Abstract
Hydrogen and carbon are biologically-regulated, essential elements that are highly mobile in the environment and the human body. As isotopes of these elements, tritium and {sup 14}C enter freely into water (in the case of tritium), plants, animals and humans. This complex behaviour means that there are substantial uncertainties in the predictions of models that calculate the transfer of tritium and {sup 14}C through the environment. The EMRAS Tritium/C14 Working Group (WG) was set up to establish the confidence that can be placed in the predictions of such models, to recommend improved modelling approaches, and to encourage experimental work leading to the development of data sets for model testing. The activities of the WG focused on the assessment of models for organically bound tritium (OBT) formation and translocation in plants and animals, the area where model uncertainties are largest. Environmental {sup 14}C models were also addressed because the dynamics of carbon and OBT are similar. The goals of the WG were achieved primarily through nine test scenarios in which model predictions were compared with observations obtained in laboratory or field studies. Seven of the scenarios involved tritium, covering terrestrial and aquatic ecosystems and steady-state and dynamic conditions. The remaining two
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Citation Formats
None.
Modelling the Environmental Transfer of Tritium and Carbon-14 to Biota and Man. Report of the Tritium and Carbon-14 Working Group of EMRAS Theme 1.
IAEA: N. p.,
2012.
Web.
None.
Modelling the Environmental Transfer of Tritium and Carbon-14 to Biota and Man. Report of the Tritium and Carbon-14 Working Group of EMRAS Theme 1.
IAEA.
None.
2012.
"Modelling the Environmental Transfer of Tritium and Carbon-14 to Biota and Man. Report of the Tritium and Carbon-14 Working Group of EMRAS Theme 1."
IAEA.
@misc{etde_22037389,
title = {Modelling the Environmental Transfer of Tritium and Carbon-14 to Biota and Man. Report of the Tritium and Carbon-14 Working Group of EMRAS Theme 1}
author = {None}
abstractNote = {Hydrogen and carbon are biologically-regulated, essential elements that are highly mobile in the environment and the human body. As isotopes of these elements, tritium and {sup 14}C enter freely into water (in the case of tritium), plants, animals and humans. This complex behaviour means that there are substantial uncertainties in the predictions of models that calculate the transfer of tritium and {sup 14}C through the environment. The EMRAS Tritium/C14 Working Group (WG) was set up to establish the confidence that can be placed in the predictions of such models, to recommend improved modelling approaches, and to encourage experimental work leading to the development of data sets for model testing. The activities of the WG focused on the assessment of models for organically bound tritium (OBT) formation and translocation in plants and animals, the area where model uncertainties are largest. Environmental {sup 14}C models were also addressed because the dynamics of carbon and OBT are similar. The goals of the WG were achieved primarily through nine test scenarios in which model predictions were compared with observations obtained in laboratory or field studies. Seven of the scenarios involved tritium, covering terrestrial and aquatic ecosystems and steady-state and dynamic conditions. The remaining two scenarios concerned {sup 14}C, one addressing steady-state concentrations in plants and the other time-dependent concentrations in animals. The WG also considered one model intercomparison exercise involving the calculation of doses following a hypothetical, short-term release of tritium to the atmosphere in a farming area. Finally, the WG discussed the nature of OBT and proposed a definition to promote common understanding and usage within the international tritium community. The models used by the various participants varied in complexity from simple specific activity approaches to dynamic compartment models and process-oriented models, in which the various transfer processes were simulated explicitly. The predictions varied by a factor of about 2 for scenarios involving continuous releases and a factor of 10 or more for short-term releases. In general, the simple and complex models performed equally well for chronic releases, but complex models were required to reproduce the observations for short-term releases. For most scenarios, the predictions tended to bracket the observations, suggesting that, in an average sense, the models reflect a good conceptual understanding of the environmental transport of tritium and {sup 14}C. In some scenarios, part of the difference between predictions and observations could be attributed to the uncertainty in the observations as well as in the predictions. Uncertainty estimates were requested as part of each scenario, and most participants submitted results for the steady-state exercises. For endpoints involving tritiated water (HTO) and {sup 14}C, these were roughly consistent with a 95% confidence interval (97.5th percentile divided by the 2.5th percentile) of a factor 3 to 4. The uncertainties in the OBT concentrations were slightly higher. Few of the participants in the dynamic scenarios determined their uncertainties. However, the scatter in the predictions and the differences between predictions and observations suggest that the 95% confidence intervals on HTO and {sup 14}C concentrations were about a factor of 10 or more. The confidence intervals were generally smaller for OBT than for HTO, reflecting the fact that, for the dynamic scenarios, HTO varies rapidly over time whereas OBT integrates. The uncertainty in the predictions of environmental tritium and {sup 14}C models can be reduced by: - ensuring that the air concentrations used to drive the models are of high quality and match the resolution and averaging requirements of the scenario. Performance was better for models that were driven by air concentrations averaged over the OBT or {sup 14}C residence time in the compartment of interest; - incorporating as much site-specific information as possible on land use, local soil properties and predominant plant cultivars and animal breeds; - implementing realistic growth curves for the plant cultivars of interest; - basing all sub-models on the physical approaches available for the disciplines in question. For example, knowledge from the agricultural sciences should be used to improve models for crop growth, photosynthesis and translocation; - recognizing and accounting for any unusual conditions (water stress, an uncommon cultivar or breed) in the model application. Further work in the following areas would help to improve tritium and {sup 14}C dose assessments: - testing and improving models for the following processes: plant uptake of HTO at night and when it is raining; OBT formation in plants at night; translocation of OBT to fruit and roots; isotopic discrimination; tritium behaviour in soils following deposition from the atmosphere; and tritium behaviour in winter; - modifying the steady-state models for chronic releases to account for the fact that fluctuations in release rates and meteorological conditions result in a state of quasi- equilibrium in the environment, rather than the complete equilibrium assumed by the models; - developing a standard conceptual model for accidental tritium releases; - investigating and understanding the large OBT/HTO ratios that have been observed in soils, plants and fish under conditions that are ostensibly at equilibrium. The ten scenarios developed by the Tritium/C14 WG provide a valuable source of test data for validating environmental tritium and {sup 14}C models.}
