Non-exchangeable Organically Bound Tritium Research Articles

Tissue-free water tritium and non-exchangeable organically bound tritium concentrations in fish near coastline during and after operation of Japan's first nuclear fuel reprocessing facility

The risks from radioactive wastewater release from nuclear facilities into the ocean are a global concern. Radioactive contaminants, such as tritium (3H), in both forms of tissue free water tritium (TFWT) and non-exchangeable organically bound tritium (NE-OBT), can be incorporated into marine biota and cause radiation doses to biota and future consumers. However, no studies have been conducted to measure both forms of 3H in marine fish as well as evaluate the residence time in the vicinity of a nuclear fuel reprocessing facility. Here, fish from a brackish lake and from the Pacific Ocean coastline of Japan, which are near such a facility, were collected between 2006 and 2021. The reprocessing facility was operational between 2006 and 2009, during which time about 300 times more tritiated water was discharged per year into the ocean compared to the period when the facility was not operational. During operation the annual release was 30 times higher than the treated water released annually from Fukushima Daiichi. As expected, TFWT and NE-OBT concentrations increased in marine fish during operations and had peak values of 3.59 ± 0.03 and 0.56 ± 0.03 Bq/L, respectively. Total dose rates to the fish were 36,000 times lower than the 10 μGy h−1 benchmark. Concentrations gradually decreased to pre-operational levels as the facility was turned off with NE-OBT taking twice as long. Fish sampled from the brackish lake tended to have more incorporated TFWT and NE-OBT concentrations than ocean fish. This indicates that ocean tides might have contributed to the accumulation of discharged tritiated water in the lake via a narrow water channel, which highlights the importance of examining all marine ecosystems in future operations. In both marine environments, the estimated committed effective dose using the highest observed data through ingestion was well below public limits (91,000 times lower).

Assessing the variability of tissue-free water tritium and non-exchangeable organically bound tritium in pine needles in Fukushima using atmospheric titrated water vapor

When conducting environmental tritium monitoring at nuclear and fusion facilities, it is important to understand how tritium concentrations vary within the soil-plant-atmosphere continuum. Past measurements of organically bound tritium (OBT) concentrations have been conducted from the standpoint of ascertaining the persistence of tritium in terrestrial vegetation, and it has been reported that OBT concentrations fluctuate depending on the influence of atmospheric sources and meteorological conditions. The present study provides information on the variability of tritium concentrations in vegetation growing close to atmospheric sources of tritiated water (HTO) in Japan. We conducted measurements of tissue-free water tritium (TFWT) and non-exchangeable OBT (NE-OBT) in pine needles collected from locations near the Tokyo Electric Power Company's Fukushima Daiichi Nuclear Power Plant from 2018 to 2021. The results we obtained indicate that TFWT concentrations fluctuate in response to short-term changes in atmospheric HTO vapor concentrations and that NE-OBT concentrations vary primarily in response to the time-averaged TFWT concentration during the pine needle growing season, which is in turn mainly determined by atmospheric HTO vapor concentrations. Although it is difficult to ascertain the time-averaged TFWT concentration during the growing season by measuring plant materials, our results show that the measurement of atmospheric HTO vapor concentrations provides an effective alternative method of doing this. We believe our findings can lead to better understanding of the potential impact on the terrestrial environment associated with the atmospheric discharge of HTO and can contribute to improving the strategy of environmental tritium monitoring.

NE-OBT and TFWT activity concentrations in wild plants in the vicinity of the PHWR nuclear power plant and control regions of the tropical monsoonal climatic region of the Indian subcontinent

The results of the first detailed study, involving a large number of samples, on water equivalent factor (WEQp), non-exchangeable organically bound tritium (NE-OBT) and tissue free water tritium (TFWT) activity concentrations in predominant plant species of the tropical monsoonal climatic region, are presented. A total of 369 samples from the vicinity of the PHWR nuclear power plant (NPP) at Kaiga, West Coast of India, and 47 samples of the control region (region not affected by local anthropogenic sources) were analysed. The WEQp varied in the range of 0.347–0.666 L kg−1 with an overall mean value of 0.540 ± 0.045 L kg−1. The NE-OBT activity concentration varied in the range of <9.8–60.9 Bq L−1 of combustion water (mean = 24.6 ± 11.5 Bq L−1) and that of TFWT in the range of 9.2–60.5 Bq L−1 (mean = 30.7 ± 10.9 Bq L−1) in the vicinity of the NPP. Rigorous statistical analysis of the data confirmed that (i) the activity concentrations of both forms of tritium decreased with the increase in the distance between the sampling location and NPP, and beyond 10 km, it was similar to that of the control region, (ii) the incorporation of tritium released from the NPP into wild plant leaves is not species-dependent, (iii) the NE-OBT activity concentration in the 5–10 km zone exhibited a dependence on the prevailing wind regime with respect to the NPP, but not in the 2.3–5 km zone which suggests that the transport of tritium, released into the atmosphere as the gaseous effluent, through diffusion is a dominating factor governing its activity concentration in the 2.3–5 km zone. The NE-OBT to TFWT specific activity concentration ratio (R-value) had a mean value of 0.82 ± 0.27 (range: 0.38–1.64) for samples collected from the vicinity of the NPP and 1.93 ± 0.50 (range: 1.35–3.19) for the control region. Recording higher NE-OBT activity concentration and R-value at the control region highlights the necessity of detailed studies to understand the mechanism of NE-OBT partitioning in the terrestrial environment.

