WSC Radioecology Research Group

1. 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

2. 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

3. 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.

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

5. 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).

6. 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.

7. Biokinetic database Water activity Dosimetry database No Yes Calculate 2 rate constants from TB1/2s At least 1 TB1/2 known? 2 TB1/2 known? No Calculate initial conditions of the system Yes No No Slope transition known % retention known? Yes Yes Refresh initial conditions of new time step using solution form previous step Calculate remaining rate constants for basic model (2 components) Calculate remaining rate constants for advanced model (3 components) Refresh initial conditions of new time step using solution form previous step Apply npn-dynamic model using CF Run a loop for series of regular time steps Write results into the spreadsheet Flow diagram

8. Basic equations

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

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

11. Consequences • Biphasic release: • Simple formulae for all the model constants:

12. 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 ):

13. Sensitivity analysis

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

15. Model inputs

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

17. 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

18. 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)

19. 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

20. Results - Short term assessment Daily time steps Tc in macrophytes - TB1/2 = 1.5 & 128 days

21. Results - Short term assessment Daily time steps Tc in winkles - TB1/2 = 142 days

22. 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.

23. 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

24. 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. Radioactivity67(3): 219-233.

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