IAEA

1
IAEAs Programme on Environmental Modelling for
RAdiation Safety(EMRAS II)WG7 Tritium Working
Group
Introduction Workshop 28-29 September 2009 EDF,
Chatou, France

• IAEA Scientific Secretary
• Volodymyr Berkovskyy, IAEA
• Working Group Leader
• Dan Galeriu, IFIN-HH,
• Romania

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Tritium

• Hydrogen is ubiquitous in the environment and is
  part of many chemical compounds, including water
  and most organic materials. As an isotope of
  hydrogen, tritium enters freely into these
  compounds and its movement through the
  environment can be inferred from the cycling of
  hydrogen. As a result, tritium behaves
  differently in a number of respects from other
  radionuclides
• In aqueous systems, tritium moves as a
  non-reactive, non-absorbed constituent with the
  bulk water flow. Accordingly, the environmental
  transport of tritium is governed in large part by
  local and global hydrologic cycles.
• As a gas, tritium moves in response to its vapor
  pressure gradient and can, under some
  circumstances, move against the water vapor flux.
• Tritium deposited from the atmosphere to soil and
  plants is readily recycled back to the atmosphere
  via evapotranspiration, forming a secondary
  airborne plume.
• The processes responsible for tritium transfer
  have time scales as short as minutes. Tritium
  can be rapidly taken up by organisms but just as
  rapidly lost. As a result, tritium transfer is
  highly dependent upon the environmental
  conditions prevailing at the time and place of
  release and the time of measurement.
• Tritium can be effectively incorporated into
  biological systems, including the human body, as
  organically bound tritium (OBT). Many
  environmental pathways to humans.
• OBT has long biological half-life in humans and
  biota
• Tritiated hydrogen, which is biologically inert,
  can be oxidized in soil to tritiated water vapor,
  which is about 20,000 times more radiotoxic.
• Although tritium is substantially heavier than
  other hydrogen isotopes, it is usually
  incorporated into larger molecules. Therefore,
  isotopic effects, although present, are not
  important in environmental tritium transport,
  except in OBT formation. BUT for OBT Very short
  range, so damage depends on where in cell, eg
  close to DNA

3
PAST RESULTS Scenario V1.05 in the Tritium WG
(BIOMOVS II 1995)

• Farmland was exposed with 1E10 Bq/m3 of HTO in
  air for one hour starting at midnight in one case
  and at 10 a.m. in the other, 30 days before the
  harvest of the various crops.
• In most cases the predicted concentrations among
  the models agreed within one order of
• magnitude and for some endpoints within two
  orders of magnitude. The higher
• discrepancy occurred after the night. Some
  processes are highlighted that may need
• further experimental work to improve the model
  performance
• HTO in soil
• 1. deposition beneath plant canopies and
  re-emission from soil, particularly in stable air
  and low wind speeds
• 2. number and thickness of soil layers needed to
  describe vertical movement in soil and between
  soil surfaces and atmosphere.
• HTO in vegetation
• 1. deposition from the atmosphere particularly at
  night when leaf stomata are closed or partly
  closed
• 2. effective rooting depth of different species.
• OBT in vegetation
• 1. rates of OBT formation, particularly at night
• 2. translocation of HTO and OBT to plant storage
  tissues, grain, tubers and roots
• 3. effect of stage of development of grain when
  release occurs.
• HTO and OBT in animal products
• 1. rates of OBT formation in animals
• 2. rates of loss of OBT from milk and meat
• 3. effect of time elapsed between release and
  slaughter on concentration in beef.

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PAST RESULTS Scenario V3.0 in the Tritium WG
(BIOMOVS II 1995)

• The importance of better understanding of the
  uptake of HTO and conversion to OBT in dark
  conditions was pointed in the scenario V3.0
  (BIOMOVS II 1995), where modelers were asked to
  predict HTO in leaves and OBT in grains and to
  compare with experimental data from FZK.
• The primary purpose of the experiment was to find
  out how much HTO enters the leaves when stomata
  close at night and, if HTO is present in leaves,
  whether it can become incorporated into OBT in
  the dark. The results clearly show that HTO
  enters plants at night and is converted to OBT in
  the dark. From the experimental results some
  transfer parameters were extracted. The velocity
  of deposition was estimated to range from 16 to
  23 mm s-1 in light and from 3 to 4 mm s-1 in the
  dark. The loss rate factor was estimated to be
  about 0.95 h-1 in light and about 0.15 h-1in the
  dark. The rate of OBT formation was estimated to
  range from 6 E-4 to 1 E-3 h-1 in light and
  between 2 E-4 and 3 E-4 h-1 in the dark. This
  rate is defined as the ratio between the OBT
  concentration at harvest and the time integrated
  HTO concentration in air moisture.
• The deposition velocity in the dark implies a
  canopy resistance of the order of 150-200 s m-1
  (using the experimental data on wind in the
  growth chamber) which suggests a stomata
  resistance of less than 1000 s m-1. If the
  stomata are completely closed, this would a
  smaller value than expected for the cuticular
  resistance. For daytime exposure, the estimated
  deposition velocity is 3-4 times higher than the
  model predictions, and consequently the canopy
  resistance seems to be overpredicted by most
  models. These assertions must be considered with
  precaution because the finite size of the growth
  chamber can influence the results if comparing
  with large canopies in the field.

