Dependence of radionuclide transfer factor on growth stage for a soil-lettuce plant system

An experiment was performed at the Nuclear Power Plant of Garigliano (Caserta, Italy) to study the transfer of radionuclides from a contaminated soil to a lettuce crop and to check the validity of the generally accepted assumption of constant uptake. Measurements of specific activities of 137Cs, 60Co, and 40K were periodically performed by sampling plants during a growth period of 48 days. The specific activity was analysed as a function of plant mass and growth time to gain information on the dependence of radionuclide sorption on plant growth stage. The results (time-dependent uptake rate) were interpreted using a dynamic model which generalizes the transfer factor. In this model, the time dependence of the transfer factor is characterized by three parameters obtained from a non linear least-squares fit to the data. The resulting values were found to be dependent on the radionuclide. The relative growth rate of lettuce plants plays an important role in the transport process: maximum radionuclide transport occurs at 12 d for 60Co and 137Cs and 28 d for 40K.