Ion cyclotron range of frequency heating of a deuterium-tritium plasma via the second-harmonic tritium cyclotron resonance.
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Absorption and Metabolism of Tritium Oxide and Tritium Gas by Bean Plants.
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Journal Article↗
Overview of tritium: characteristics, sources, and problems.
Tritium has certain characteristics that present unique challenges for dosimetry and health-risk assessment. For example, in the gas form, tritium can diffuse through almost any container, including those made of steel, aluminum, and plastics. In the oxide form, tritium can generally not be detected by commonly used survey instruments. In the environment, tritium can be taken up by all hydrogen-containing molecules, distributing widely on a global scale. Tritium can be incorporated into humans through respiration, ingestion, and diffusion through skin. Its harmful effects are observed only when it is incorporated into the body. Several sources contribute to the inventory of tritium in our environment. These are 1) cosmic ray interaction with atmospheric molecules; 2) nuclear reactions in the earth's crust; 3) nuclear testing in the atmosphere during the 1950s and 1960s; 4) continuous release of tritium from nuclear power plants and tritium production facilities under normal operation; 5) incidental releases from these facilities; and 6) consumer products. An important future source will be nuclear fusion facilities expected to be developed for the purpose of electricity generation. The principal health physics problems associated with tritium are 1) the determination of the parameters for risk estimation with further reduction of their uncertainties (e.g., relative biological effectiveness and dose-rate dependency); 2) risk estimation from complex exposures to tritium in gas form, tritium in oxide form, tritium surface contamination, and other tritium-contaminated forms, with or without other ionizing radiations and/or nonionizing radiations; 3) the dose contributions of elemental tritium in the lung and from its oxidized tritium in the gastrointestinal tract; 4) prevention of tritium (in oxide form) intake and enhancement of tritium (oxide form) excretion from the human body; 5) precise health effects information for low-level tritium exposure; and 6) public acceptance of tritium leakage and waste disposal from reactors and fuel reprocessing plants.
Environmental Pollutants↗
Percutaneous absorption of tritium-gas-contaminated pump oil.
One of the radiological problems encountered in tritium handling facilities is the hazards associated with tritium's ability to label and degrade organic materials. Experiments in which male hairless rats have been contaminated with tritium-gas-contaminated pump oil have demonstrated that tritium deposited on the skin provides an input of organically bound tritium and tritiated water in the body. The accumulation of organically bound tritium at the point of contact in the skin and in various tissues influenced tritium excretion in urine and feces. The retention of tritium in the body showed that tritium was mainly metabolized and assimilated as organically bound tritium. The distribution of tritiated water was rapid and uniform in the whole-body. Analyses of tritium excreted in animal urine and feces showed that a significant level of organically bound tritium was excreted shortly after exposure. The highest concentration of tritium activity was measured in the exposed area of the skin. An increased level of tritium accumulation in the liver and kidneys was seen. Dose calculations showed that the exposed skin had the highest dose, and the skin dose was primarily due to the retention of organically bound tritium at the point of contact. The interpretation of these data has indicated that the retention of short-term organically bound tritium in the skin may be a dominant factor for dosimetry purposes.
Absorption↗
Theoretical organically bound tritium dose estimates.
This paper illustrates a theoretical approach to estimating the dose associated with the ingestion of both organically bound tritium and tissue free water tritium relative to the ingestion of only tissue free water tritium. Organically bound tritium, specifically non-exchangeable OBT, can result in an increased dose relative to that from exchangeable organically bound tritium and tissue free water tritium because of the longer biological half-life of the former resulting in a dose conversion factor that is twice that of the latter. Non-exchangeable organically bound tritium is tritium that is bound to carbon whereas exchangeable organically bound tritium is tritium bound to oxygen, nitrogen, or sulfur. Tissue free water ranges from 85+% in most fruits and vegetables down to approximately 10% in grains. The remaining edible food mass consists, in part, of exchangeable and nonexchangeable hydrogen that is incorporated into carbohydrates, proteins and fat. The potential organically bound tritium content of several common food items was calculated knowing the amount of bound and unbound hydrogen that exists in these foods and by assuming that the hydrogen to tritium ratio is the same for the "free water" and bound hydrogen compartments. The theoretical ratio of dose from ingestion of organically bound tritium and tissue free water tritium to dose from ingestion of only tissue free water tritium was calculated to be on average within 12%, 30%, and 261% of experimentally based values for fruits and vegetables, meats and eggs, and grains, respectively. The difference is attributed to the T:H ratio being a function of the kinetics associated with the assimilation of tritium into the tissues.
