nuclear accidents.

Assessment of the tritium distribution in the vegetation cover in the areas of underground nuclear explosions at the Semipalatinsk test site

This paper provides results of assessment of the tritium distribution in the vegetation cover in the areas of underground nuclear explosions at the Semipalatinsk Test Site (STS). The research was conducted at the former "Degelen" test site along the streams that are one of the main channels of tritium migration from underground nuclear explosions epicenters. The dominant plant species Carex supine and Achnatherum splendens that belong to different ecological groups in relation to humidity were selected as representatives of the vegetation cover. The TFWT (tissue free water tritium) and OBT (organically bound tritium) activity concentrations in the vegetation cover were measurement. TFWT activity concentration in the samples of both plant species had high values with an average of up to 30 kBq kg^-1. The OBT activity concentration was 1-2 orders of magnitude lower than the TFWT in all plant samples. The TFWT and OBT activity concentrations in vegetation samples are closely correlated (r = 0.75, p < 0.05). No statistically significant difference was found between the content of tritium in the samples of Carex supine and those of Achnatherum splendens taken at the same locations. OBT/HTO ratios for vegetation samples of both species were close to equilibrium ratio used in environmental transfer models. In some cases, OBT/HTO ratios were significantly lower than one, which indicates that simple environmental transfer models may not accurately predict the behavior of HTO and OBT in different environmental compartments. The average OBT/HTO ratio for soil samples (0.6 ± 0.1) close to the equilibrium value indicates the equilibrium condition at the research area. The obtained [OBT]_plant/[OBT]_soil ratios indicate that soil organic matter accumulates tritium from year to year. However, in some locations with high tritium contamination ratios [OBT]_plant/[OBT]_soil were more than one due to OBT activity in soils is almost the same as OBT activity in plants. It was found that the nature of the spatial distribution of tritium in the vegetation cover in the areas of underground nuclear explosions is complex, and obviously depends on the location of the tunnels in which nuclear tests were conducted, as well as on the peculiarities of the hydrological regime of underground and surface waters, which are the main channels of tritium migration in the research area. Thus, the vegetation cover reflects the spatial distribution of tritium contamination in the sites of underground nuclear explosions and can be used as an indicator of the radiation situation when monitoring radiation-hazardous areas.

Tritium distribution in typical plants around tritium laboratory in south-west of China

A low-temperature desorption method was developed to remove the tissue free water tritium (TFWT) in plants, which dramatically reduces the sample-preparation time from several days to only 2-3 h. Two factors, including spatial distribution and seasonal variation, that influence the tritium distribution and the organic bound tritium (OBT)/TFWT ratios in plants were investigated based on three different kinds of subtropical-basin plants located near the decommissioned nuclear facility in south-west of China. Under the same environmental condition, higher tritium concentration was seen in pine needle (PN) compared with flat bamboo-root (FBT) and wild cotton (WC), while these three plants show similar location- and season-dependent trend of tritium distribution. The location-dependent investigation shows that OBT and TFWT concentration are significantly higher in plants growing in the downwind direction of the tritium laboratory than that in plants growing in the upwind and cross direction. In different seasons, the TFWT in plants reached peak in winter and valley in summer, while the OBT increased continuously with the season changing from spring to winter. The ratios of OBT/TFWT were investigated to evaluate the ability of plants to concentrate tritium into organic matter, which were 0.97-2.72 for PN, 0.89-1.64 for FBR, and 0.80-1.62 for WC.

Experimental investigation of D2 conversion to DHO in soil near the Cernavoda nuclear power plant site in Romania

