ASSESSING THE SPECIATION AND THE BIOGEOCHEMICAL PROCESSES AFFECTING THE MOBILITY OF SELENIUM FROM A GEOLOGICAL REPOSITORY OF RADIOACTIVE-WASTES TO THE BIOSPHERE

In the scope of the studies carried out on the geological disposal of radioactive wastes, it is essential to assess the migration behaviour of long lived radionuclides as a function of the physicochemical condi tions (pH, redox potential, temperature, pressure, ...), the nature of the host rocks and the chemical composition of the underground waters . In this study, we have considered the case of selenium which is an e lement of importance because of its long lived under its Se-79 isotope . A review is first performed on the behaviour of selenium from availa ble studies in natural systems in considering particularly the inorgan ic Se forms. Later on, these results are transposed to the physicochem ical conditions occurring in the vicinity of a geological repository. The most stable forms in this context would be Se(-II), Se(0) and Se(I V). Several parameters can govern the mobility of these Se species suc h as the pH, the potential of the water and the presence of solid phas es containing iron, manganese or aluminium (oxy)hydroxides on which Se (IV) can bz sorbed. Selenide and selenite mobility can also be retarde d by precipitation reactions. This is particularly true for Se(-II) in presence of iron(II) and sulfides. The hypothetical presence of micro organisms is also considered because of its importance on the fare of the Se species.