Abstract
Molten chloride salts are ionic liquids in which the anions and cations exhibit network formation. An attractive salt system for use in molten salt reactors is NaCl-UCl3, an ionic liquid with complex non-ideal thermodynamic behaviour due to the formation of short-range order. The relationship between local structure and thermodynamic properties is investigated in this work, in which molecular dynamics simulations using the Polarizable Ion Model (PIM) and thermodynamic modelling by means of the CALPHAD method are combined. The system is simulated for a wide range of temperatures and compositions and various properties are derived derived from molecular dynamics data: density/molar volume, thermal expansion, heat capacity and excess properties including excess molar volume, mixing enthalpy and excess heat capacity. Generally, there is good agreement with previously published experimental data. An in depth analysis of the local structure of the liquid is performed for multiple temperatures. This analysis demonstrates the transition from a molecular liquid consisting of primarily Na+, Cl−, UCl63-/UCl74- at low UCl3 content to a polymeric liquid at high UCl3 content, manifesting itself in the formation of species like U2Cl126-, U3Cl178-, U4Cl2210- etc. Exceeding 40% UCl3, the liquid consists of a three-dimensional network of corner or edge-sharing uranium polyhedra. The output of the MD simulations and experimental data are incorporated into a coupled structural-thermodynamic model for the NaCl-UCl3 system based on the quasi-chemical formalism in the quadruplet approximation, that provides a physical description of the melt and reproduces (in addition to the thermodynamic data) the chemical speciation of uranium polymeric species predicted from the simulations.
Highlights
Chloride salts have attracted an increasing interest in recent years as fuel and coolant system for the generation of nuclear fission Molten Salt Reactors (MSRs), in particular for designs with a fast spectrum
A coupled structural-thermodynamic description of the melt is made following the principles described by Smith et al [12] to reflect the fact that mixtures rich in UCl3 (>40% UCl3 fraction) are found to be ‘‘polymeric”, meaning that the melt entirely consists of species with a nucleation of more than 4
Using molecular dynamics simulations with the polarizable ion model, several thermo-physical properties of the NaCl-UCl3 salt system have been determined, namely density/molar volume, thermal expansion, heat capacity, and excess properties including excess molar volume, mixing enthalpy and excess heat capacity
Summary
Chloride salts have attracted an increasing interest in recent years as fuel and coolant system for the generation of nuclear fission Molten Salt Reactors (MSRs), in particular for designs with a fast spectrum. UCl3 serves as the primary fissile material, while NaCl is an attractive carrier salt due to its high solubility for actinides [4], availability and low cost. The chemical composition of the fuel continuously changes, as the fission process introduces new reactive dissolved, precipitated or gaseous species, depending on local thermodynamic equilibrium conditions. The MSR salt system is a complex multi-component system, that shows complex non-ideal thermodynamic behaviour at the high temperatures used in the reactor [4].
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