place = {IAEA}
year = {2012}
month = {Jun}
}
title = {Modelling the Environmental Transfer of Tritium and Carbon-14 to Biota and Man. Report of the Tritium and Carbon-14 Working Group of EMRAS Theme 1}
author = {None}
abstractNote = {Hydrogen and carbon are biologically-regulated, essential elements that are highly mobile in the environment and the human body. As isotopes of these elements, tritium and {sup 14}C enter freely into water (in the case of tritium), plants, animals and humans. This complex behaviour means that there are substantial uncertainties in the predictions of models that calculate the transfer of tritium and {sup 14}C through the environment. The EMRAS Tritium/C14 Working Group (WG) was set up to establish the confidence that can be placed in the predictions of such models, to recommend improved modelling approaches, and to encourage experimental work leading to the development of data sets for model testing. The activities of the WG focused on the assessment of models for organically bound tritium (OBT) formation and translocation in plants and animals, the area where model uncertainties are largest. Environmental {sup 14}C models were also addressed because the dynamics of carbon and OBT are similar. The goals of the WG were achieved primarily through nine test scenarios in which model predictions were compared with observations obtained in laboratory or field studies. Seven of the scenarios involved tritium, covering terrestrial and aquatic ecosystems and steady-state and dynamic conditions. The remaining two scenarios concerned {sup 14}C, one addressing steady-state concentrations in plants and the other time-dependent concentrations in animals. The WG also considered one model intercomparison exercise involving the calculation of doses following a hypothetical, short-term release of tritium to the atmosphere in a farming area. Finally, the WG discussed the nature of OBT and proposed a definition to promote common understanding and usage within the international tritium community. The models used by the various participants varied in complexity from simple specific activity approaches to dynamic compartment models and process-oriented models, in which the various transfer processes were simulated explicitly. The predictions varied by a factor of about 2 for scenarios involving continuous releases and a factor of 10 or more for short-term releases. In general, the simple and complex models performed equally well for chronic releases, but complex models were required to reproduce the observations for short-term releases. For most scenarios, the predictions tended to bracket the observations, suggesting that, in an average sense, the models reflect a good conceptual understanding of the environmental transport of tritium and {sup 14}C. In some scenarios, part of the difference between predictions and observations could be attributed to the uncertainty in the observations as well as in the predictions. Uncertainty estimates were requested as part of each scenario, and most participants submitted results for the steady-state exercises. For endpoints involving tritiated water (HTO) and {sup 14}C, these were roughly consistent with a 95% confidence interval (97.5th percentile divided by the 2.5th percentile) of a factor 3 to 4. The uncertainties in the OBT concentrations were slightly higher. Few of the participants in the dynamic scenarios determined their uncertainties. However, the scatter in the predictions and the differences between predictions and observations suggest that the 95% confidence intervals on HTO and {sup 14}C concentrations were about a factor of 10 or more. The confidence intervals were generally smaller for OBT than for HTO, reflecting the fact that, for the dynamic scenarios, HTO varies rapidly over time whereas OBT integrates. The uncertainty in the predictions of environmental tritium and {sup 14}C models can be reduced by: - ensuring that the air concentrations used to drive the models are of high quality and match the resolution and averaging requirements of the scenario. Performance was better for models that were driven by air concentrations averaged over the OBT or {sup 14}C residence time in the compartment of interest; - incorporating as much site-specific information as possible on land use, local soil properties and predominant plant cultivars and animal breeds; - implementing realistic growth curves for the plant cultivars of interest; - basing all sub-models on the physical approaches available for the disciplines in question. For example, knowledge from the agricultural sciences should be used to improve models for crop growth, photosynthesis and translocation; - recognizing and accounting for any unusual conditions (water stress, an uncommon cultivar or breed) in the model application. Further work in the following areas would help to improve tritium and {sup 14}C dose assessments: - testing and improving models for the following processes: plant uptake of HTO at night and when it is raining; OBT formation in plants at night; translocation of OBT to fruit and roots; isotopic discrimination; tritium behaviour in soils following deposition from the atmosphere; and tritium behaviour in winter; - modifying the steady-state models for chronic releases to account for the fact that fluctuations in release rates and meteorological conditions result in a state of quasi- equilibrium in the environment, rather than the complete equilibrium assumed by the models; - developing a standard conceptual model for accidental tritium releases; - investigating and understanding the large OBT/HTO ratios that have been observed in soils, plants and fish under conditions that are ostensibly at equilibrium. The ten scenarios developed by the Tritium/C14 WG provide a valuable source of test data for validating environmental tritium and {sup 14}C models.}
place = {IAEA}
year = {2012}
month = {Jun}
}