137 CS Desorption within an estuary: modelling and experiments

International audienceWorld nuclear production will increase sharply until 2035 and most of the planned reactors will be located along a river or by the sea. Any accidental release of radionuclides into the aquatic environment may thus reach the ocean, and the need to anticipate its consequences remains more significant than ever. IRSN has developed a chain of models for simulating the behavior of a dissolved 137Cs release on a river-sea continuum. Two independent models for the fluvial and marine environment were coupled using a box-model describing the hydrodynamic and geochemical estuarine processes affecting the 137Cs within an estuary. The study site was the Rhone River, bordered by 4 nuclear power plants and with a stratified estuary. The box model connects the flows from the river to the sea and takes into account desorption processes. Field data were used to represent hydrodynamic processes and for model validation. Laboratory experiments were performed to specify the role of salinity in the intensity and kinetics of 137Cs desorption. The salinity occurring when freshwaters meet marine systems induces a desorption of 137Cs from the particles, starting above 3-4 PSU and increasing inversely with the duration of the adsorption phase (ageing effect). A dynamic exchange model involving 2 sites of different affinities onto the particles was set to predict the distribution of 137Cs, and the role of ageing effect is also presently tested for other radionuclides (110Ag, 54Mn, 60Co) and preliminary results will be presented. This process will be critical in the event of very low river flow when a salt-wedge can go up the river for tens of kilometers. This can favor desorption of 40% of the particulate 137Cs, thus modifying its potential impact both in the coastal zone and in the terminal part of the river