The current Canadian and Romanian model predictions for tritium dose following an atmospheric tritiated hydrogen gas (HT) release is based on a default Canadian Standards Association (CSA) conversion factor of HT to tritiated water (HTO) of 4.3%. The determination of an empirical site specific value for the conversion factor was essential for the CANDU Cernavoda Nuclear Power Plant (NPP) in Romania to verify if the CSA value is appropriate for use at this site. Given the role of soil characteristics on the conversion of HT to HTO, on-site experiments would provide the best evaluation of the conversion factor. The objective of the study was to define the soil HT to HTO conversion parameters specific to the Cernavoda NPP site. In June 2016, a series of experiments were conducted to meet this objective. First, the in situ deposition velocity of D₂ gas, as a surrogate for HT gas, was obtained using an exposure chamber. Diffusion of D₂ into the soil was then evaluated based on the measurements of DHO concentrations in the exposed soil. As soil microbes play a role in the conversion of HT to HTO, this work included a microbiological characterization of the soil, which targeted total soil bacteria (cultivable and gene-based) and hydrogen oxidizing bacteria (cultivable and gene-based). The fraction of hydrogen oxidizing cultivable soil bacteria represented 14-20% of the total cultivable bacteria population estimated as 2.8-29.2 × 10⁵ cfu/g of soil. The empirically derived HT to HTO conversion factor was lower than the default value (4.3%). It fell between 0.9% and 2.0%. The default value is therefore more conservative than the Cernavoda site-specific derived value obtained from the study.

Tritiated Water Exposure in Zebrafish (Danio rerio): Effects on the Early-Life Stages

Tritium, a radioactive isotope of hydrogen of natural and anthropogenic origin, is ubiquitously present in the environment. Effluents of nuclear centers of production are significant anthropogenic sources. With the upcoming project of thermonuclear fusion, tritium releases in the environment may increase. It is therefore important to characterize the ecological risk linked to tritium. The effects of tritiated water (HTO) were therefore studied in zebrafish larvae exposed for 10 d to different dose rates, 1.1 × 10² , 4.1 × 10² , and 3.8 × 10³  µGy/h for larvae corresponding, respectively, to a water contamination of 10⁴ , 10⁵ , and 10⁶  Bq/mL of HTO. Those dose rates were higher than 10 μGy/h, which is the threshold recommended to start monitoring ecosystems where radiological contaminants are present. Mortality, embryo-larval development, immune toxicity, genotoxicity, neurotoxicity, and alterations of tissues were investigated. The results showed that HTO exposure induced DNA damage and reactive oxygen species production and modulated the expression of genes involved in detoxification processes. Moreover, modifications of the muscular tissues (degradation of myofibrils at 4 d post fertilization and disorganization of mitochondria at later stages) were observed. The results differed with HTO dose rates and with developmental stages. These results will drive future research for the development of new HTO-sensitive biomarkers and will allow us to progress in the characterization of the modes of action of tritium in fish. Environ Toxicol Chem 2020;39:648-658. © 2019 SETAC.

Evidence for tritium persistence as organically bound forms in river sediments since the past nuclear weapon tests

Tritium of artificial origin was initially introduced to the environment from the global atmospheric fallout after nuclear weapons tests. Its level was increased in rainwaters by a factor 1000 during peak emissions in 1963 within the whole northern hemisphere. Here we demonstrate that tritium from global atmospheric fallout stored in sedimentary reservoir for decades as organically bound forms in recalcitrant organic matter while tritium released by nuclear industries in rivers escape from such storages. Additionally, we highlight that organically bound tritium concentrations in riverine sediments culminate several years after peaking emission in the atmosphere due to the transit time of organic matter from soils to river systems. These results were acquired by measuring both free and bound forms of tritium in a 70 year old sedimentary archive cored in the Loire river basin (France). Such tritium storages, assumed to be formed at the global scale, as well as the decadal time lag of tritium contamination levels between atmosphere and river systems have never been demonstrated until now. Our results bring new lights on tritium persistence and dynamics within the environment and demonstrate that sedimentary reservoir constitute both tritium sinks and potential delayed sources of mobile and bioavailable tritium for freshwaters and living organisms decades after atmospheric contamination.