Administration, Oral↗
Quantitative film autoradiography for tritium: methodological considerations.
The details of quantitative film autoradiography for tritium using tritium plastic standards were examined with respect to 3 issues: tritium tissue equivalent (TE) calibration; correction of autoabsorption differences for gray and white matter; and the use of carbon-14 standard sources as a method for quantifying tritium tissue images. Both 3H-tissue and 3H-plastic sources produced linear log-log relationships of 3H-concentration (nCi/mg source weight) vs source optical density (OD) over a reproducible OD range (0.080-0.800). The curves for both 3H-tissue and 3H-plastic were parallel; uncalibrated 3H-plastic standards exhibited a 4-fold higher autoabsorption over 3H-tissue sources for OD values in the linear range. Using chloroform extraction of brains from rats treated with either [14C]deoxyglucose ([14C]DG) or [3H]deoxyglucose ([3H]DG), we found neither isotope loss nor redistribution after defatting (30% reduction of tissue dry weight). After chloroform extraction, the OD values from both gray and white matter structures containing carbon-14 were unaltered. Gray matter OD values increased by 28.7 +/- 5.6% (mean +/- S.D.) in structures containing tritium; white matter structures containing tritium exhibited a 115.9 +/- 29.3% increase in OD after chloroform extraction. The increase in OD after chloroform extraction was a fixed percent for any given tritium OD value from unextracted tissue when the value was within the linear range of 0.080-0.800 OD units. The magnitude of the higher white matter autoabsorption for tritium was confirmed using tritium impregnated cow brain pastes of variable gray/white mixtures. Chloroform extraction of tissue from [3H]DG treated rats was therefore a suitable procedure for direct correction of regionally heterogeneous autoabsorption of tritium. Finally, the rates of image generation for tritium and carbon-14 sources were compared. The rate of increase of OD with increasing exposure time was found to be equal for 3H-tissue and 3H-plastic images; sources of carbon-14 in plastic, however, exhibited more accelerated rates of image generation when compared to tritium sources (i.e. 3H- and 14C-images did not covary with exposure time). The effect of non-covariance on tritium TE calibrated standards was the overestimation of OD values for 14C-plastic standards with increasing times of exposure (comparison of 4 week images to 1 week images showed errors of 35-40%). Use of carbon-14 sources to quantify tritium-generated images therefore required recalibration of 14C-plastic for all exposure times of interest.
Absorption↗
Dosimetric implications of atmospheric dispersal of tritium near a heavy-water research reactor facility.
An estimate of the tritium dose to the public in the vicinity of the heavy water research reactor facility at AECL-Chalk River Laboratories, Ontario, Canada, has largely been accomplished from analyses on regularly-collected samples of air, precipitation, drinking water and foodstuffs (pasture, fruit, vegetables and milk) and environmental dose models. To increase the confidence with which public doses are calculated, tritium doses were estimated directly from the ratio of tritiated species in urine samples from members of the general public. Single cumulative 24 h urine samples from a few adults living in the vicinity of the heavy-water research reactor facility at Chalk River Laboratories, Canada were collected and analysed for tritiated water and organically bound tritium. The participants were from Ottawa (200 km east), Deep River (10 km west) and Chalk River Laboratories. Tritiated water concentrations in urine ranged from 6.5 Bq.l-1 for the Ottawa resident to 15.9 Bq.l-1 for the Deep River resident, and were comparable to the ambient levels of tritium-in-precipitation at their locations. The ultra-low levels of organically bound tritium in urine from these same individuals were measured by 3He-ingrowth mass spectrometry and were 0.06 Bq.l-1 (Ottawa) and 0.29 Bq.l-1 (Deep River). For Chalk River Laboratories workers, tritiated water concentrations in urine ranged from 32 Bq.l-1 to 9.2 x 10(4) Bq.l-1, depending on the ambient levels of tritium in their workplace. The organically bound tritium concentrations in urine from the same workers were between 0.08 Bq.l-1 and 350 Bq.l-1. With a model based on the ratio of tritiated water to organically bound tritium in urine, the estimated dose arising from organically bound tritium in the body for the Ottawa and Deep River residents was about 26% and 50%, respectively, of the body water tritium dose. The workers in a reactor building at Chalk River Laboratories had less than 10% dose contribution from organically bound tritium, but had higher overall tritium dose due to frequent intakes of tritiated water vapour in the workplace. The results of this study suggest that most of the tritium dose to workers at Chalk River and general population near Chalk River is the result of tritiated water intakes and not due to dietary intake of organically bound tritium.
Adult↗
Dose contribution from metabolized organically bound tritium after chronic tritiated water intakes in humans.
