Equilibrium and non-equilibrium partition of tritium between organics and tissue water of different biological systems

Sensitivity analysis in a radiological impact assessment of a nuclear power plant discharge. A comparison of the Morris, Spearman and Sobol' approaches

This paper compares the Morris, Spearman and Sobol' methods of sensitivity analysis in radiological risk assessment. The determination of the most influential parameters on model with regards to the propagation of their uncertainties to output variables, is of greatest interest. This study aims to determine the relative importance of parameters uncertainties on the dose calculation uncertainty in the framework of a scenario of routine discharges discussed in the context of an IAEA working group. The scenario considers atmospheric and liquid discharges of three different types of radionuclides (¹⁴C, tritium as HTO and ^110mAg) from a nuclear power plant located by the side of a river. It is concluded that the most reliable and practical method according to the ability of ranking influential parameters and the easiness of its application is the Spearman method. As key result, the three first influential variables for annual total dose for all pathways and all radionuclides were the water dissolved inorganic carbon concentration, the volatilisation rate constant and the soil layer solid liquid distribution in ¹⁴C.

Tritiated water (HTO) uptake and loss in maize plant after short-term exposure of atmospheric HTO vapor at daytime and nighttime

Maize plants were exposed to atmospheric tritiated water (HTO) in the field for 1 h during the day and night under different growth periods. The uptake of HTO vapor by leaves and tissue free water tritium (TFWT) loss in maize plants were investigated. Rate constants of uptake of HTO from the atmosphere by maize leaves were 0.10 ± 0.02-0.21 ± 0.01 h^-1 during the daytime exposure and 0.035 ± 0.001-0.13 ± 0.01 h^-1 in the nighttime exposure. The uptake rate tended to decrease as the exposure increased and harvest neared. At the end of exposure, the TFWT concentration was highest in the leaf and lowest in the ear, the TFWT concentration in different parts of maize varied by a factor of 3-28 for daytime release and 2-30 for nighttime release. After the end of every exposure, TFWT was lost rapidly from different maize tissues for one day and then at a much slower rate until to harvest. At harvest, the TFWT concentrations varied with different maize tissues by factors of 1-4. The decrease was by factors of 4-351 for daytime exposure and 6-605 for nighttime exposure. The reduction of TFWT varied with the exposure time by the factors of 1-10 depending on plant parts. The reduction factor was lowest in kernel. This result suggests that the HTO vapor uptake rates in maize leaves and TFWT loss rates in the ear may be lower than those in C₃ plants.

Tritium forms discrimination in ryegrass under constant tritium exposure: From seed germination to seedling autotrophy

Uncertainties remain regarding the fate of atmospheric tritium after it has been assimilated in grasslands (ryegrass) in the form of TFWT (Tissue Free Water Tritium) or OBT (Organically Bound Tritium). One such uncertainty relates to the tritium forms discrimination during transfer from TFWT to OBT resulting from photosynthesis (OBT_photo), corresponding to the OBT_photo/TFWT ratio. In this study, the OBT/TFWT ratio is determined by experiments in the laboratory using a ryegrass model and hydroponic cultures, with constant activity of tritium in the form of tritiated water (denoted as HTO) in the "water" compartment (liquid HTO) and "air" compartment (HTO vapour in the air). The OBT_photo/TFWT ratio and the exchangeable OBT fraction are measured for three parts of the plant: the leaf, seed and root. Plant growth is modelled using dehydrated biomass measurements taken over time in the laboratory and integrating physiological functions of the plant during the first ten days after germination. The results suggest that there is no measurable discrimination of tritium in the plant organic matter produced by photosynthesis.

Isotopic fractionation of tritium in biological systems

Isotopic fractionation of tritium is a highly relevant issue in radiation protection and requires certain radioecological considerations. Sound evaluation of this factor is indeed necessary to determine whether environmental compartments are enriched/depleted in tritium or if tritium is, on the contrary, isotopically well-distributed in a given system. The ubiquity of tritium and the standard analytical methods used to assay it may induce biases in both the measurement and the signification that is accorded to the so-called fractionation: based on an exhaustive review of the literature, we show how, sometimes large deviations may appear. It is shown that when comparing the non-exchangeable fraction of organically bound tritium (neOBT) to another fraction of tritium (e.g. tritiated water) the preparation of samples and the measurement of neOBT reported frequently led to underestimation of the ratio of tritium to hydrogen (T/H) in the non-exchangeable compartment by a factor of 5% to 50%. In the present study, corrections are proposed for most of the biological matrices studied so far. Nevertheless, the values of isotopic fractionation reported in the literature remain difficult to compare with each other, especially since the physical quantities and units often vary between authors. Some improvements are proposed to better define what should encompass the concepts of exchangeable and non-exchangeable fractions.

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.

Empirical insights and considerations for the OBT inter-laboratory comparison of environmental samples

Organically bound tritium (OBT) is an important tritium species that can be measured in most environmental samples, but has only recently been recognized as a species of tritium in these samples. Currently, OBT is not routinely measured by environmental monitoring laboratories around the world. There are no certified reference materials (CRMs) for environmental samples. Thus, quality assurance (QA), or verification of the accuracy of the OBT measurement, is not possible. Alternatively, quality control (QC), or verification of the precision of the OBT measurement, can be achieved. In the past, there have been differences in OBT analysis results between environmental laboratories. A possible reason for the discrepancies may be differences in analytical methods. Therefore, inter-laboratory OBT comparisons among the environmental laboratories are important and would provide a good opportunity for adopting a reference OBT analytical procedure. Due to the analytical issues, only limited information is available on OBT measurement. Previously conducted OBT inter-laboratory practices are reviewed and the findings are described. Based on our experiences, a few considerations were suggested for the international OBT inter-laboratory comparison exercise to be completed in the near future.