A brief history of origins and contents of Organically Bound Tritium (OBT) and 14C in the sediments of the Rhône watershed

Tritium (³H) and Carbon-14 (¹⁴C) are radionuclides of natural (cosmogenic) origin that have also been introduced into the environment by humans since the middle of the last century. They are therefore not compounds that have only recently been released into the environment and they do not pose a recognized health threat due to their low radiotoxicity. However, they hold an important place among current concerns because they are being discharged into the environment by the nuclear industry in large quantities compared to other radionuclides. Those both radionuclides partly integrate organic matter during metabolic processes (i.e., photosynthesis) leading to organically bound forms that can be found in sediments. Organically bound tritium (OBT) analyses carried out on the sediments of the Rhône and its tributaries indicate a significant and historical tritium labelling of sedimentary particles all along the Rhône river, as well as in several northern tributaries, in particular the Ognon and the Tille rivers (tributaries of the Saone), the Doubs River and the Loue River (a tributary of the Doubs) and the Arve river. The recorded levels (10 to over 20,000 Bq/L) are very likely to be related to the presence of synthetic tritiated particles (technogenic tritium), which were used in the past in watchmaking workshops. Although overall contamination levels decrease from north to south in the Rhône watershed and fade over time, particularly due to the radioactive decay of tritium, this contamination source of technogenic tritium in the Rhône watersheds is currently still not negligible. Carbon-14 analyses show that the Rhône sediments generally display ¹⁴C levels close to the atmospheric reference values (231 Bq·kg^-1 of C in 2015) or even lower in most of cases, and show sporadic and weak labelling near nuclear facilities. The low ¹⁴C levels in the Rhône sediments are most likely related to the solid contributions from tributaries draining areas that are rich in fossil organic matter, and therefore devoid of ¹⁴C. In the Rhône watershed, the presence in solid particles of tritium in a form organically bound to synthetic compounds and of petrogenic (fossil) organic carbon, can potentially alter the apparent assimilation rates to the food chain of these two radionuclides.

Studies of particle size distribution of Non-Exchangeable Organically Bound Tritium activities in the soil around Qinshan Nuclear Power Plant

The NE-OBT (Non-Exchangeable Organically Bound Tritium) in the soil plays a significant role in tritium migration and transformation. In order to further understand the NE-OBT activity in the soil, the particle size, vertical profile and spatial distribution of the NE-OBT activities in the soil were determined around the Qinshan Nuclear Power Plant (NPP) in China. The experimental results indicated that the NE-OBT preferred to concentrate in the soil particle sizes of 53-250 μm within the soil depth of 5 cm-25 cm. The NE-OBT activity showed significantly vertical variations, however, its largest activity did not appear at the surface soil (0-5 cm). Meanwhile, the NE-OBT had a significant spatial distribution, its activity decreased with the increasing distance from the NPP, especially from the HWRs. In this study, the NE-OBT activities have no significant relationship to the organic matter content in the soil. But the vertical profile distribution of the NE-OBT activity has a strong correlation with the NE-OBT/HTO ratio in the soil, which reflect the capability of living organisms converting HTO into NE-OBT. According to these analyses, we supposed that the NE-OBT in the soil may be derived from the microbial transformation of HTO.

Towards speciation of organically bound tritium and deuterium: Quantification of non-exchangeable forms in carbohydrate molecules

An original methodology to quantitatively explore exchangeability of hydrogen isotopes in carbohydrate molecules is proposed. To access the speciation of organically bound hydrogen isotopes, isotopic exchanges were carried out under a soft path regime in the vapor phase at 20 °C with set (D,T/H) vapor pressure ratios. When steady states were reached, the fraction of exchangeable hydrogen of microcrystalline cellulose, alpha-cellulose and wheat grains were obtained and ranged from 13 to 31% (versus a theoretical value of 30%). In cellulose, and more specifically in microcrystalline cellulose, the molecular hydrogen bonds as well as the different conformations of the network seemed to decrease the hydroxyl groups of glucose units available for isotopic exchange. On the contrary, the assumed enzymatic hydrolysis of the constitutive molecules of wheat starch into low-molecular weight carbohydrate molecules enhanced the exchangeable pool. An average value of the activity between non-exchangeable organically bound tritium (NE-OBT) and non-exchangeable organically bound hydrogen was calculated for wheat grains, (TH)_NE  = 0.55 ± 0.03 Bq.g^-1 of hydrogen atoms.