Our earlier study of acute tritiated water intakes in humans has demonstrated that the dose contribution from metabolized organically bound tritium is less than 10% of the body water dose. To further demonstrate that the dose contribution from the organically bound tritium per unit intake of tritiated water is the same, regardless of whether the intake is acute (all at once) or chronic (spread over time), urine samples from six male radiation workers with chronic tritiated water intakes were collected and analyzed for tritium. These workers have a well-documented dose history and a well-controlled tritium bioassay database, providing assurance that their tritium intakes were in the form of tritiated water. Each month for a full calendar year, urine samples were collected from each exposed worker. The monthly concentration of tritiumin-urine for each exposed worker was no lower than 10(4) Bq L(-1) but no higher than 10(5) Bq L(-1). These urine samples were analyzed for tritiated water and organically bound tritium to determine the ratio of these tritiated species in urine. The average ratio of tritiated water to organically bound tritium in urine for each exposed worker was 330 +/- 129 (range, 297-589). In calculating the dose to these workers, we assumed that, under steady-state conditions, the ratio of the specific activity of tritium (3H activity per gH) in the organic matter and water fractions of urine is representative of the ratio of the specific activity of tritium in the organic matter and water fractions of soft tissue. A mathematical model was developed and used to estimate the dose increase from the metabolized organically bound tritium based on the ratio of tritiated water to organically bound tritium in urine. The resulting average dose from the organically bound tritium was 6.9 +/- 3.1% (range, 4.7-9.9%) of the body water dose for the six male workers, and agrees well with the value obtained from our acute tritiated water intakes study in humans. The observed dose contribution from organically bound tritium, relative to body water dose, is in agreement with current recommendations of assigning 10% of total body water dose for organically bound tritium in soft tissues after tritiated water intakes.
Drinking↗
Dose contribution from metabolized organically bound tritium after acute tritiated water intakes in humans.
Urine samples from eight male radiation workers who had an unplanned acute tritiated water intake were measured for tritium-in-urine up to 300 d post-exposure. During the first month or so post-exposure, these individuals increased their fluid intakes to accelerate the turnover rate of tritium in the body for dose mitigation. Their daily fluid intakes reverted to normal levels in the latter period of the study. A non-linear regressional analysis of the tritium-in-urine data showed that the average biological half-life of tritium in body water, with standard deviation, was 6.3 +/- 1.0 d (range, 5.0-8.1 d) and 8.4 +/- 2.0 d (range, 6.2-12.8 d) during the respective periods of increased fluid intake and the later period of normal fluid intake. A longer term component of tritium excretion was also observed with average biological half-life of 74 +/- 18 d (range, 58-104 d), indicating the incorporation of tritium, and its retention, in the organic fractions of the body. A mathematical model was developed and used to estimate the dose increase from the metabolized organically bound tritium on the basis of the kinetics of tritium-in-urine. The model accounts for a change in the rates of urinary excretion caused by variable fluid intakes. The average dose to the body, for the eight male workers, due to the metabolized organically bound tritium was estimated to be 6.2 +/- 1.3% (range, 3.5% to 8.9%) of the committed effective dose due to tritium in the body water. This value for the dose increase from organically bound tritium is in the range of the current recommendations of the International Commission on Radiological Protection, i.e., organically bound tritium incorporated into the body contributes about 10% of the dose to the body water following tritiated water intakes.
Adult↗
Improving tritium exposure reconstructions using accelerator mass spectrometry.
Direct measurement of tritium atoms by accelerator mass spectrometry (AMS) enables rapid low-activity tritium measurements from milligram-sized samples and permits greater ease of sample collection, faster throughput, and increased spatial and/or temporal resolution. Because existing methodologies for quantifying tritium have some significant limitations, the development of tritium AMS has allowed improvements in reconstructing tritium exposure concentrations from environmental measurements and provides an important additional tool in assessing the temporal and spatial distribution of chronic exposure. Tritium exposure reconstructions using AMS were previously demonstrated for a tree growing on known levels of tritiated water and for trees exposed to atmospheric releases of tritiated water vapor. In these analyses, tritium levels were measured from milligram-sized samples with sample preparation times of a few days. Hundreds of samples were analyzed within a few months of sample collection and resulted in the reconstruction of spatial and temporal exposure from tritium releases. Although the current quantification limit of tritium AMS is not adequate to determine natural environmental variations in tritium concentrations, it is expected to be sufficient for studies assessing possible health effects from chronic environmental tritium exposure.
Environmental Exposure↗
Tritium isotope effects in adenosylcobalamin-dependent glutamate mutase: implications for the mechanism.
