Tritium Dynamics Research Articles

Irradiation experiments of titanium beryllide samples in the WWR-K reactor

This paper presents the results primary characterization of unirradiated titanium beryllide samples and neutron analysis of irradiation experiments of titanium beryllide Be12Ti samples in the WWR-K reactor. The XRD analysis of the samples was carried out. The surface morphology of the samples was studied using optical and electron microscopy. Two options of the irradiation capsule design are considered. The first design is designed for low-temperature irradiation, and the second design is for high-temperature irradiation. The effect of the capsule design on the neutron irradiation conditions of titanium beryllide samples is shown. The dynamics of tritium, helium-4 and helium-3 generation in samples is given. The values of the specific energy release of samples and structural materials of the capsule are calculated. The rate of displacements per atom in the irradiated material is estimated.

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.

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/m3 (the garden was not in a major wind sector). Mean air concentrations during the day on workdays (HTO 10.3 Bq/m3, HT 5.8 Bq/m3) were higher than at other times (0.7–2.6 Bq/m3). 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.

Preparatory Steps for a Robust Dynamic Model for Organically Bound Tritium Dynamics in Agricultural Crops

Some nuclear facilities have large tritium loads and subsequent environmental emissions and consequently, the radiological impact assessment is mandatory. In case of the unplanned (potential) accidents there are extremely large uncertainties of the model predictions for crop contamination and dose due to many factors affecting the Organically Bound Tritium (OBT) dynamics in crops. A full mechanistic model is difficult to develop due to the complexity of the processes involved in tritium transfer and the environmental conditions. The preparatory steps for a robust model are discussed in the present study, considering the role of the dry matter and photosynthesis contributing to the OBT dynamics in crops.

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.

Importance of root HTO uptake in controlling land-surface tritium dynamics after an-acute HT deposition: a numerical experiment

To investigate the role of belowground root uptake of tritiated water (HTO) in controlling land-surface tritium (T) dynamics, a sophisticated numerical model predicting tritium behavior in an atmosphere-vegetation-soil system was developed, and numerical experiments were conducted using the model. The developed model covered physical tritiated hydrogen (HT) transport in a multilayered atmosphere and soil, as well as microbial oxidation of HT to HTO in the soil, and it was incorporated into a well-established HTO-transfer organically bound tritium (OBT)-formation model. The model performance was tested through the simulation of an existing HT-release experiment. Numerical experiments involving a hypothetical acute HT exposure to a grassland field with a range of rooting depths showed that the HTO release from the leaves to the atmosphere, driven by the root uptake of the deposited HTO, can exceed the HTO evaporation from the ground surface to the atmosphere when root water absorption preferentially occurs beneath the ground surface. Such enhanced soil-leaf-atmosphere HTO transport, caused by the enhanced root HTO uptake, increased HTO concentrations in both the surface atmosphere and in the cellular water of the leaf. Consequently, leaf OBT assimilation calculated for shallow rooting depths increased by nearly an order of magnitude compared to that for large rooting depths.

Tritium dynamics in large fish – a model test

Tritium can represent a key radionuclide in the aquatic environment, in some cases, contributing significantly to the doses received by aquatic non-human biota and by humans due to aquatic releases. Recently, the necessity to have a robust assessment of tritium routine and accidental risk emissions for large nuclear installations increased the interest in the topic. In the present study, the recent experiments concerning tritium transfer in adult rainbow trout are described. The updated model concerning the dynamics of tritium transfer in aquatic food chain (AQUATRIT model) developed by the authors is applied and tested for these experimental data. The model predicts the experimental data with a factor of 2 to 3 and the potential improvements of the model are discussed. The present model results emphasize that in the field conditions, the major factors influencing the OBT biological loss rate are the temperature and the prey availability while, the OBT uptake is mainly influenced by the fish growth rates. The main goals of this study are to enhance the robustness of aquatic models for tritium risk assessment and to fulfil a gap for aquatic pathways in environment.