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Environmental and Radiological Impact of
Accidental Tritium Release
Review of past conclusions

• Philippe Guétat, Luc Patryl
• CEA - France

8th International Conference on Tritium Science
and Technology September 16-21, 2007 Rochester,
New York
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Conclusions for Scientistes
Review of past conclusions

• General features are known but
• Are we able to do better than a factor 10 ?
• What fundamental parameters should be known in
  the vicinity of a tritium plant ?
• Some experiments to realize
• deposition velocity
• Air-plant exchanges during the night
• Parts coming from air and from soil.

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Review of past conclusions
TRITIUM and the ENVIRONMENT SOURCES MEASUREMEN
T and TRANSFER Ph GUETAT, CEA Thanks for their
help to C Douche, JC Hubinois, N. Baglan , D
Galeriu, Ph. Davis, W Raskob
CEA/DAM/VA UE scientific seminar emerging issues
on tritium 13/11/2007
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Conclusions for environment RD
Review of past conclusions

• Tritium does not concentrate in food chain
• About models
• Variability remains very large in case of
  accident especially in rain and night cases.
• Modification needed for wheat modelling -
  realistic approach
• About experimental Data
• Translocation of organic matter from leaves to
  edible part of the vegetable.
• Case of the night for experimental data.
• What about Tritiated particulates ?
• About modelers
• The present Tritium scientific community is very
  small,
• have to synthesize what is absolutely needed in
  models for acute release.
• This community could disappear from EU in the few
  next years.

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Review of past conclusions
TRITIUM RADIOECOLOGY AND DOSIMETRY - TODAY AND
TOMORROW D. Galeriu, P. Davis, W. Raskob, A.
Melintescu IFIN-HH Romania AECL Canada
IKET Germany Invited lecture
8th International Conference on Tritium Science
and Technology September 16-21, 2007 Rochester,
New York
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Review of past conclusions
TOWARD CONCLUSIONS

• The 1990 Aiken list was amended in 1997 by W.
  Raskob and P. Barry. Sensitivities and hence
  importance in this list vary with both inputs
  and end points. Site- and task-specific analyses
  must be done to identify the most important
  processes in a given application.
• Areas Requiring Further Work
• plant uptake of HTO at night
• rates of OBT formation in plants,
• particularly at night
• dispersion in soil
• reemission from soil and plants
• rates of OBT formation and loss
• in animals
• translocation to fruits and roots
• tritium behavior in winter
• HTO concentrations in the
• environment following an HT release.

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Review of past conclusions
With present knowledge, it can be argued that the
expected dose to members of the public from
routine tritium releases is unlikely to be higher
than 30 µSv/y for todays nuclear facilities.
Accidental releases of HT or aquatic HTO releases
have much lower radiological impact than an
accidental atmospheric release of HTO. EU
guidance on response to accidental releases is as
follows
No emergency action beyond 800 m
No delayed action at any time beyond 3 km
No long-term action (longer than 1 year) beyond 800 m
Restriction on the consumption of foodstuff and crops limited in terms of timescale and ground area
Limited economic impact
The next generation of models for accidental HTO
releases must be improved to decrease
uncertainties and to cope with tighter regulatory
requirements.
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Requirements for the Next Generation of Dynamic
HTO Models
Review of past conclusions

• Reliable atmospheric transport and dispersion
  codes (particle models) with good representation
  of reemission and inclusion of turbulence,
  topography etc
• Changing environmental conditions must be taken
  into account
• Several sub-models are needed to describe the
  behaviour of tritium in soil and crops
• The crop sub-model is most important and here the
  plant physiological parameters must be
  considered
• Conversion processes from HT to HTO and further
  to OBT have to be modelled
• Sub-models have to be based on physical
  approaches knowledge from other disciplines
  should be used to derive general dependencies
  based on data for other substances than tritium
• Site-specific information on land use, soil types
  and crop genotypes should be applied, together
  with realistic habits for the maximally exposed
  individual.

