The European consortium SACSESS aims at studying the safety of nuclear waste reprocessing techniques dedicated to the separation of actinides. One of the methods considered is called pyroprocessing. In this method, the fuel bars are immersed in a bath of molten salts, directly on the way out from the reactor. By applying a series of voltage differences to the electrodes, one would like to deposit the target elements at the electrodes in a selective fashion. While the separability of the ions is governed by thermodynamics and in particular by the ratio of their activity coefficients, knowledge of transport phenomena is also necessary for the implementation of this technique. In this work, we study by classical molecular dynamics, dynamic and structural properties of mixtures of molten chloride salts MCl3-(LiCl-KCl)eut (M3+=Y3+, Tb3+, La3+, U3+) or fluoride salts NF3/4-(LiF-AlF3)eut (N3+/4+ = Y3+, La3+,Th4+,U4+) in which the relative composition of the basic melt is kept at the eutectic ratio, 58.2:41.8% for the chlorides and 85:15/65:35% for the fluorides. The simulations are performed using a polarizable force field extracted from DFT simulations and therefore of ab initio accuracy. We calculate dynamic quantities such as the viscosity and the characteristic relaxation times of the systems and structural properties such as the radial distribution functions, the coordination numbers of the cations, the number of anions shared between two cations, etc., as a functions of the concentration of the MCl3/NF3/4 salt and of temperature. We also estimate the energy of the cation-anion bonds from the potential of mean force extracted from the radial distribution functions. We find that the average coordination number of the cations and the viscosity are anticorrelated with the energy of the cation-anion bonds. The most viscous systems are instead characterized by the presence of a high number of anions shared between two cations. This favors the the formation of an extend network able to resist shear stresses.

Structure and dynamics of mixtures of molten salts for the pyroprocessing of nuclear waste

250th National Meeting (2015)

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