An updated review on tritium in the environment

Various studies indicated more or less recently that organically bound tritium (OBT) formed from gaseous or liquid tritium releases into the environment potentially accumulates in organisms contradicting hypotheses associated to methods used to assess the biological impact of tritium on humans (ASN, 2010). Increasing research works were then performed during the last decade in order to gain knowledge on this radionuclide expected to be increasingly released by nuclear installations in the near future within the environment. This review focusses on publications of the last decade. New unpublished observations revealing the presence of technogenic tritium in a sedimentary archive collected in the upper reaches of the Rhône river and findings from the Northwestern Mediterranean revealing in all likelihood the impact of terrigenous tritium inputs on OBT levels recorded in living organisms are also presented. Identifying and understanding the physicochemical forms of tritium and the processes leading to its persistence in environmental compartments would explain most observations regarding OBT concentrations in organisms and definitively excludes that tritium would "bio accumulate" within living organisms.

Role of soil-to-leaf tritium transfer in controlling leaf tritium dynamics: Comparison of experimental garden and tritium-transfer model results

Environmental transfer models assume that organically-bound tritium (OBT) is formed directly from tissue-free water tritium (TFWT) in environmental compartments. Nevertheless, studies in the literature have shown that measured OBT/HTO ratios in environmental samples are variable and generally higher than expected. The importance of soil-to-leaf HTO transfer pathway in controlling the leaf tritium dynamics is not well understood. A model inter-comparison of two tritium transfer models (CTEM-CLASS-TT and SOLVEG-II) was carried out with measured environmental samples from an experimental garden plot set up next to a tritium-processing facility. The garden plot received one of three different irrigation treatments - no external irrigation, irrigation with low tritium water and irrigation with high tritium water. The contrast between the results obtained with the different irrigation treatments provided insights into the impact of soil-to-leaf HTO transfer on the leaf tritium dynamics. Concentrations of TFWT and OBT in the garden plots that were not irrigated or irrigated with low tritium water were variable, responding to the arrival of the HTO-plume from the tritium-processing facility. In contrast, for the plants irrigated with high tritium water, the TFWT concentration remained elevated during the entire experimental period due to a continuous source of high HTO in the soil. Calculated concentrations of OBT in the leaves showed an initial increase followed by quasi-equilibration with the TFWT concentration. In this quasi-equilibrium state, concentrations of OBT remained elevated and unchanged despite the arrivals of the plume. These results from the model inter-comparison demonstrate that soil-to-leaf HTO transfer significantly affects tritium dynamics in leaves and thereby OBT/HTO ratio in the leaf regardless of the atmospheric HTO concentration, only if there is elevated HTO concentrations in the soil. The results of this work indicate that assessment models should be refined to consider the importance of soil-to-leaf HTO transfer to ensure that dose estimates are accurate and conservative.

Uncertainty of current understanding regarding OBT formation in plants

Radiological impact models are important tools that support nuclear safety. For tritium, a special radionuclide that readily enters the life cycle, the processes involved in its transport into the environment are complex and inadequately understood. For example, tritiated water (HTO) enters plants by leaf and root uptake and is converted to organically bound tritium (OBT) in exchangeable and non-exchangeable forms; however, the observed OBT/HTO ratios in crops exhibit large variability and contradict the current models for routine releases. Non-routine or spike releases of tritium further complicate the prediction of OBT formation. The experimental data for a short and intense atmospheric contamination of wheat are presented together with various models' predictions. The experimental data on wheat demonstrate that the OBT formation is a long process, it is dependent on receptor location and stack dynamics, there are differences between night and day releases, and the HTO dynamics in leaf and ear is a very important contributor to OBT formation.

