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, June 2007

Introduction Interest in recent years regarding protection of non-human biota Different approaches: Environment Agency R&D 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

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 R&D 128 methodologies. Encode the model in a simple spreadsheet which assesses for lists of radionuclides and biota over time.

Model Design Environment (seawater) Slow phase Fast phase Organism Fast Uptake Slow Uptake Fast Release Slow Release Radioactive decay

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 131 I from L. littorea (Wilson et al., 2005).

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.

Flow diagram Biokinetic database Water activity Dosimetry database Calculate initial conditions of the system 2 T B1/2 known? Slope transition known No Yes % retention known? Calculate 2 rate constants from T B1/2 s At least 1 T B1/2 known? No Apply npn-dynamic model using CF No Yes Calculate remaining rate constants for basic model (2 components) Yes Calculate remaining rate constants for advanced model (3 components) Run a loop for series of regular time steps Yes Refresh initial conditions of new time step using solution form previous step Write results into the spreadsheet

Basic equations

General solution: Involves Laplace transformation, algebraic manipulation and some substitutions ( , , d’s and ƒ’s are functions of the rate constants).

Model parameterisation Initial conditions: Approximation 1 (organism is a faster accumulator than the medium): Approximation (organism holds less activity than the medium):

Consequences Biphasic release: Simple formulae for all the model constants:

Calculation of "x" If we know the % retained at time  (f 100 ) : If we know when the release curve closes in to slope of the final phase (factor f ):

Sensitivity analysis

Basis for the dosimetry Same as EA R&D 128 and FASSET (aquatic)

Model inputs

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

Model Outputs Reference organisms Phytoplankton Zooplankton Macrophyte Winkle Benthic mollusc Small benthic crustacean Large benthic crustacean Pelagic fish Benthic fish Nuclides 99 Tc 125 I, 129 I & 131 I 134 Cs & 137 Cs 238 Pu, 239 Pu & 241 Pu 241 Am Weighted and un-weighted external and internal doses and activity concentrations within biota produced

Validation 99 Tc activity in lobsters: comparison with model by Olsen and Vives i Batlle (2003) 129 I activity in winkles: comparison with model by Vives i Batlle et al. (2006)

Results - Long term assessment Pu benthic mollusc - T B1/2 = 474 days Tc large benthic crustacean - T B1/2 = 56.8 & 114 days Benthic mollusc Dynamic model Annual time steps

Results - Short term assessment Tc in macrophytes - T B1/2 = 1.5 & 128 days Daily time steps

Results - Short term assessment Tc in winkles - T B1/2 = 142 days Daily time steps

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.

Conclusions Successfully production of a dynamic model that makes assessments to biota more realistic Simple, user-friendly spreadsheet format similar to R&D 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

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 99 Tc in lobsters (Homarus gammarus) from the West Cumbrian coast. J. Environ. Radioactivity 67(3):

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