HT Deposition Research Articles

Impact of ultralow yttrium concentration on formation, morphology and optical properties of DC magnetron co-sputtered yttrium-doped ZnO films

In this article, we reveal the effect of ultralow concentration of Y dopant on the formation, structure peculiarities and optical properties of yttrium-doped ZnO (YZO) films. The YZO films were deposited on glass substrates at 250 °C and at room temperature (HT and RT, correspondingly) with the magnetron co-sputtering method in a direct current (DC) regime. The Y content in the ZnO matrix was varied by DC power applied to the Y target, and did not exceed 0.14 and 0.11 at.% for RT and HT regimes, correspondingly. For the first time it was revealed that the presence of ultralow Y dopant in the co-sputtering process at RT abruptly increases the growth rate of the YZO films by 20%, enlarges the size of crystallites and degree of texture, and cardinally changes the morphology. For the HT regime, the smooth increasing of the deposition rate and the morphological changing of the YZO films with an increase in the Y content were registered. The deposited YZO films demonstrated a high level of optical transparency in the visible and IR region. Ultralow Y doping of ZnO led to decreasing of the bandgap width. The ZnO and YZO films possessed the resistivity typical for insulators even at the highest Y content at both RT and HT deposition regimes.

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.

Realistic and Practical Modeling of Tritium Deposition to Bare Soil

This paper develops a theory of tritium deposition, which is then combined with the experimental results, depicting the temperature and moisture dependence of the HT oxidation rate. The model describes the atmosphere-to-soil HT deposition process in terms of the atmospheric transport in the surface boundary layer, the transport in soil and the oxidation process by microbial activity. This model is favorably validated with the field data from the Canadian tritium release experiment. Based on this fundamental but sophisticated model, a practical model of HT deposition velocity is developed as a simple function of the soil temperature and the soil moisture. This model drastically reduces the uncertainty in the deposition velocity from a few orders of magnitude to a factor of 2.

Studies of HT Deposition to Unsaturated Soil

ABSTRACTThis paper presents further results of studies of tritium oxidation in unsaturated soil by microorganisms. The objective of the study was to develop a laboratory technique to study the kinetics of HT deposition to soil due to its oxidation and the kinetics of HTO retention in local soils, which are used for agriculture and forestry.Kinetics of HT to HTO oxidation and deposition to soil has been studied in laboratory conditions. An experimental cell was developed to prepare a mixture of air, water vapor and tritium gas and to pump the mixture through the soil sample under study. The activity of HTO converted in the soil sample during a certain period of time was used to determine the oxidation rate. This rate varies, depending on the catalytic and/or biological activity of the soil material. Theoretical considerations have shown that the deposition rate can be expressed by the effective rate of oxidation, which formally corresponds to the first-order HT oxidation.The rate of HT to HTO conversion and deposition to soil is required for assessment of consequences of HT release into the atmosphere.

Studies of HTO transport and oxidation in unsaturated soil

A paper presents results of studies of HTO transport in unsaturated soil in the vicinity of long-term emission source and tritium oxidation by soil microorganisms. The objective of the first study was to determine the vertical profile of tritium from surface to 10m-depth using boreholes. The boreholes were sampled every 1m for measurements of tritium in the soil water. The objective of the second study was to determine tritium oxidation activity of soil in laboratory environment depending on temperature, tritium activity, humidity, etc. A special apparatus in which a mixture of air, water vapour and tritium gas is pumped through the soil material under study was used to determine soil oxidation activity. The amount of HT converted into HTO in the soil sample during a certain period of time was used to determine the tritium reaction rate. This rate varies, depending on the catalytic and/or biological activity of the material. The knowledge of the reaction rate can help to estimate HT deposition rate of HT onto the surface of an undisturbed soil sample. A theoretical treatment based on simplifying assumptions has shown that the deposition rate can be expressed by the reaction rate and the effective diffusion coefficient of HT in the porous material. The studies are performed in frame of ISTC project 654.

A new method for determining HT deposition velocity and dependence of the velocity on environmental factors.

A new method was developed to determine HT deposition velocity and to investigate how environmental factors affect the deposition processes including the diffusion into soil and the biochemical conversion by microorganisms to HTO in soil. This method, using a chamber and two tritium samplers, enables determination of the HT deposition velocity in situ without a large scale experiment and an additional tritium source. The main problem concerning the use of a flow-through method is the possible effect of flow rate on flux measurement. Since the tritium sampler used in this study has many collecting columns, the flow rate is limited. Experimental results showed no significant effect of flow rate over a low range from 0.48 to 0.95 l.min-1. From the increase of HT deposition velocity with increase of water content, it was indicated that biochemical oxidation rather than physical diffusion dominantly affects the HT deposition velocity under examined site conditions. No clear relationship between the HT deposition velocity and atmospheric temperature, relative humidity or soil temperature was observed. The soil temperature was only of secondary importance as a factor controlling the HT deposition velocity.

