Dosimetric implications of atmospheric dispersal of tritium near a heavy-water research reactor facility

Development of a Novel Passive Monitoring Technique to Showcase the 3D Distribution of Tritiated Water (HTO) Vapor in Indoor Air of a Nuclear Facility

Tritiated water (HTO), a ubiquitous byproduct of the nuclear industry, is a radioactive contaminant of major concern for environmental authorities. Although understanding spatiotemporal heterogeneity of airborne HTO vapor holds great importance for radiological safety as well as diagnosing a reactor's status, comprehensive HTO distribution dynamics inside nuclear facilities has not been studied routinely yet due to a lack of appropriate monitoring techniques. For current systems, it is difficult to simultaneously achieve high representativeness, sensitivity, and spatial resolution. Here, we developed a passive monitoring scheme, including a newly designed passive sampler and a tailored analytical protocol for the first comprehensive 3D distribution characterization of HTO inside a nuclear reactor facility. The technique enables linear sampling in any environment at a one-day resolution and simultaneous preparation of hundreds of samples within 1 day. Validation experiments confirmed the method's good metrological properties and sensitivity to the HTO's spatial dynamics. The air in TU Wien's reactor hall exhibits a range of 3H concentrations from 75-946 mBq m-3 in the entire 3D matrix. The HTO release rate estimated by the mass-balance model (3199 ± 306 Bq h-1) matches the theoretical calculation (2947 ± 254 Bq h-1), suggesting evaporation as the dominant HTO source in the hall. The proposed method provides reliable and quality-controlled 3D monitoring at low cost, which can be adopted not only for HTO and may also inspire monitoring schemes of other indoor pollutants.

Studies on diurnal variation of atmospheric tritium concentration at a sampling location near to PHWR site in Semi-Arid Zone, India

Tritium (³H) is one of the important long-lived radioisotopes in the gaseous effluent released from Pressurised Heavy Water Reactors (PHWR). For the first time, the studies on diurnal variation of atmospheric tritium concentration was carried out using an in-house developed automatic air moisture collection sampler at Kakrapar Gujarat site, India, where PHWRs are operational. Correlation of diurnal variation of atmospheric tritium concentration with meteorological parameters such as absolute humidity, ambient temperature and wind speed is studied and presented in this paper. Positive and significant correlation of atmospheric ³H concentration was observed with respect to the absolute humidity. Negative correlation was observed in ambient temperature and wind speed. The diurnal maximum and minimum of ³H concentration was found during 21.00-23.00 h (night) and 13.00-16.00 h (day), respectively.

Experimental database on water equivalent factor (WEQp) and organically bound tritium activity for tropical monsoonal climate region of South West Coast of India

Tritium in the form of tritiated water is easily incorporated into terrestrial biota as tissue free water tritium (TFWT). A part of TFWT is converted into organically bound tritium (OBT) through metabolic processes. For the computation of NE-OBT activity (expressed as Bq L^-1 of combustion water) in terrestrial plants, knowledge on 'water equivalent factor (WEQ_p)', defined as the volume of water produced from the combustion of 1 kg of the dry sample, is essential. On a global scenario, experimental data are not available on this parameter. This paper presents (i) a method for determination of WEQ_p by combustion method using a tube furnace system, (ii) a large database (N = 294) on WEQ_p parameter for samples of tropical monsoonal climate region of the Indian subcontinent, and (iii) NE-OBT activity in terrestrial biota samples (N = 186) collected from the vicinity of a PHWR nuclear power plant of India. The data generated in this study on WEQ_p serves for the validation of the data compiled in IAEA (2009 and 2010), which are estimated based on the hydrogen content of protein, fat and carbohydrates, and the fractions of protein, fat and carbohydrates. The WEQ_p varied in the ranges of 0.492-0.678 L kg^-1 (GM = 0.569 Bq L^-1, GSD = 1.06), 0.520-0.630 L kg^-1 (GM = 0.557 Bq L^-1, GSD = 1.02) 0.473-0.633 L kg^-1 (GM = 0.562 Bq L^-1, GSD = 1.02) for non-leafy vegetables, leafy vegetables, and fruits, respectively. A comparison between the experimental WEQ_p data with those compiled in the IAEA report revealed that the maximum deviation between the two data sets is <10%. The NE-OBT activity in the food samples collected from 2.3 to 20 km zone around NPP had a geometric mean (GM) value of 25.4 Bq L^-1 (GSD = 1.6, N = 186). Variations in NE-OBT activity with different seasons of the year are discussed.

