DIFFER seminar: Multi-scale modelling of molten salt fuel system

Multi-scale modelling of molten salt fuel systems: from structure to thermodynamic properties (Anna Smith, TUDelft)

Abstract
Molten salts are received increasing attention worldwide as key materials for sustainable and low-carbon energy technologies, including for fission reactors, and in particular for the Molten Salt Reactor (MSR) designs. Because of their appealing thermo-physical properties (e.g. low melting and high boiling points, low vapour pressure, thermal stability, high heat capacity and thermal conductivity etc.), molten salts are considered in MSRs both for the fuel and coolant. Fluoride and chloride salts are to this date the two main candidates, depending on the main objective pursued for the envisaged nuclear reactor, e.g. actinide burner, thorium breeder etc. Typical compositions are LiF-ThF₄-UF₄ or NaCl-MgCl₂-PuCl₃. During irradiation, numerous fission products are also generated, making the fuel a very complex multi-components system, that shows non-ideal thermodynamic behaviour at the temperatures used in the reactor (800-1000 K). The use of multiscale models and computer simulation codes able to predict performance under normal operation and accidental conditions is critical for the safety analysis and reactor operation.

Because of the challenges inherent to the work with molten salt materials, which are hygroscopic, highly corrosive at high temperatures, and in the case of nuclear applications radioactive, the available knowledge on the physico-chemical properties is still far from complete. Among these, the local structure properties of the salts at high temperature are particularly relevant, as the formation of short-range order is observed in the liquid, which has a direct impact on the transport (e.g. viscosity, thermal conductivity) and excess thermodynamic properties.

In this presentation, I will discuss the structure properties and trends observed in trivalent and tetravalent actinide containing salts, based on our most recent work and state-of-the-art knowledge in the literature. I will also present a new modelling approach of the molten salt properties from a microscopic (local structure) to macroscopic (thermodynamic properties) scale, that integrates information from (i) in-situ high temperature Extended X-ray Absorption Fine Structure spectroscopy measurements of the local structure of the melt, (ii) phase diagram equilibria and (iii) excess thermodynamic properties, together with (iv) the output of molecular dynamics simulations. I will illustrate with results obtained in our group on LiF-BeF₂, AF-MF₄ (A=Li, Na, K, and M = Zr, Th, U), and NaCl-UCl₃ systems, looking at the behaviours in molecular and polymeric types of liquids.