The VATO project: An original methodology to study the transfer of tritium as HT and HTO in grassland ecosystem

Tritium (³H) is mainly released into the environment by nuclear power plants, military nuclear facilities and nuclear reprocessing plants. The construction of new nuclear facilities in the world as well as the evolution of nuclear fuel management might lead to an increase of ³H discharges from the nuclear industry. The VATO project was set up by IRSN (Institut de Radioprotection et de Sûreté Nucléaire) and EDF (Electricité de France) to reduce the uncertainties in the knowledge about transfers of ³H from an atmospheric source (currently releasing HT and HTO) to a grassland ecosystem. A fully instrumented technical platform with specifically designed materials was set up downwind of the AREVA NC La Hague reprocessing plant (Northwest of the France). This study, started in 2013, was conducted in four main steps to provide an hourly data set of ³H concentrations in the environment, adequate to develop and/or validate transfer models. It consisted first in characterizing the physico-chemical forms of ³H present in the air around the plant. Then, ³H transfer kinetics to grass were quantified regarding contributions from various compartments of the environment. For this purpose, an original experimental procedure was provided to take account for biases due to rehydration of freeze-dried samples for the determination of OBT activity concentrations in biological samples. In a third step, the ³H concentrations measured in the air and in rainwater were reconstructed at hourly intervals. Finally, a data processing technique was used to determine the biological half-lives of OBT in grass.

Tritium dynamics in soils and plants grown under three irrigation regimes at a tritium processing facility in Canada

The dynamics of tritium released from nuclear facilities as tritiated water (HTO) have been studied extensively with results incorporated into regulatory assessment models. These models typically estimate organically bound tritium (OBT) for calculating public dose as OBT itself is rarely measured. Higher than expected OBT/HTO ratios in plants and soils are an emerging issue that is not well understood. To support the improvement of models, an experimental garden was set up in 2012 at a tritium processing facility in Pembroke, Ontario to characterize the circumstances under which high OBT/HTO ratios may arise. Soils and plants were sampled weekly to coincide with detailed air and stack monitoring. The design included a plot of native grass/soil, contrasted with sod and vegetables grown in barrels with commercial topsoil under natural rain and either low or high tritium irrigation water. Air monitoring indicated that the plume was present infrequently at concentrations of up to about 100 Bq/m(3) (the garden was not in a major wind sector). Mean air concentrations during the day on workdays (HTO 10.3 Bq/m(3), HT 5.8 Bq/m(3)) were higher than at other times (0.7-2.6 Bq/m(3)). Mean Tissue Free Water Tritium (TFWT) in plants and soils and OBT/HTO ratios were only very weakly or not at all correlated with releases on a weekly basis. TFWT was equal in soils and plants and in above and below ground parts of vegetables. OBT/HTO ratios in above ground parts of vegetables were above one when the main source of tritium was from high tritium irrigation water (1.5-1.8). Ratios were below one in below ground parts of vegetables when irrigated with high tritium water (0.4-0.6) and above one in vegetables rain-fed or irrigated with low tritium water (1.3-2.8). In contrast, OBT/HTO ratios were very high (9.0-13.5) when the source of tritium was mainly from the atmosphere. TFWT varied considerably through time as a result of SRBT's operations; OBT/HTO ratios showed no clear temporal pattern in above or below ground plant parts. Native soil after ∼20 years of operations at SRBT had high initial OBT that persisted through the growing season; little OBT formed in garden plot soil during experiments. High OBT in native soil appeared to be a signature of higher past releases at SRBT. This phenomenon was confirmed in soils obtained at another processing facility in Canada with a similar history. The insights into variation in OBT/HTO ratios found here are of regulatory interest and should be incorporated in assessment models to aid in the design of relevant environmental monitoring programs for OBT.

An improved combustion apparatus for the determination of organically bound tritium in environmental samples

This paper reports an improved combustion apparatus for the determination of organically bound tritium in environmental samples. The performance of this apparatus including the recovery rate and reproducibility was investigated by combusting lettuce and pork samples. To determine the factors for the different recovery rates of lettuce and pork and investigate whether the samples were completely oxidized, the ashes and exhaust gases produced by the combustion were analyzed. The results indicate that the apparatus showed an excellent performance in the combustion of environmental samples. Thus, the improvements conducted in this study were effective.

