A DYNAMIC-MODEL FOR RADIONUCLIDE TRANSFER FROM WATER TO FRESH-WATER FISH

A dynamic model for radionuclide transfer from water to fish is presen ted, from its design stages to its in situ validation. Four steps are proposed in order to apply the model, for predictive purposes, to Cs-1 37 and Ru-106 in case of the Rhone (S.E. France), downstream of the Ma rcoule fuel reprocessing plant. The first step consists of an experime ntal laboratory study conducted on a species which is representative o f the second order trophic level (Cyprinus carpio L.). Compartmental a nalysis then allows construction of the conceptual model of the organi sm. 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 resu lts. A numerical method which allows consideration of all the fluctuat ions recorded in the radionuclide concentration in the water during th e exposure phase is shown. Direct Cs-137 transfer is modeled on the ba sis of a single compartment, characterized by the exchange kinetics co nstants A(2)=0.224 d(-1) and lambda(2)-0.0065 d(-1). For Ru-106, two c ompartments are necessary in order to model the exchange kinetics. The y are characterized by the constants A(1)=1.319 d(-1) and lambda(1)=0. 638 d(-1), A(2)=0.198 d(-1) and lambda(2)-0.0261 d(-1). For a theoreti cal 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 Cs-137 and 10 L Kg w.w.(-1) for Ru-106. The correspondin g long biological half- lives are 106 days and 27 days. A different tr ansfer model expression is necessary based on the radionuclide tissue distribution differences within the organism. The major distribution o f the Cs-137 in muscle which is a growth target tissue, implies buildi ng the conceptual transfer model on the assumption of the proportional ity between the mass of water intervening in the transfer and the mass of the organism. For the Ru-106 preferentially accumulating in a low growth organ (digestive tract), the basic assumption is that of the in tervention of a constant mass of water in the transfer. The entire mod el, i.e. the basic assumption on the influence of the growth of the or ganism, the number of compartments, the kinetic parameters, was valida ted by adapted in situ experimentation. The validation tested with a m onthly time step was satisfactory for the two radionuclides.