A model approach for tritium dynamics in wild mammals

Tritium (3 H) transfer into environment must be modelled differently than the transfer of other radionuclides released from nuclear reactors because hydrogen represents the building blocks of life. A solid understanding of 3 H behaviour is essential because 3 H may be released in large quantities from CANDU (CANada Deuterium Uranium) reactors and from future thermonuclear reactors. Recently, the authors published a complex dynamic metabolic model for 3 H and 14 C transfer in farm and wild animals, but the model applications for wild biota were restricted to too few examples and mostly for 14 C transfer. In this study, the model is applied to few selected wild animals for 3 H uptake. Despite the lack of any experimental data for wild animals, the results presented in this study are less uncertain than for many other radionuclides and can provide a useful estimation for biota radioprotection.

Recent Progresses in Tritium Radioecology and Dosimetry

In this paper, some aspects of recent progress in tritium radioecology and dosimetry are presented, with emphasis on atmospheric releases to terrestrial ecosystems. The processes involved in tritium transfer through the environment are discussed, together with the current status of environmental tritium models. Topics include the deposition and reemission of HT and HTO, models for the assessment of routine and accidental HTO emissions, a new approach to modeling the dynamics of tritium in mammals, the dose consequences of tritium releases and aspects of human dosimetry. The need for additional experimental data is identified, together with the attributes that would be desirable in the next generation of tritium codes.

Tritium Transfer in Pigs - A Model Test

In the frame of IAEA EMRAS (Environmental Modelling for Radiation Safety) programme, there was developed a scenario for models’ testing starting with unpublished data for a sow fed with OBT for 84 days. The scenario includes model predictions for the dynamics of tritium in urine and faeces and HTO and OBT in organs at sacrifice. There have been done two inter-comparison exercises and most of the models succeeded to give predictions better than a factor 3 to 5, excepting faeces. There has been done an analysis of models’ structure, performance and limits in order to be able to build a model of moderate complexity with a reliable predictive power, able to be applied for human dosimetry, also, when OBT data are missing.

The Dynamics of Tritium - Including OBT - in the Aquatic Food Chain

Tritiated water spills by nuclear installations result in uptake in aquatic organisms. The radionuclide uptake model BURN (developed by NRG, modified), considers not only tritium as tritiated water (HTO) but also the conversion into organically bound tritium (OBT). Comparison with the original BURN mode showed that the modified model gave more realistic results in terms of concentration levels, and consequently for dose assessment as result of ingestion of fishery products. For more accurate modelling, seasonal effects and half-life estimates asa function of body weight and water temperature must be taken into account. A first attempt is given, although limited empirical data gives reason to further investigation of this significant effect.

Activities of the EMRAS Tritium/C14 Working Group

A new model evaluation program, Environmental Modeling for Radiation Safety (EMRAS), was initiated by the International Atomic Energy Agency in September 2003. EMRAS includes a working group (WG) on modeling tritium and C-14 transfer through the environment to biota and man. The main objective of this WG is to develop and test models of the uptake, formation and translocation of organically bound tritium (OBT) in food crops, animals and aquatic systems. To the extent possible, the WG is carrying out its work by comparing model predictions with experimental data to identify the modeling approaches and assumptions that lead to the best agreement between predictions and observations. Results for scenarios involving a chronically contaminated aquatic ecosystem and short-term exposure of soybeans are presently being analyzed. In addition, calculations for scenarios involving chronically contaminated terrestrial food chains and hypothetical short-term releases are currently underway, and a pinetree scenario is being developed. The preparation of datasets on tritium dynamics in large animals and fish is being encouraged, since these are the areas of greatest uncertainty in OBT modeling. These activities will be discussed in this paper.