Canadian inter-laboratory organically bound tritium (OBT) analysis exercise

Tritium emissions are one of the main concerns with regard to CANDU reactors and Canadian nuclear facilities. After the Fukushima accident, the Canadian Nuclear Regulatory Commission suggested that models used in risk assessment of Canadian nuclear facilities be firmly based on measured data. Procedures for measurement of tritium as HTO (tritiated water) are well established, but there are no standard methods and certified reference materials for measurement of organically bound tritium (OBT) in environmental samples. This paper describes and discusses an inter-laboratory comparison study in which OBT in three different dried environmental samples (fish, Swiss chard and potato) was measured to evaluate OBT analysis methods currently used by CANDU Owners Group (COG) members. The variations in the measured OBT activity concentrations between all laboratories were less than approximately 20%, with a total uncertainty between 11 and 17%. Based on the results using the dried samples, the current OBT analysis methods for combustion, distillation and counting are generally acceptable. However, a complete consensus OBT analysis methodology with respect to freeze-drying, rinsing, combustion, distillation and counting is required. Also, an exercise using low-level tritium samples (less than 100 Bq/L or 20 Bq/kg-fresh) would be useful in the near future to more fully evaluate the current OBT analysis methods.

Levels of tritium in soils and vegetation near Canadian nuclear facilities releasing tritium to the atmosphere: implications for environmental models

Concentrations of organically bound tritium (OBT) and tritiated water (HTO) were measured over two growing seasons in vegetation and soil samples obtained in the vicinity of four nuclear facilities and two background locations in Canada. At the background locations, with few exceptions, OBT concentrations were higher than HTO concentrations: OBT/HTO ratios in vegetation varied between 0.3 and 20 and values in soil varied between 2.7 and 15. In the vicinity of the four nuclear facilities OBT/HTO ratios in vegetation and soils deviated from the expected mean value of 0.7, which is used as a default value in environmental transfer models. Ratios of the OBT activity concentration in plants ([OBT]plant) to the OBT activity concentration in soils ([OBT]soil) appear to be a good indicator of the long-term behaviour of tritium in soil and vegetation. In general, OBT activity concentrations in soils were nearly equal to OBT activity concentrations in plants in the vicinity of the two nuclear power plants. [OBT]plant/[OBT]soil ratios considerably below unity observed at one nuclear processing facility represents historically higher levels of tritium in the environment. The results of our study reflect the dynamic nature of HTO retention and OBT formation in vegetation and soil during the growing season. Our data support the mounting evidence suggesting that some parameters used in environmental transfer models approved for regulatory assessments should be revisited to better account for the behavior of HTO and OBT in the environment and to ensure that modelled estimates (e.g., plant OBT) are appropriately conservative.

Determination of changes to TFWT and OBT concentrations in potatoes and Swiss chard as a result of preparation for human consumption

Ingestion is one of the most important pathways to consider for calculating tritium dose to human beings. The objective of this study is to determine changes to TFWT and OBT concentrations in food as a result of its preparation for consumption. The contribution of OBT to the total tritium dose can be reduced by the oxidation of OBT during food preparation. The results show that preparation for consumption can result in reductions of up to 46% in TFWT concentration and 54% in OBT concentration in potato, and 22% in TFWT concentration and 57% in OBT concentration in Swiss chard.

Apparent enrichment of organically bound tritium in rivers explained by the heritage of our past

The global inventory of naturally produced tritium (3H) is estimated at 2.65 kg, whereas more than 600 kg have been released during atmospheric nuclear tests (NCRP, 1979; UNSCEAR, 2000) constituting the main source of artificial tritium throughout the Anthropocene. The behaviour of this radioactive isotope in the environment has been widely studied since the 1950s, both through laboratory experiments and, more recently, through field observations (e.g., Cline, 1953; Kirchmann et al., 1979; Daillant et al., 2004; McCubbin et al., 2001; Kim et al., 2012). In its "free" forms, [i.e. 3H gas or 3H hydride (HT); methyl 3H gas (CH3T); tritiated H2O or 3H-oxide (HTO); and Tissue Free Water 3H (TFWT)], tritium closely follows the water cycle. However, 3H bound with organic compounds, mainly during the basic stages of photosynthesis or through weak hydrogen links, is less exchangeable with water, which explains its persistence in the carbon cycle as re underlined recently by Baglan et al. (2013), Jean-Batiste and Fourré (2013), Kim et al. (2013a,b). In this paper, we demonstrate that terrestrial biomass pools, historically contaminated by global atmospheric fallout from nuclear testing, have constituted a significant delayed source of organically bound tritium (OBT) for aquatic systems, resulting in an apparent enrichment of OBT as compared to HTO. This finding helps to explain concentration factors (tritium concentration in biota/concentration in water) greater than 1 observed in areas that are not directly affected by industrial radioactive wastes, and thus sheds light on the controversies regarding tritium 'bioaccumulation'. Such apparent enrichment of OBT is expected to be more pronounced in the Northern Hemisphere where fallout was most significant, depending on the nature and biodegradability of terrestrial biomass at the regional scale. We further believe that OBT transfers from the continent to oceans have been sufficient to affect tritium concentrations in coastal marine biota (i.e., near river inputs). Our findings demonstrate that the persistence of terrestrial organic (3)H explains imbalances between organically bound tritium and free (3)H in most river systems in particular those not impacted by releases from nuclear facilities.