Distributions of tritium with different chemical form in the soil around Qinshan Nuclear Power Plant

Tritium with different chemical forms have different effects on the human body. In tritium migration and conversion, soil plays a significant role. In this study, our samples are from the surrounding environment of the Qinshan Nuclear Power Plant, in addition, Qinshan Phase III Nuclear Power Plant is the only heavy water reactor nuclear power plant (CANDU type, 700 MWe) in China. Both tritiated water (HTO) and non-exchangeable organically bound tritium (NE-OBT) were measured at the surface soil (0–2 cm) at 8 sampling sites and their depth profiles in the subsurface 30 cm of soil in two sampling sites around Qinshan Nuclear Power Plant in 2015, 2016 and 2018. The distribution analysis showed the further away from the nuclear power plant, the lower activity concentration of HTO at the surface soil. Also, in the depth profile, the HTO activity concentration decreased as the depth increase. Through soil analysis at different depths of sampling points A and D, it is concluded that the activity concentration of NE-OBT is the largest in the surface soil of the selected sampling points, which is similar to the HTO of most sampling points.

Non-intrusive and reliable speciation of organically bound tritium in environmental matrices

Measurements of tritium in its various forms within the environment and especially in organic matter are keys to improving the current understanding of its environmental behavior and distribution. Validated or standardized analytical procedures for tritium determination methods have now been developed for several forms of tritium in environmental samples, yet an analytical lack remains regarding the quantifications of exchangeable and non-exchangeable forms of organically bound tritium (OBT) fractions. The present work therefore aims to provide a means of developing a standardized method for OBT fraction determination by evaluating the robustness and relevance of two methods (intrusive and non-intrusive methods) developed for non-exchangeable OBT quantification on a broad panel of pertinent environmental matrices. The validity and reliability of a non-intrusive method has thus been confirmed through a robust comparative study. Moreover, its relevance for standardization purposes is discussed, while the fundamental weakness of the conventional and most widespread method is highlighted and directly quantified for the first time in relying on many demonstrated biases.

Determination of non-exchangeable organically bound tritium concentration in reference material of pine needles (NIST 1575a)

Non-Exchangeable Organically Bound Tritium (Nx-OBT) in a biological reference material: NIST 1575a Pine Needles was determined by radiometry and noble gas mass spectrometry. Nx-OBT concentration ranged from 1.08 to 1.45 Bq L−1-combustion water (CW) (n = 4) with mean value (± S.D.) of 1.25 ± 0.15 Bq L−1-CW in radiometry and ranged from 1.12 to 1.35 Bq L−1-CW (n = 3) with mean value of 1.22 ± 0.25 Bq L−1-CW in mass spectrometry. For the Pine Needles reference material, there is no proposed value about Nx-OBT, but our results showed a good agreement with the data determined by different methods. Thus, it is useful material for the quality control of Nx-OBT measurements.

Distribution of non-exchangeable organically bound tritium activities at the surface soil around Qinshan Nuclear Power Plant

The tritium deposited in the soil formed the non-exchangeable organically bound tritium (NE-OBT). Most of the NE-OBT locates at the surface. In this study, higher NE-OBT activity appeared at the sampling sites in the prevailing wind direction. The distribution of NE-OBT activity in soil particle with different particle size implied that the NE-OBT might originate from soil organic matter. According to the correlation analyses, in the soil with particle size less than 53 μm, the NE-OBT might originate from soil organic matter, and in the soil with particle size between 250 and 2000 μm, it originated from HTO.