The transfer of tritium between adenosylcobalamin and substrate in the reaction catalyzed by glutamate mutase was examined to investigate the possibility of a protein-based radical intermediate. There was no evidence that tritium was transferred to the protein during the reaction, as tritium neither became stably bound to the protein nor exchanged with water. The kinetics of tritium transfer from adenosylcobalamin to 3-methylaspartate was investigated. Both the transfer of tritium to product and the exchange of enzyme-bound and free coenzyme contribute to the kinetics of tritium loss from adenosylcobalamin. By varying the experimental conditions, the rates of both coenzyme exchange and tritium transfer could be measured. Exchange of adenosylcobalamin with enzyme is very slow, k off = 0.01 s-1, which may reflect a conformational change in the coenzyme and/or protein involved in forming active holo enzyme. The rate constants for the loss of tritium from adenosylcobalamin and the appearance of tritium in 3-methylaspartate are much faster and very similar, k = 0.67 +/- 0.05 s-1 and k = 0.50 +/- 0.05 s-1, respectively, consistent with the transfer of tritium occurring directly between coenzyme and substrate. The isotope effect, calculated from the rate constants for tritium transfer, and kcat, determined for the overall reaction under the same conditions, are between 13.5 and 18. These values are typical of primary isotope effects seen for enzymes in which hydrogen transfer is substantially rate limiting. A protein radical, therefore, appears unlikely to feature in the mechanism of this enzyme.
Adenosine Triphosphatases↗
Tritium radiobiology and relative biological effectiveness.
During the past decade, a large number of radiobiological studies have become available for tritium--many of them focusing on the relative biological effectiveness of tritium beta rays. These and previous studies indicate that tritium in body water produces the same spectrum of radiogenic effects (e.g., cancer, genetic effects, developmental abnormalities, and reproductive effects) observed following whole-body exposure to penetrating radiations such as gamma rays and x rays. However, tritium beta rays are of greater biological effectiveness than gamma rays and x rays. For example, tritium in the oxide form is about 2 to 3 times more effective at low doses or low dose rates than gamma rays from 137Cs or 60Co. When tritium is bound to organic molecules, relative biological effectiveness values may be somewhat larger than those for tritium in oxide form. Tritium administered to animals or to cells in vitro as tritiated amino acids results in relative biological effectiveness values that appear similar to those obtained for tritium in oxide form; however, if administered as tritiated thymidine, the relative biological effectiveness values appear to be about two-fold higher. It is clear from the wealth of tritium data now available that relative biological effectiveness values for tritium beta rays are higher than the quality factor of unity generally used in radiation protection.
Humans↗
A metabolic derivation of tritium transfer coefficients in animal products.
Tritium is a potentially important environmental contaminant originating from the nuclear industry, and its behaviour in the environment is controlled by that of hydrogen. Animal food products represent a potentially important source of tritium in the human diet and a number of transfer coefficient values for tritium transfer to a limited number of animal products are available. In this paper we present an approach for the derivation of tritium transfer coefficients which is based on the metabolism of hydrogen in animals. The derived transfer coefficients separately account for transfer to and from free (i.e. water) and organically bound tritium. A novel aspect of the approach is that tritium transfer can be predicted for any animal product for which the required metabolic input parameters are available. The predicted transfer coefficients are compared to available independent data. Agreement is good (R2=0.97) with the exception of the transfer coefficient for transfer from tritiated water to organically bound tritium in ruminants. This may be attributable to the particular characteristics of ruminant digestion. We show that tritium transfer coefficients will vary in response to the metabolic status of an animal (e.g. stage of lactation, diet digestibility etc.) and that the use of a single transfer coefficient from diet to animal product is inappropriate. It is possible to derive concentration ratio values from the estimated transfer coefficients which relate the concentration of tritiated water and organically bound tritium in an animal product to their respective concentrations in the animals diet. These concentration ratios are shown to be less subject to metabolic variation and may be more useful radioecological parameters than transfer coefficients. For tritiated water the concentration ratio shows little variation between animal products ranging from 0.59 to 0.82. In the case of organically bound tritium the concentration ratios vary between animal products from 0.15 (goat milk) to 0.67 (eggs).
Animals↗
Use of tritium accelerator mass spectrometry for tree ring analysis.