Tritium in urine from members of the general public and occupationally exposed workers in Lund, Sweden, prior to operation of the European Spallation Source

A powerful neutron source, the European Spallation Source (ESS), is currently under construction in Lund, Sweden (~90 000 inhabitants). Levels of tritium (³H) in urine were estimated in members of the public in Lund and employees at the ESS using liquid scintillation counting, to obtain baseline levels before the start of operation of the ESS. These were compared with levels in other occupationally exposed radiation workers. Both the spallation reaction in the ESS tungsten target and the activation of various materials by the protons produced by the 5 MW linear accelerator will generate tritium, which will be released into the atmosphere mainly as tritiated water (HTO). Urinary HTO activity concentrations were determined in a total of 55 individuals belonging to four different categories: ESS employees, neighbours of the ESS, members of the general public in Lund and exposed workers from other facilities. The participants were asked to provide information on their beverage intake the day before urine sampling. The urine samples were filtered on activated charcoal and distilled before analysis. The effect of sample preparation on the isotope fractionation of urine samples was investigated by isotope ratio mass spectrometry (IRMS) of ²H/¹H, which showed no influence. IRMS was also used to investigate if the ratio between the stable hydrogen isotopes (²H/¹H) could provide useful data of the origin, and hence the tritium concentration, of various types of drinking water. Urinary HTO activity concentrations determined using liquid scintillation counting (LSC) were found to be below the minimum detectable activity (MDA) of 2.1 Bq⋅L^-1 for most of the participants. Five of the workers actively handling organic tritiated material were found to have activity concentrations between 3.5 and 11 Bq⋅L^-1, which were higher than the average value in local tap water of 1.5 ± 0.6 Bq⋅L^-1. The results will be used to evaluate the radiological impact on the population from future releases of tritium resulting from the operation of the ESS.

Cytogenetic damage analysis in mice chronically exposed to low-dose internal tritium beta-particle radiation

The aim of this study was to carry out a comprehensive examination of potential genotoxic effects of low doses of tritium delivered chronically to mice and to compare these effects to the ones resulting from equivalent doses of gamma-irradiation. Mice were chronically exposed for one or eight months to either tritiated water (HTO) or organically bound tritium (OBT) in drinking water at concentrations of 10 kBq/L, 1 MBq/L or 20 MBq/L. Dose rates of internal β-particle resulting from such tritium treatments were calculated and matching external gamma-exposures were carried out. We measured cytogenetic damage in bone marrow and in peripheral blood lymphocytes (PBLs) and the cumulative tritium doses (0.009 - 181 mGy) were used to evaluate the dose-response of OBT in PBLs, as well as its relative biological effectiveness (RBE). Neither tritium, nor gamma exposures produced genotoxic effects in bone marrow. However, significant increases in chromosome damage rates in PBLs were found as a result of chronic OBT exposures at 1 and 20 M Bq/L, but not at 10 kBq/L. When compared to an external acute gamma-exposure ex vivo, the RBE of OBT for chromosome aberrations induction was evaluated to be significantly higher than 1 at cumulative tritium doses below 10 mGy. Although found non-existent at 10 kBq/L (the WHO limit), the genotoxic potential of low doses of tritium (>10 kBq/L), mainly OBT, may be higher than currently assumed.

Determination of total tritium in urine from residents living in the vicinity of nuclear power plants in Qinshan, China

To estimate the tritium doses of the residents living in the vicinity of a nuclear power plant, urine samples of 34 adults were collected from residents living near the Qinshan nuclear power plant. The tritium-in-urine (HTO plus OBT) was measured by liquid scintillation counting. The doses of tritium-in-urine from participants living at 2, 10 and 22 km were in a range of 1.26-6.73 Bq/L, 1.31-3.09 Bq/L and 2.21-3.81 Bq/L, respectively, while the average activity concentrations of participants from the three groups were 3.53 ± 1.62, 2.09 ± 0.62 and 2.97 ± 0.78 Bq/L, respectively. The personal committed effective doses for males were 2.5 ± 1.7 nSv and for females they were 2.9 ± 1.3 nSv. These results indicate that tritium concentrations in urine samples from residents living at 2 km from a nuclear power plant are significantly higher than those at 10 km. It may be the downwind direction that caused a higher dose in participants living at 22 km. All the measured doses of tritium-in-urine are in a background level range.

The risk of leukaemia in young children from exposure to tritium and carbon-14 in the discharges of German nuclear power stations and in the fallout from atmospheric nuclear weapons testing

