Redissolution and long-term transport of radionuclides released from a contaminated sediment: a numerical modelling study

A numerical model based upon residual circulation has been developed to simulate the long-term (years) transport of non conservative radionuclides in the English Channel. The transfer of radionuclides between water, suspended matter and bottom sediments has been described in terms of kinetic transfer coefficients, so that the model can be applied in situations out of equilibrium. The model has been used to study and compare the effects of direct discharges of radionuclides, the case of a continuous release and the redissolution of radionuclides from a contaminated sediment. These numerical experiments have been carried out for two radionuclides with a different geochemical behaviour: 137Cs and 239,240Pu. It has been found that natural equilibrium in the partition of radionuclides between the liquid and solid phases is reached in the case of instantaneous releases and in the case of redissolution from a contaminated sediment. However, in the case of a continuous release such partition remains out of natural equilibrium. Also, the behaviour of 137Cs and 239,240Pu is similar, but the time scales of the processes are clearly different due to the different affinities of both radionuclides to remain fixed to the solid phases.