Public concerns over the health effects associated with low-level and long-term exposure to tritium released from industrial point sources have generated the demand for better methods to evaluate historical tritium exposure levels for these communities. The cellulose of trees accurately reflects the tritium concentration in the source water and may contain the only historical record of tritium exposure. The tritium activity in the annual rings of a tree was measured using accelerator mass spectrometry to reconstruct historical annual averages of tritium exposure. Milligram-sized samples of the annual tree rings from a Tamarix located at the Nevada Test Site are used for validation of this methodology. The salt cedar was chosen since it had a single source of tritiated water that was well-characterized as it varied over time. The decay-corrected tritium activity of the water in which the salt cedar grew closely agrees with the organically bound tritium activity in its annual rings. This demonstrates that the milligram-sized samples used in tritium accelerator mass spectrometry are suited for reconstructing anthropogenic tritium levels in the environment.
Cellulose↗
Tritium sampling and measurement.
Current methods for sampling and measuring tritium are described. Although the basic techniques have not changed significantly over the last 10 y, there have been several notable improvements in tritium measurement instrumentation. The design and quality of commercial ion-chamber-based and gas-flow-proportional-counter-based tritium monitors for tritium-in-air have improved, an indirect result of fusion-related research in the 1980s. For tritium-in-water analysis, commercial low-level liquid scintillation spectrometers capable of detecting tritium-in-water concentrations as low as 0.65 Bq L-1 for counting times of 500 min are available. The most sensitive method for tritium-in-water analysis is still 3He mass spectrometry. Concentrations as low as 0.35 mBq L-1 can be detected with current equipment. Passive tritium-oxide-in-air samplers are now being used for workplace monitoring and even in some environmental sampling applications. The reliability, convenience, and low cost of passive tritium-oxide-in-air samplers make them attractive options for many monitoring applications. Airflow proportional counters currently under development look promising for measuring tritium-in-air in the presence of high gamma and/or noble gas backgrounds. However, these detectors are currently limited by their poor performance in humidities over 30%.
Environmental Pollutants↗
Metabolism and dosimetry of tritium.
This document was prepared as a review of the current knowledge of tritium metabolism and dosimetry. The physical, chemical, and metabolic characteristics of various forms of tritium are presented as they pertain to performing dose assessments for occupational workers and for the general public. For occupational workers, the forms of tritium discussed include tritiated water, elemental tritium gas, skin absorption from elemental tritium gas-contaminated surfaces, organically bound tritium in pump oils, solvents and other organic compounds, metal tritides, and radioluminous paints. For the general public, age-dependent tritium metabolism is reviewed, as well as tritiated water, elemental tritium gas, organically bound tritium, organically bound tritium in food-stuffs, and tritiated methane.
Environmental Exposure↗
Organically bound tritium.
Tritium released into the environment may be incorporated into organic matter. Organically bound tritium in that case will show retention times in organisms that are considerably longer than those of tritiated water which has significant consequences on dose estimates. This article reviews the most important processes of organically bound tritium production and transport through food networks. Metabolic reactions in plant and animal organisms with tritiated water as a reaction partner are of great importance in this respect. The most important production process, in quantitative terms, is photosynthesis in green plants. The translocation of organically bound tritium from the leaves to edible parts of crop plants should be considered in models of organically bound tritium behavior. Organically bound tritium enters the human body on several pathways, either from the primary producers (vegetable food) or at a higher tropic level (animal food). Animal experiments have shown that the dose due to ingestion of organically bound tritium can be up to twice as high as a comparable intake of tritiated water in gaseous or liquid form. In the environment, organically bound tritium in plants and animals is often found to have higher specific tritium concentrations than tissue water. This is not due to some tritium enrichment effects but to the fact that no equilibrium conditions are reached under natural conditions.
Animals↗
Tritium intake by exposure to plastic case watches.
In recent years tritium has been used in plastic case watches as permanent light sources on watch dials. To measure the release of tritium through the plastic cases, 82 different waterproofed watches were immersed in a water bath for 24 h, and the tritium concentration of the water was measured. The mean tritium release rate was 24,400 Bq d(-1) ranging from 110-162,000 Bq d(-1). Parallel measurements were made to determine the tritium concentration in the urine of 108 wearers of plastic case watches. The mean tritium concentration in urine was 197 Bq L(-1) up to 1,133 Bq L(-1). The whole body dose resulting from exposure to plastic case is negligibly small but given the pathway of skin absorption, the annual skin dose is 3-4 times higher than the dose limit for the public. Plastic case watches are collector's items and are often kept with other watches in glass cabinets or other containers. Storage of a large number of such watches in one container causes tritium to diffuse through the plastic cases and to contaminate watches that did not contain tritium at first. If the container is more or less airtight, the tritium concentration in the container and the tritium release rate from the watches can reach levels up to 4 MBq d(-1).
Humans↗