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Modelling the transfer of 3H and 14C into the
environment - lessons learnt from IAEAs EMRAS
project
Review of past conclusions

• A. Melintescu and D. Galeriu
• Horia Hulubei National Institute for Physics
  and Nuclear Engineering, Bucharest-Magurele,
  ROMANIA

International Conference on RADIOECOLOGY
ENVIRONMENTAL RADIOACTIVITY 15 20 June 2008,
Bergen, Norway
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Suggestions for improving H-3 and C-14 accidental
release models
Review of past conclusions

• Models must include more reliable atmospheric
  transport and dispersion with turbulence data and
  topography, as well as improved area source for
  re-emission
• HT conversion into HTO in soils must be analysed
  starting from basic science and modelled
  accordingly with local soil properties
• The influence of environmental condition on the
  transfer of tritium to plants must be included
  and generic models must separate wet, dry, and
  hot or cold situations
• Knowledge from agricultural science must be
  incorporated, including physiologically based
  crop growth modelling (photosynthesis, partition
  of newly formed dry matter, genotype influence,
  evapotranspiration)
• For OBT production at night it must develop an
  improved model based on a deeper analysis of
  plant processes
• Translocation in fruits and roots must be
  modelled using knowledge in agricultural
  research tests with experimental data are
  needed
• Robust operational models based on energy
  metabolism are needed for transfer in animals
• The predictions for contamination of eggs or
  broilers must be experimentally checked
• For cold climate, tritium behaviour in winter,
  including washout by snow, dry deposition to snow
  and the fate of tritium in the snow pack must be
  studied
• A further reduction of uncertainties must be
  based on the ability to use site-specific
  information on land use, local soil properties
  and predominant crop genotype characteristics,
  together with realistic assumptions concerning
  habits of the maximally exposed individual.

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WG7- PARIS

• The Working Group focuses on the development of a
  dynamic reference model that allows the
  estimation exposure to tritium subsequent to
  accidental releases. For this purpose, the
  processes involved in the transfer of tritium in
  the environment will be analyzed in dependence on
  the environmental conditions, season and time of
  the day. A main issue is the integration of
  actual weather data to enable reliable estimation
  of the tritium behavior.
• Our meeting must
• - discuss and harmonize the views of
  participants concerning the approaches for
  developing the conceptual model for tritium
  accidents (atmospheric and aquatic)
• - agree on the structure and scope of the
  conceptual model
• - identify potential gaps in knowledge and
  expertise, which should be addressed during the
  model development
• - define the structure of the technical
  document and share tasks according to the
  expertise of each participant and the interests
  of his/her organization or institute
• - elaborate the work plan for developing the
  conceptual model and
• - distribute specific tasks to be accomplished
  and reported at the next EMRAS II Technical
  Meeting (2529 January 2010).

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Task groups

• Task Group I covering -    Tritium washout-   
  HT/HTO deposition-reemission
• -    Actual evaporation and transpiration
  and connected HTO
• concentration dynamics-    HTO
  uptake and retention in plant in rain condition
  -    Movement of HTO to deeper soil layers-   
  Winter case (particularly deposition on snow and
  how
• to deal with snow)Task Group II
  covering -    Use of growth models - define the
  minimal needs-    OBT formation in night -   
  Translocation of OBT from leaves to edible plant
  partsTask Group III -    Modelling the
  transfer in aquatic food chain

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Questionnaire

• 11 members of WG7 responded to the questionnaire.
  From their replies, some preliminary conclusions
  can be reached
• There is interest in both liquid and atmospheric
  releases.
• About half of the respondents have an interest in
  HT emissions.
• Plants of interest include
• pasture, lucerne, vegetables (leafy and root
  vegetables),
• rice, wheat, corn, tomatoes, potatoes,
  apples and citrus fruits, grapes.
• For Cernavoda add sunflower and sugar beat
• Animals of interest include cow ( milk), sheep
  ( milk), beef, goat, pork, chicken, fish,
  boars.
• All agree that the local climate and soils have a
  large influence.
• Some prefer compartmental models with
  site-specific parameters. There is an increased
  interest in process level modeling of minimal
  complexity.
• To be conservative is the requirement, but with
  no details on how to control the robustness.

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Expert view (IAEA)

• It is especially important to focus on the
  uncertainties and sensitivities that are involved
  in modeling the behavior of tritium in the
  environment after accidents.
• Although we know much about the behavior of water
  in the environment, the reliable prediction of
  tritium concentrations in environmental  media
  subsequent to an accident is the result of the
  complex interaction of a number of factors that
  are subject to hourly, daily and annual
  fluctuations. Due to these large uncertainties
  related to the environmental conditions at the
  time of the accidental release, predictions are
  unavoidably associated with considerable
  uncertainties.
• However, these inherent problems in tritium
  modeling are not clear to everybody. Therefore,
  it would be very important for the work to
• to identify the main contributors to uncertainty
• to identify the critical periods during the year
  in relation to resulting exposures to tritium
• to identify the important and sensitive
  parameters, having in mind hourly, daily and
  annual variations  in parameters/processes
• to explore the practical  possibilities in
  determining those parameters  
• to get an idea about the achievable
  reliability of tritium modelling under practical,
  this means under accidental field conditions
• to get a clear idea for which phases of the
  tritium accident the application of a tritium
  model is desirable and useful.