Method validation and uncertainty evaluation of organically bound tritium analysis in environmental sample

The analytical method for organically bound tritium (OBT) was developed in our laboratory. The optimized operating conditions and parameters were established for sample drying, special combustion, distillation, and measurement on a liquid scintillation spectrometer (LSC). Selected types of OBT samples such as rice, corn, rapeseed, fresh lettuce and pork were analyzed for method validation of recovery rate reproducibility, the minimum detection concentration, and the uncertainty for typical low level environmental sample was evaluated. The combustion water recovery rate of different dried environmental sample was kept at about 80%, the minimum detection concentration of OBT ranged from 0.61 to 0.89 Bq/kg (dry weight), depending on the hydrogen content. It showed that this method is suitable for OBT analysis of environmental sample with stable recovery rate, and the combustion water yield of a sample with weight about 40 g would provide sufficient quantity for measurement on LSC.

HT to HTO conversion and field experiments near Darlington Nuclear Power Generating Station (DNPGS) site

The Canadian input parameters related to tritiated hydrogen gas (HT) used in tritium dose models are currently based on experiments performed at the Chalk River Laboratories (CRL) site in 1986, 1987 and 1994. There is uncertainty in how well other sites experiencing atmospheric HT releases are represented by these data. In order to address this uncertainty, HT to HTO conversion factors were evaluated at different locations near the Darlington Nuclear Power Generating Station (DNPGS) site using various experimental approaches. These were D2 gas exposure chamber experiments, atmospheric tritium measurements, and HTO and OBT measurements in vegetation and soil. In addition to these field experiments, chamber experiments were conducted using HT gas on field soil samples. The suggested Canadian input parameters for atmospheric tritium releases estimate the total fraction of HT oxidized in air and in soil, at the site, to be up to a maximum of 2.4%. Based on the more limited data obtained near DNPGS in early spring, this fraction would likely be closer to 0.5%. The result suggests that current parameters provide a conservative estimate for the DNPGS site.

Isotopic fractionation of tritium in biological systems

Isotopic fractionation of tritium is a highly relevant issue in radiation protection and requires certain radioecological considerations. Sound evaluation of this factor is indeed necessary to determine whether environmental compartments are enriched/depleted in tritium or if tritium is, on the contrary, isotopically well-distributed in a given system. The ubiquity of tritium and the standard analytical methods used to assay it may induce biases in both the measurement and the signification that is accorded to the so-called fractionation: based on an exhaustive review of the literature, we show how, sometimes large deviations may appear. It is shown that when comparing the non-exchangeable fraction of organically bound tritium (neOBT) to another fraction of tritium (e.g. tritiated water) the preparation of samples and the measurement of neOBT reported frequently led to underestimation of the ratio of tritium to hydrogen (T/H) in the non-exchangeable compartment by a factor of 5% to 50%. In the present study, corrections are proposed for most of the biological matrices studied so far. Nevertheless, the values of isotopic fractionation reported in the literature remain difficult to compare with each other, especially since the physical quantities and units often vary between authors. Some improvements are proposed to better define what should encompass the concepts of exchangeable and non-exchangeable fractions.