An overview of organically bound tritium experiments in plants following a short atmospheric HTO exposure

The need for a less conservative, but reliable risk assessment of accidental tritium releases is emphasized in the present debate on the nuclear energy future. The development of a standard conceptual model for accidental tritium releases must be based on the process level analysis and the appropriate experimental database. Tritium transfer from atmosphere to plants and the subsequent conversion into organically bound tritium (OBT) strongly depends on the plant characteristics, seasons, and meteorological conditions, which have a large variability. The present study presents an overview of the relevant experimental data for the short term exposure, including the unpublished information, also. Plenty of experimental data is provided for wheat, rice, and soybean and some for potato, bean, cherry tomato, radish, cabbage, and tangerine as well. Tritiated water (HTO) uptake by plants during the daytime and nighttime has an important role in further OBT synthesis. OBT formation in crops depends on the development stage, length, and condition of exposure. OBT translocation to the edible plant parts differs between the crops analyzed. OBT formation during the nighttime is comparable with that during the daytime. The present study is a preliminary step for the development of a robust model of crop contamination after an HTO accidental release.

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.

Organically bound tritium (OBT) for various plants in the vicinity of a continuous atmospheric tritium release

In order to quantify tritium impact on the environmental, we studied vegetation continuously exposed to a tritiated atmosphere. We chose lichens as bio-indicators, trees for determination of past tritium releases of the Valduc Centre, and lettuce as edible vegetables for dose calculation regarding neighbourhood. The Pasquill and Doury models from the literature were tested to estimate tritium concentration in the air around vegetable for distance from the release point less than 500 m. The results in tree rings show that organically bound tritium (OBT) concentration was strongly correlated with tritium releases. Using the GASCON model, the modelled variation of OBT concentration with distance was correlated with the measurements. Although lichens are recognized as bio-indicators, our experiments show that they were not convenient for environmental surveys because their age is not definitive. Thus, tritium integration time cannot be precisely determined. Furthermore, their biological metabolism is not well known and tritium concentration appears to be largely dependent on species. An average conversion rate of HTO to OBT was determined for lettuce of about 0.20-0.24% h(-1). Nevertheless, even if it is equivalent to values already published in the literature for other vegetation, we have shown that this conversion rate, established by weekly samples, varies by a factor of 10 during the different stages of lettuce development, and that its variation is linked to the biomass derivative.

Tritium doses from chronic atmospheric releases: a new approach proposed for regulatory compliance

Regulatory models for atmospheric releases of tritium approved by the Environmental Protection Agency (CAP88, AIRDOS-PC, and COMPLY) calculate doses only from tritiated water (HTO) taken into the body. They do not deal with dose from emissions of tritiated hydrogen gas (HT) and conversion of HT to HTO in the environment, nor do they address the dose from ingesting tritium incorporated into organic compounds. A simple model (NEWTRIT) is proposed that accounts for all pathways to dose from atmospheric releases of HT and HTO. The model is formulated in terms of the tritium-to-hydrogen ratio in each environmental compartment. With each transfer, a small reduction in the ratio is introduced to reflect the dilution that occurs in nature. Conversion of HT to HTO in the environment is modeled using the latest experimental data. Concentrations of organically bound tritium are calculated in foodstuffs based on amounts of hydrogen in proteins, fats, and carbohydrates. Concentrations in foodstuffs and doses calculated by NEWTRIT are consistent with the predictions of existing regulatory models. In addition, the HTO component of NEWTRIT is tested using public bioassay data and the HT component is tested using results from a model intercomparison study for a hypothetical HT release. Although tritium doses probably have not been underestimated by regulatory models that account only for HTO (due to the high degree of conservatism built into these models), the explicit treatment of HT and organically bound tritium proposed here will make the dose assessments more comprehensive, defensible, and scientifically acceptable. Because NEWTRIT includes all pathways to dose and predicts conservative doses, it is a suitable model to replace the tritium models currently used for compliance.

Deposition of gaseous radionuclides to fruit

14C, 35S and 3H are released to the environment during the operation of gas-cooled reactors and were identified as radionuclides of interest by the BIOMASS Fruits Working Group. This paper provides a review of the deposition, uptake, allocation and loss of these radionuclides with respect to fruit and conceptual models for gaseous radionuclides. It is concluded that the mechanisms for the uptake of CO35S, HTO and 14CO2 are well understood and that their deposition velocities have been quantified. There is also a reasonable body of work on the translocation of 14C once in the crop, but much less for 35S and 3H, which are considered to follow source-sink relationships. The loss rates of the three radionuclides show large differences, with tritium lost rapidly in the form of HTO but retained longer when converted to OBT. The losses of 14C are less and those of sulphur are minimal post fixation. When fruit crops alone are considered, the quantity of information is further reduced but predictions on possible behaviour of these radionuclide species can be made from the current knowledge.