WSC Radioecology Research Group

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WSC Radioecology Research Group
A new methodology for the assessment of radiation
doses to biota under non-equilibrium conditions
J. Vives i Batlle, R.C. Wilson, S.J. Watts, S.R.
Jones, P. McDonald and S. Vives-Lynch
EC PROTECT Workpackage 2 Workshop, Vienna, 27 -
29 June 2007
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Introduction
Interest in recent years regarding protection of
non-human biota

• Different approaches
• Environment Agency RD 128
• FASSET/ERICA
• RESRAD - Biota, Eden, EPIC-DOSES3D, etc.

All have one common theme Assume equilibrium
within the system they are modelling
Current work builds on previous work but takes it
to the next stage Non-equilibrium conditions
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Objectives

• Model the retention behaviour observed for many
  organisms and radionuclides.
• Express model rate constants as a function of
  known parameters from the literature.
• Ensure the model automatically reduces to the old
  CF-based approach in the non-dynamic case.
• Incorporate dosimetry compatible with FASSET and
  EA RD 128 methodologies.
• Encode the model in a simple spreadsheet which
  assesses for lists of radionuclides and biota
  over time.

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Model Design
Environment (seawater)
Fast Release
Slow Release
Slow Uptake
Fast Uptake
Radioactive decay
Radioactive decay
Organism
Slow phase
Fast phase
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Multi-phasic release

• Some organisms have fast followed by slow
  release, represented by two biological half-lives

Typical biphasic retention curve, representing
the depuration of 131I from L. littorea (Wilson
et al., 2005).
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Model options

• Three cases are possible
• No biological half-lives known ? use instant
  equilibration with a CF (current method).
• One biological half-life known ? use simple
  dynamic 2-compartment model.
• Two biological half-lives known ? use fully
  dynamic 3-compartment model.

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Flow diagram
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Basic equations
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Basic equations

• General solution

• Involves Laplace transformation, algebraic
  manipulation and some substitutions (?, ?, ds
  and s are functions of the rate constants).

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Model parameterisation

• Initial conditions

• Approximation 1 (organism is a faster accumulator
  than the medium)

• Approximation (organism holds less activity than
  the medium)

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Consequences

• Biphasic release

• Simple formulae for all the model constants

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Calculation of "x"

• If we know the retained at time ? (f100)

• If we know when the release curve closes in to
  slope of the final phase (factor f )

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Sensitivity analysis
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Basis for the dosimetry

• Same as EA RD 128 and FASSET (aquatic)

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Model inputs
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Biokinetic Parameters

• Current data defaults from literature
• User can edit with site-specific data

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Model Outputs

• Reference organisms
• Phytoplankton
• Zooplankton
• Macrophyte
• Winkle
• Benthic mollusc
• Small benthic crustacean
• Large benthic crustacean
• Pelagic fish
• Benthic fish

• Nuclides
• 99Tc
• 125I, 129I 131I
• 134Cs 137Cs
• 238Pu, 239Pu 241Pu
• 241Am

Weighted and un-weighted external and internal
doses and activity concentrations within biota
produced
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Validation

• 99Tc activity in lobsters comparison with model
  by Olsen and Vives i Batlle (2003)

• 129I activity in winkles comparison with model
  by Vives i Batlle et al. (2006)

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Results - Long term assessment
Annual time steps
Benthic mollusc
Dynamic model
Pu benthic mollusc - TB1/2 474 days Tc large
benthic crustacean - TB1/2 56.8 114 days
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Results - Short term assessment
Daily time steps
Tc in macrophytes - TB1/2 1.5 128 days
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Results - Short term assessment
Daily time steps
Tc in winkles - TB1/2 142 days
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Time-integrated doses

• differences between the integrated dose rates
  obtained from the two approaches increase with
  slowness of response of the organism to an input
  of radioactivity, due to the smoothing effect of
  the dynamic method.

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Conclusions

• Successfully production of a dynamic model that
  makes assessments to biota more realistic
• Simple, user-friendly spreadsheet format similar
  to RD 128
• Model is rigorously tested and validated against
  CF and dynamic research models
• Can be edited with site-specific data
• Expandable for extra nuclides and organisms

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References

• Vives i Batlle, J., Wilson, R.C., Watts, S.J.,
  Jones, S.R., McDonald, P. and Vives-Lynch, S.
  Dynamic model for the assessment of radiological
  exposure to marine biota. J. Environ.
  Radioactivity (submitted).
• Vives i Batlle, J., Wilson, R. C., McDonald, P.,
  and Parker, T. G. (2006) A biokinetic model for
  the uptake and release of radioiodine by the
  edible periwinkle Littorina littorea. In P.P.
  Povinec, J.A. Sanchez-Cabeza (Eds) Radionuclides
  in the Environment, Volume 8. Elsevier, pp. 449
  462.
• Olsen, Y.S. and Vives i Batlle, J. (2003). A
  model for the bioaccumulation of 99Tc in lobsters
  (Homarus gammarus) from the West Cumbrian coast.
  J. Environ. Radioactivity 67(3) 219-233.

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Acknowledgements
The authors would like to thank the Nuclear
Decommissioning Authority (NDA), UK, for funding
this project.