Investigations and Modeling of Tritium Reemission from Soil

Although the process of tritium reemission from soils after HT or HTO deposition is principally understood, the prediction of initial values and time courses of reemission rates on the basis of readily available data is still insufficient, especially when high time resolutions are required. Theoretical and experimental investigations discussed here show that for a yet limited number of environmental conditions good model performance is reached. In the case of evaporation conditions the initial reemission rate after HT deposition can be coupled to the evaporation rate, when the tritium profile in soil is of exponential shape, characterized by a mean scaling length. After HTO deposition the ideal profile can be described by an error function with a scaling length about 1/3 of that of the HT case. Hence, the initial reemission rate is 3 times higher. Time courses can be modeled by a diffusion approach applying the same diffusion coefficient as for the deposition process.

Oxidation and Dispersion of Ht in the Environment

The short-range environmental dispersion and oxidation of a release of tritiated hydrogen (HT) to the atmosphere has been studied in a field experiment. Emphasis was placed on the processes leading to the appearance of tritiated water (HTO) vapor in the atmosphere because HTO is much more radiotoxic than HT. The following conclusions were reached: No evidence was found for the rapid conversion of HT to HTO in the atmosphere; HTO observed in air, during and after the release, arose mainly from HT oxidation in the soil followed by emission of HTO; HT deposition velocities to soil ranged from 0.041 cm s-1 to 0.13 cm s-1, consistent with previous chamber measurements; the rate of HTO loss from soil, averaged over 21 d, was less than 1% h-1; and HTO concentrations in vegetation water initially increased with time after the release, then by 48 h decreased exponentially at a rate similar to soils.

HTO Reemission from Soil after HT Deposition and Dose Consequences of HT Releases

With respect to inhalation doses the reemission of HTO from the area source established in soil after HT releases is the most important process. In two field experiments performed in France and Canada reemission was studied by successive evaluations of tritium contents in soil and corresponding air samples. It was found that the reemission rate is essentially dependent upon the specific activities in air and soil. Evaporation conditions support reemission, while dew formation and rainfall as well as HTO diffusion into deeper soil layers have the opposite effect. Dose consequences of a HT release were evaluated from time-integrated tritium concentrations in air and breathing rates as well as by analyses of urine samples of the experimenter.

HT Dispersion and Deposition in Soil after Experimental Releases of Tritiated Hydrogen

During two experimental field releases of tritiated hydrogen, performed in France and Canada, a series of measurements was carried out to trace the pathways of tritium in the environment. Informati...

Modelling Elemental Tritium Deposition, Conversion and Reemission Using Ontario Hydro’s Tritium Dispersion Code

A mathematical model and associated computer code is being developed to assess the potential radiological impact of atmospheric releases of elemental tritium, HT, and tritium oxide, HTO, from nuclear facilities. Of the two forms of tritium under consideration, the behaviour of HT in the environment is more complex. This paper describes the theoretical basis of the Ontario Hydro Tritium Dispersion Code (OHTDC) for modelling HT deposition to soil, conversion of HT to HTO and reemission of HTO from soil. The OHTDC predictions were compared to field measured HT and HTO concentrations in air during the HT release experiments at Chalk River Nuclear Laboratories, Canada in August 1986 and June 1987, and good agreement was found. These results validate the code for radiological safety assessments of HT and HTO emissions from fusion facilities such as the International Thermonuclear Experimental Reactor (ITER) and other nuclear sites that manage tritium.

Tritium Release Experiment in France Results Concerning HT/HTO Conversion in the Air and Soil

A controlled release of 256 TBq of pure dry tritium through a 40-m high stack was performed in France, on October 15, 1986. Air measurements were assigned to the Health Physics Division (SPR) from the Centre of Bruyeres-le-Chatel and soil measurements in particular for deposition velocity and reemission rate, to the Division for Studies and Research on the Environment (SERE) of the Institute of Protection and Nuclear Safety (IPSN). The main conclusions were: concordance between code predictions and air measurements; HTO increase in the air originating from soil effects; HT deposition velocity: 1 ..-->.. 5 E-4 ms/sup -1/, reemission rate: 5 % per hour.

Development of a Tritium Dispersion Code

This paper describes the development and verification of a computer code designed to calculate the radiation dose to man following acute or chronic atmospheric releases of tritium gas and oxide from a point source. The Ontario Hydro Tritium Dispersion Code calculates tritium concentrations in air, soil, and vegetation and doses to man resulting from inhalation/immersion and ingestion of food, milk meat and water. The deposition of HT to soil, conversion of HT to HTO by soil enzymes and resuspension of HTO to air have been incorporated into the terrestrial compartment model and are unique features of the code. Sensitivity analysis has identified the HT deposition velocity and the equivalent water depth of the vegetation compartment as two parameters which have a strong influence on dose calculations. Tritium concentrations in vegetation and soil calculated by the code were in reasonable agreement with experimental results. The radiological significance of including the mechanisms of HT to HTO conversion and resuspension of HTO to air is illustrated.