A versatile model for tritium transfer from atmosphere to plant and soil

The need to increase the predictive power of risk assessment for large tritium releases implies a process level aproach for model development. Tritium transfer for atmosphere to plant and the conversion in organically bound tritium depend strongly on plant characteristics, season, and meteorological conditions. In order to cope with this large variability and to avoid also, expensive calibration experiments, we developped a model using knowledge of plant physiology, agrometeorology, soil sciences, hydrology, and climatology. The transfer of tritiated water to plant is modelled with resistance approach including sparce canopy. The canopy resistance is modelled using Jarvis-Calvet approach modified in order to directly use the canopy photosynthesis rate. The crop growth model WOFOST is used for photosynthesis rate both for canopy resistence and formation of organically bound tritium, also. Using this formalism, the tritium transfer parameters are directely linked to known processes and parameters from agricultural sciences. The model predictions for tritium in wheat are closed to a factor two to experimental data without any calibration. The model also is tested for rice and soybean and can be applied for various plants and environmental conditions. For sparce canopy the model uses coupled equations between soil and plants. There are three phases in the dynamics of tritium in SVAT (soil-vegetation- atmosphere transport). The first one treats the period of active deposition when the cloud of tritiated water travels the study area and the atmospheric concentration is the driving source of tritium. The last one starts few days after cloud passage, when the soil water tritium is the driving force. The middle stage takes care of the reemission of HTO from vegetation and surface soil into the atmosphere, a fast process immediately after cloud passage, slowing down later. The active and transition phase are sensitive to actual meteorological parameters (sunshine, humidity, temperature, and rainfall) as were as on plant physiological characteristics and growth stage. In the later stage the processes to be considered are movement of tritiated water in rooth soil, depth distribution of roots, evapotranspiration and plant photosynthesis. This can be modeled with a slow dynamics, using climatic data and approximate dynamics of some plant parameters. After an atmospheric dry deposition episode the tritiaded water concentration in plant is decresing fast, while the OBT concentration in whole plant will decrease very slow but will be translocated to storage plant parts. For crops harvested one time in the year, most of tritium in the harvested plant will be in form of OBT, while for continously harvestd plants as grass and leafy vegetable, in the first few days after the accident, the concetration of HTO is high. An operational model must include both situations under various agrometeorological conditions. The well-known model UFOTRI was built to cope with these requirements and senitivity and uncertainty has been analysed (1) showing reliability for maximum exposed individual (worse case) as required for a facil design and licencing. This study and discussions in the IAEA-EMRAS tritium group

Tritium transport and cycling in the environment.

The transport and cycling of tritium in the environment can be understood in terms of the role of hydrogen in the environment. Physical and chemical isotopic effects, while present in some transport mechanisms and chemical reactions, are not important factors in environmental tritium dynamics. In addition, because of the role of hydrogen in metabolism and its ubiquitousness in the environment, organisms have not evolved mechanisms to accumulate or concentrate hydrogen or its isotopes in food chains. Therefore, biomagnification of tritium is not a factor in food chain transfer. The lack of significant isotopic effects or biomagnification means that tritium transport and cycling in the environment can be predicted on the basis of the transport processes, hydrogen content, and chemical transformation of hydrogen and its compounds in the environment.

Tritium Dynamics in Mice Exposed to Tritiated Water and Diet

The contributions of tritiated water (3HHO) and dietary tritiated amino acids to the steady-state specific activities of tissue water tritium and organically bound tritium in mice were essentially independent and additive. Following exposure (56 d), organically bound tritium clearance was resolved into two distinct compartments. The first, with a half-life of 1-2 d, likely represented exchangeable organically bound tritium, and the second, with a half-life of 20-30 d, likely represented nonexchangeable organically bound tritium. Since organically bound tritium was cleared much more slowly than tissue water tritium, organically bound tritium was the principal determinant in estimated radiation doses to mice following exposure.

文部省科学研究費補助金(昭和58〜60年度)エネルギー特別研究(核融合)研究成果報告会の概要

The Summary Meeting on Nuclear Fusion Research by Grant-in-Aid (1983-1985) of Monbusho has been held on Aug. 25-27, 1986 at Gakushikaikan in Tokyo. More than 300 papers, including the research activities in the following subjects have been presented.I. Reactor Materials and Plasma-Wall Interactions1. Heavy irradiation effects, 2. Plasma-wall interactions, 3. Compatibility of non-metal materials including ceramicsII. Science, Technology and Biological Effects of Tritium1. Science and technology of tritium, 2. Dynamics of tritium in environment, 3. Biological effects of tritiumIII. Fundamentals of Reactor Plasma Control1. Diagnostics of high-temperature plasma, 2. Fundamentals of implosion engineering for inertial confinement, 3. Fundamentals of plasma heating, 4. Physics of new schemes for plasma confinementIV. Technology of Superconducting Magnet1. Superconducting magnet materials, 2. Fundamental phenomena of superconductivity, 3. Magnet technologyV. Fusion Reactor Blanket Engineering1. Fusion neutronics, 2. Heat transfer, fluid dynamics and structural engineeringVI. Design and Evaluation of Fusion Reactor1. System design, 2. Technological assessment of fusion energy, 3. Fusion theory.

Tritium dynamics in vegetables: Experimental results

Tritium dynamics in vegetables: Experimental results