Towards the end of 2007, the results were published from a case-control study (the "KiKK Study") of cancer in young children, diagnosed <5 years of age during 1980-2003 while resident near nuclear power stations in western Germany. The study found a tendency for cases of leukaemia to live closer to the nearest nuclear power station than their matched controls, producing an odds ratio that was raised to a statistically significant extent for residence within 5 km of a nuclear power station. The findings of the study received much publicity, but a detailed radiological risk assessment demonstrated that the radiation doses received by young children from discharges of radioactive material from the nuclear reactors were much lower than those received from natural background radiation and far too small to be responsible for the statistical association reported in the KiKK Study. This has led to speculation that conventional radiological risk assessments have grossly underestimated the risk of leukaemia in young children posed by exposure to man-made radionuclides, and particular attention has been drawn to the possible role of tritium and carbon-14 discharges in this supposedly severe underestimation of risk. Both (3)H and (14)C are generated naturally in the upper atmosphere, and substantial increases in these radionuclides in the environment occurred as a result of their production by atmospheric testing of nuclear weapons during the late 1950s and early 1960s. If the leukaemogenic effect of these radionuclides has been seriously underestimated to the degree necessary to explain the KiKK Study findings, then a pronounced increase in the worldwide incidence of leukaemia among young children should have followed the notably elevated exposure to (3)H and (14)C from nuclear weapons testing fallout. To investigate this hypothesis, the time series of incidence rates of leukaemia among young children <5 years of age at diagnosis has been examined from ten cancer registries from three continents and both hemispheres, which include registration data from the early 1960s or before. No evidence of a markedly increased risk of leukaemia in young children following the peak of above-ground nuclear weapons testing, or that incidence rates are related to level of exposure to fallout, is apparent from these registration rates, providing strong grounds for discounting the idea that the risk of leukaemia in young children from (3)H or (14)C (or any other radionuclide present in both nuclear weapons testing fallout and discharges from nuclear installations) has been grossly underestimated and that such exposure can account for the findings of the KiKK Study.

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.

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.

Relation between the tritium in continuous atmospheric release and the tritium contents of fruits and tubers

Concentrations of organically bound tritium (OBT) and tissue-free water tritium (TFWT, also referred to as HTO) in fruits and tubers were measured at a garden plot in the vicinity of the source of chronic airborne tritium emissions during the 2008, 2010, and 2011 growing seasons. A continuous record of HTO concentration in the air moisture was reconstructed from the continuous record of Ar-41 ambient gamma radiation, as well as from frequent measurements of air HTO by active samplers at the garden plot and Ar-41 and air HTO monitoring data from the same sector. Performed measurements were used for testing the modified Specific Activity (SA) model based on the assumption that the average air HTO during the pod-filling period provides an appropriate basis for estimating the levels of OBT present in pods, fruits and tubers. It is established that the relationship between the OBT of fruits and tubers and the average air HTO from a 15-20 day wide window centred at the peak of the pod-filling period is consistent throughout the three analysed years, and could be expressed by the fruit or tuber's OBT to air-HTO ratio of 0.93 ± 0.21. For all three years, the concentration of HTO in fruits and tubers was found to be related to levels of HTO in the air, as averaged within a 3-day pre-harvest window. The variability in the ratio of plant HTO to air HTO appears to be three times greater than that for the OBT of the fruits and tubers. It is concluded that the OBT of fruits and tubers adequately follows an empirical relationship based on the average level of air HTO from the pod-filling window, and therefore is clearly in line with the modified SA approach.

Analysis of urine for pure beta emitters: methods and application

Bioassay for individual radionuclides is an essential and first step in estimation of radiation risk to nuclear facilities workers and people who are exposed to the contaminated environment in the event of a nuclear accident or radiological attack. Urine is a frequently used biological sample for this purpose. Tritium and (14)C are important radionuclides for workers in nuclear reactors and radiopharmaceutical laboratories. A method for the determination of tritium and (14)C in organic and inorganic forms in urine has been developed. It involves activated charcoal absorption of organic matter followed by combustion to separate tritiated water from organically-bound tritium. Inorganic (14)C from organically-bound (14)C, the separated tritium and (14)C were measured using liquid scintillation counting. Iodine-129, a long-lived beta emitter, is normally released to the atmosphere during the operation of nuclear facilities, especially in reprocessing plants. The high concentration of iodine in the thyroid makes this radionuclide an important source of exposure to exposed populations. A simple method has been developed in this work for the determination of (129)I in urine by anion exchange preconcentration, extraction purification and liquid scintillation counting. Using accelerator mass spectrometry, urine samples can be analyzed for low level (129)I in both organic and inorganic forms after active charcoal adsorption and solvent extraction separation. Condensed water collected daily from the reactor hall in a Danish research reactor and monthly urine samples from the staff working in the reactor building were collected from 2003-2010 and analyzed using this method, and the results are presented and discussed.

Free water 3H concentrations in serum samples collected during 1969-1992 in Akita, Japan

The measurements for human and environmental samples from the 1960's and 1970's are important to understand the long-term transfer of 3H from the environment to the human body. The authors have previously reported 3H concentrations in diet samples collected in Akita Prefecture during 1969-1988. Serum samples from persons living in Akita Prefecture during 1969-1992 were recently obtained. The samples were originally gathered for medical examinations and stored in freezers at -20 degrees C. Composite samples from 100 persons on average were made for analysis. The free water 3H (FWT) concentrations in those samples were determined and compared with 3H concentrations in diet samples and precipitation. The long-term variation pattern of the FWT concentrations in the serum samples was similar to patterns in the diet samples and precipitation, but the FWT concentrations in the serum samples were slightly higher than those in the latter two. A single compartment model calculation showed that the apparent mean residence time of serum FWT was 1.4 y using precipitation as an input to the compartment.