Preliminary research on method of tree ring’s NE-OBT measurement using AMS

The non-exchangeable organically bound tritium (NE-OBT) concentration in the cellulose of tree rings can accurately reflects the tritiated water concentration in the local environment and often contains the only historical record of tritium exposure. However, there is few study about NE-OBT measurement using AMS around the world, especially in China. Furthermore, in order to measure tritium in tree rings exactly, it is necessary to solve several key problems during sample preparation. In this study, the optimized sample preparations procedure are investigated. The purity of cellulose extracted by improved sodium hypochlorite pretreatment method is about 83.9%, considerably higher than the 64.8% conventional methods. Pure TiH2 samples reaching requirements of AMS measurement have been made. The isotope fractionation is stabilized during sample preparation. Preliminary results of sample analysis using a compact AMS with 200 kV accelerator are presented. Currently, the minimum T/H ratio that can be accurately measured is only about 10−12, limited by hydrogen contents in samples and instrument conditions. Further works will be done in order to achieve lower detection limits and better accuracy.

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.

Occurrence of HTO and NE-OBT in soil in the vicinity of the Qinshan Nuclear Power Plant

In this study, concentrations of tritiated water (HTO) and non-exchangeable organically bound tritium (NE-OBT) and the NE-OBT/HTO ratios were determined in soil around Qinshan Nuclear Power Plant (NPP), Zhejiang, China, and their vertical profile and spatial distribution were investigated. Results showed variations in both the HTO and NE-OBT concentrations in the vertical profile. The HTO concentrations generally decreased firstly and then increased with the increasing depths, but the trend was not significant. The NE-OBT concentrations in the surface soils (0–5 cm) were considerably lower than those in the deep soils (5–25 cm) at all the sites. The NE-OBT/HTO ratios also show a variable vertical profile, initially increasing and then decreasing with increasing soil depth. Both the HTO and NE-OBT concentrations decreased with the increasing distance to the HWRs in deep soil layers (5–25 cm), revealing that the released tritium from the NPP affected the spatial distribution.

Occurrence of organically bound tritium in the Mohelno lake system

This study was focused on the 3H activity levels in the unique “tritium valley” around the Mohelno reservoir, which receives outlet cooling waters from the Dukovany nuclear power plant. Tritium activity levels above the background reference value were found in water from the reservoir and from the effluent part of the Jihlava water, in air moisture and in plant tissues tissue free water tritium (TFWT), and non-exchangeable organically bound tritium (NE-OBT). These zones were discernible that had noticeably different TFWT and NE-OBT values: (1) littoral zones, (2) slopes above the reservoir, (3) plateaus above the reservoir/river.

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.

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

The objective of this study is to quantify the relative amounts of exchangeable organically bound tritium (OBT) and non-exchangeable OBT in various vegetables. A garden plot at Perch Lake, where tritium levels are slightly elevated due to releases of tritium from a nearby nuclear waste management area and Chalk River Laboratories (CRL) operations, was used to cultivate a variety of vegetables. Five different kinds of vegetables (lettuce, cabbage, tomato, radish and beet) were studied. Exchangeable OBT behaves like tritium in tissue free water in living organisms and, based on past measurements, accounts for about 20% of the total tritium in dehydrated organic materials. In this study, the percentage of the exchangeable OBT was determined to range from 20% to 57% and was found to depend on the type of vegetables as well as the sequence of the plants exposure to HTO.

Development and validation of a dynamical atmosphere–vegetation–soil HTO transport and OBT formation model

A numerical model simulating transport of tritiated water (HTO) in atmosphere-soil-vegetation system, and, accumulation of organically bound tritium (OBT) in vegetative leaves was developed. Characteristic of the model is, for calculating tritium transport, it incorporates a dynamical atmosphere-soil-vegetation model (SOLVEG-II) that calculates transport of heat and water, and, exchange of CO 2. The processes included for calculating tissue free water tritium (TFWT) in leaves are HTO exchange between canopy air and leaf cellular water, root uptake of aqueous HTO in soil, photosynthetic assimilation of TFWT into OBT, and, TFWT formation from OBT through respiration. Tritium fluxes at the last two processes are input to a carbohydrate compartment model in leaves that calculates OBT translocation from leaves and allocation in them, by using photosynthesis and respiration rate in leaves. The developed model was then validated through a simulation of an existing experiment of acute exposure of grape plants to atmospheric HTO. Calculated TFWT concentration in leaves increased soon after the start of HTO exposure, reaching to equilibrium with the atmospheric HTO within a few hours, and then rapidly decreased after the end of the exposure. Calculated non-exchangeable OBT amount in leaves linearly increased during the exposure, and after the exposure, rapidly decreased in daytime, and, moderately nighttime. These variations in the calculated TFWT concentrations and OBT amounts, each mainly controlled by HTO exchange between canopy air and leaf cellular water and by carbohydrates translocation from leaves, fairly agreed with the observations within average errors of a factor of two.

