Thermochemical investigation of molten fluoride salts for Generation IV nuclear applications - an equilibrium exercise

Abstract

The concept of the Molten Salt Reactor, one of the so-called Generation IV future reactors, is that the fuel, a fissile material, which is dissolved in a molten fluoride salt, circulates through a closed circuit. The heat of fission is transferred to a second molten salt coolant loop, the heat of which drives the generator and turbines to produce electricity. The salt is periodically purified and refuelled on-site, without interrupting the reactor cycle, so that very high burn-ups can be reached, theoretically. Other advantages are the efficient power production, because of the high temperature of the salt and the small fissile inventory needed to start the cycle. Typical inlet and outlet temperatures are 565 °C, respectively 700°C. Knowledge of the thermodynamic properties of the fluoride salt mixtures is of major importance for the reactor design. But there are innumerable possible compositions and it would be impossible to measure the properties of all of them. The aim of this PhD project was to assess the chemical systems of various salt compositions of interest with aid of a thermodynamic software package and where possible, to provide experimental data to test and improve the thermodynamic models that describe and predict the phase behavior of the system. Assessing a system means that known thermodynamic data, like the heat capacity function, the enthalpy and entropy of formation of the components of interest are collected and carefully selected. Extra information is added in the form of available experimental data, from literature as well as from own work. These data are used to optimize the unknown data, which are most of the time the excess Gibbs energy parameters, which are necessary to define the shape of a phase diagram. A complete internally consistent thermodynamic database has been obtained, including the data from own thermal and calorimetric experiments. From this database, phase diagrams were calculated and the phase behavior of the molten salt mixture in a given temperature range could be predicted. In addition, physicochemical properties that are interesting for the reactor design, such as vapor pressure, density and viscosity, were included in the model, enabling to make precise predictions on the behavior of the molten salt in an operating reactor.