OBT/HTO ratio in agricultural produce subject to routine atmospheric releases of tritium

The mean expected value of the OBT/HTO ratio (i.e. generic ratio) is derived in this study on the joint basis of a long-term study conducted at Atomic Energy of Canada Limited (AECL)'s Chalk River Laboratories (CRL), model simulations targeted at filling gaps in a yet incomplete timeline of CRL measurements and a reference dataset comprised of numerous experiments reported in the literature. Cultivar variability and disparity in site-specific settings are covered by the reference dataset. Dynamical variability caused by meteorology has been a specific target of the long-term experimental campaign at CRL, where the former two types of variability were eliminated. The distribution of OBT/HTO ratios observed at CRL appears to be a fairly good match to the distribution of OBT/HTO ratios from the literature. This implies that dynamical variability appears important in both cases. Dynamics of atmospheric HTO at CRL is comprised of a sequence of episodes of atmospheric HTO uptake and re-emission of plant HTO. The OBT/HTO ratio appears sensitive to the proportion of the duration of these two episodes: the lesser the frequency (and duration) of plume arrivals, the higher the expected mean OBT/HTO ratio. With the plume arrival frequency defined by the typical wind rose, one would encounter a mean OBT/HTO ratio close to 2. It is important to note that this number is seen both in the reference dataset, and in the continuous timeline of HTO and OBT reconstructed from CRL observations by dynamical interpolation (modelling). Many datasets (including that of CRL) targeted at the OBT/HTO ratio are biased high compared to the suggested number. This could be explained by scarce measurements of the low OBT/HTO ratios in the short phase of uptake of atmospheric HTO by the plant.

Measurement of tritium in the free water of milk : spotting and quantifying some biases and proposing ways of improvement

As one of the three natural isotopes of hydrogen, tritium is ubiquitous and may potentially be present in any water or organic molecule that constitutes a biological matrix. Milk is one of the most frequently monitored foodstuffs in the vicinity of chronic release of radionuclides, as it is a very common food product and also because it integrates deposition on large areas of grass or crops at a local scale. Different parameters have been studied to assess their impact on the reliability of tritium measurements in the free water of milk. The volume of the sample, the technique used to extract the water and the level of dehydration modulate the results but in different ways: dispersion of results and under- or over-estimation of the tritium activity. The influence of sample storage and preparation has also been investigated. Methodological improvements of tritium measurements in the free water of milk are proposed. An original fractionation effect during distillation of milk is also described.

Current understanding of organically bound tritium (OBT) in the environment

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

Organically Bound Tritium (OBT) in Soil at Different Depths Around Chalk River Laboratories (CRL), Canada

Atomic Energy of Canada Limited (AECL) Chalk River Laboratories (CRL) is a large nuclear research and test establishment with nuclear and non-nuclear facilities located in Chalk River, Ontario. The CRL Environmental Monitoring Program is designed to demonstrate that radiological exposure resulting from releases from the CRL site remain below the public dose limit specified in the regulations (1 mSv/year). This study was conducted to consolidate environmental effects following a continuous atmospheric tritium release observed at CRL.

Soil samples were collected at depths of up to 20 cm using soil probes at the CRL site and surrounding areas. The samples were sectioned at 5 cm intervals, and HTO and OBT concentrations were measured in the samples. Prevailing winds at CRL are from NW and SE, which was suggested to be in close relationship with tritium distribution in environmental samples such as soils and plant leaves. The HTO concentration was the highest in surface soil water and plant leaves at a given sampling point. This result suggests that the concentration of tritium in surface soil water and in plants tissue free water essentially reflects the surrounding atmospheric tritium concentration.

OBT concentrations in soil were measured at the historical HT release site, Plant Road, Mattawa Road and three background sites near CRL. The top layer of soil generally had the highest OBT concentration among collected soil samples. This result suggests that OBT concentrations are different from HTO concentrations at the same site and can be representative of previously released environmental tritium at the sampling point. The relationship between the OBT concentration in soil and the amount of tritium released into the environment will be useful for the evaluation of environmental tritium effects and the fate of tritium in the terrestrial ecosystem. The study points out that HTO shows shorter-term dynamic conditions, whereas OBT shows longer-term steady-state conditions.