Themes: Geological Sciences, Earth, Universe
location: Research Laboratory for Radionuclide Transfer in Aquatic Ecosystems (LRTA) - Cadarache (13)
Master degree in Geosciences, Geochemistry, Environmental Chemistry, Water Sciences
Age limit: 26 years old unless otherwise stated.
The assessment of the propagation of liquid effluents discharged into aquatic environments by the nuclear industry is a major issue for the management of water resources, the quality of environments and communication to the media and to the population. It also contributes to the expertise of authorized discharges, current and future dismantling operations as well as that of accident situations. To assess medium and long-term fate scenarios, the calculation codes must combine hydro-sedimentary models and solid / liquid fractionation of effluents. For the latter, it is the distribution coefficient (Kd) that is most often used, it corresponds to the activity in the solid phase over that in the liquid phase. Most of the work in rivers assumes that this equilibrium is instantaneous and reversible, but several recent studies show that a model with two reversible stages would probably reproduce the observations better. This question of characterizing these kinetics, and the geochemical (types of particles) or physical (grain size) parameters that control them, is now essential to be able to better predict the behavior of radionuclides in natural waters. The thesis will be based on adsorption / desorption experiments conducted in laboratory (Cadarache site) for radionuclides of scientific and operational interest (released by the NPPs): Cs, Co, Mn, Ag. These experiments will be carried out on samples of suspended matter and sediments from the Rhône, Loire and Garonne rivers, which are or will be sampled. The adsorption kinetics will be measured on time scales ranging from an hour to over a month. Other stable elements (Cu, Zn) could be studied in order to have a more complete approach, as they have a particular affinity for organic matter. The data obtained will be integrated into two types of dynamic models which will be tested: 1) the E-K model which describes the liquid / solid exchanges by a fraction in instantaneous equilibrium and a dynamic fraction and 2) a model with two dynamic fractions. The latter may be more suited to situations with rapid kinetics such as near-field adsorption of liquid discharges. The two approaches will allow a comparison of these models. The candidate will have to set up and dimension the experiments to feed these two concepts by taking into account several influencing variables determined experimentally and by thermodynamic modeling. Among these variables, the particle size and the nature of the particles will be integrated, and for this each sample will be characterized geochemically (clays, organic matter, Fe, Al and Mn contents, mineralogy) and physically. The work of the doctorate will focus primarily on carrying out experiments and acquiring data, with modeling work being complementary.