A follow up of the decrease of non exchangeable organically bound tritium levels in the surroundings of a nuclear research center

In the past decades limited amounts of tritium were handled on the CEA site of Bruyères le Châtel with authorised atmospheric releases. A small fraction of the tritium released entered into environmental samples under three forms: (i) as part of free water (TFWT – Tissue Free Water Tritium), or associated with organic matter in two ways; either (ii) bound to the oxygen and nitrogen atoms of the material as exchangeable organically bound tritium (E-OBT), or (iii) bound to carbon atoms as non exchangeable organically bound tritium (NE-OBT). The first two components provide only a picture of atmospheric tritium concentrations at the sampling time as they are in equilibrium with atmospheric moisture and soil humidity. Unlike these exchangeable forms, however, NE-OBT is tightly bound to the organic matter and provides an integrated record of atmospheric tritium during the growing phase of the vegetation. We mapped NE-OBT in tree leaf samples in an area of about 25 × 30 km 2 around the centre of the CEA site and compared the results with those obtained during a previous sampling exercise in 1989. At this time, the activity levels were almost ten times higher than those observed presently in a similar area almost 20 years later which is consistent with the decrease of atmospheric releases issued from the centre. As the activity levels are now close to environmental background specific attention was also paid to the analytical procedure to ensure reliable low level NE-OBT detection.

Determination of the Non Exchangeable-Organically Bound Tritium (NE-OBT) Fraction in Tree Leaf Samples Collected Around a Nuclear Research Center

Tritium exists in environmental samples in three forms: (i) Tissue Free Water Tritium (TFWT) and associated with the organic matter (OBT) under two forms; (ii) bound to oxygen and nitrogen atoms into the material (EOBT); (iii) bound to carbon atoms into the material (NEOBT). The developed analytical procedure allows obtaining accurate and reproducible information for the various tritium fraction determinations.Aiming to follow the distribution and the integration of NE-OBT in the vicinity of a nuclear research centre down to environmental level the analytical procedure was optimized to reduce possible contamination during critical stages such as the E-OBT elimination. Therefore, a new process using steam was designed and investigated leading to promising results.A broad study was initiated to study potential impact of tritium on tree leaves sampled in the vicinity of a nuclear research centre within a radius of about 20 km. Moreover both plane tree and oak leaves have been sampled to establish the NE-OBT mapping. Therefore, for several locations they were sampled twofold for comparison. Appropriate statistical tests allow assessing that the type of tree does not influence the NE-OBT integration in our experimental conditions.

Tritium Determination at Trace Level: Which Strategy to Determine Accurately HTO and OBT in Environmental Samples?

Focusing on environmental tritium levels, measurements have been made for several natural water and leaf samples from an area where no tritium industrial discharge was known to occur. Therefore, to obtain sufficiently accurate data tritium was determined from large samples. Moreover, tritium measurement at environmental level requires appropriate methodology to avoid any contamination. Both tissue free water tritium (TFWT) and organically bound tritium (OBT) were determined for biological samples and compared to preliminary or literature data. In this paper, the authors describe both:- a mobile extraction water device allowing therefore to realise the extraction step under the sampling site conditions to get rid of any contamination.- a sensitive method for low level non-exchangeable OBT determination by a combination of a suitable sample treatment, a large capacity combustion apparatus and low background liquid scintillation spectrometry. Then, owing to validate this approach the authors give an application of this methodology to the determination of both fractions determined on tree leaves originating from the lower Rhone valley. The results demonstrate both the suitability of the procedure as tritium concentration in leaves and natural waters exhibit environmental level concentration and also that the OBT background in the studied area is very close to the one measured in the south west of France.

Uptake of Tritium in the Organically Bound Form into the Biomass of the Soil

Laboratory experiments and field observations have shown that elemental tritium HT is completely converted into tritiated water (HTO) by the activity of soil microorganisms. No organically bound tritium (OBT) is formed initially, but some tritium is taken up into soil biomass by general biosynthesis. Only a small fraction of tritium is directly incorporated from HTO into OBT with the non-exchangeable portion becoming the dominant source of tritium in OBT. In practice a small fraction of non-exchangeable OBT must be separated by vacuum distillation from a large amount of soil water containing HTO. The method has been tested by labelling the soil water with HTO and H/sub 2//sup 18/O, respectively. Several steps are necessary to obtain a complete yield. During the incubation experiments a continuous loss of HTO by an exchange between soil water and air water vapour must be taken into account. Uptake of tritium into biomass of about 0.1% per week was observed. The biological synthesis and consequently the uptake of tritium from HTO into OBT can be stimulated by the addition of energy sources such as glucose.