A dynamic model for radionuclide transfer from water to freshwater fish

A dynamic model for radionuclide transfer from water to fish is presented, from its design stages to its in situ validation. Four steps are proposed in order to apply the model, for predictive purposes, to 137 Cs and 106 Ru in case of the Rhone (S.E. France), downstream of the Marcoule fuel reprocessing plant. The first step consists of an experimental laboratory study conducted on a species which is representative of the second order trophic level (Cyprinus carpio L.). Compartmental analysis then allows construction of the conceptual model of the organism. The number of compartments necessary to represent the organism and the kinetic parameters quantifying the transfer studied are estimated using a standardized procedure for the statistical processing of results. A numerical method which allows consideration of all the fluctuations recorded in the radionuclide concentration in the water during the exposure phase is shown. Direct 137Cs transfer is modeled on the basis of a single compartment, characterized by the exchange kinetics constants A2=0.224 d-1 and λ2=0.0065 d-1. For106 Ru, two compartments are necessary in order to model the exchange kinetics. They are characterized by the constants A1=1.319 d-1 and λ1=0.638 d-1, A2=0.198 d-1 and λ2=0.0261 d-1. For a theoretical fish having zero growth, and for constant concentration in water, these transfer kinetics lead to concentration factor values of 34 L Kg w.w.-1 for 137Cs and 10 L Kg w.w.-1 for 106Ru. The corresponding long biological half- lives are 106 days and 27 days. A different transfer model expression is necessary based on the radionuclide tissue distribution differences within the organism. The major distribution of the 137Cs in muscle which is a growth target tissue, implies building the conceptual transfer model on the assumption of the proportionality between the mass of water intervening in the transfer and the mass of the organism. For the 106Ru preferentially accumulating in a low growth organ (digestive tract), the basic assumption is that of the intervention of a constant mass of water in the transfer. The entire model, i.e. the basic assumption on the influence of the growth of the organism, the number of compartments, the kinetic parameters, was validated by adapted in situ experimentation. The validation tested with a monthly time step was satisfactory for the two radionuclides.