Recent developments for molten salt systems in Thermochimica

The modified quasi-chemical model in the quadruplet approximation has been implemented in the open-source equilibrium thermodynamics library Thermochimica, enabling single point equilibrium calculations and sophisticated multi-physics simulations of molten salt nuclear reactor systems. Here, the derivations necessary to obtain the chemical potentials of the quadruplet species required for Gibbs energy minimization are provided. The implementation is verified via code-to-code benchmarking against FactSage. A scheme to increase computational efficiency of multi-physics simulations including Thermochimica is described and its effectiveness for molten salt systems demonstrated. Finally, a multi-physics simulation of a molten salt nuclear fuel system is presented as a demonstration problem: ORIGEN-S is used to calculate the isotopic evolution of a fuel-loaded FLiBe mixture with fission and activation products as Thermochimica predicts the phase evolution and number of moles of Cs in various phases.     

Experimental reinvestigation and thermodynamic description of Bi-Te binary system

The Bi-Te phase diagram was determined by equilibrium alloy method, combined with electron probe microanalysis (EPMA), X-ray diffraction (XRD) and thermal analysis (DSC). The experimental result shows that there is a β-phase with a large composition range at low temperature, while Bi₂Te and Bi₄Te₃ are relatively stable in the solid-liquid region. A consistent phase diagram that covers the experimental findings has been achieved. Based on the new experimental phase diagram, coupling with the reported thermodynamic data, the thermodynamic optimization of the Bi-Te binary system was carried out with the help of CALPHAD approach. A group of reasonable thermodynamic parameters was obtained.     

Assessment of atomic mobility for BCC Ti-Mn and Ti-Al-Mn alloys

The atomic mobility in BCC Ti-Mn and Ti-Al-Mn alloys was assessed by using the DICTRA software after critically evaluating all available experimental diffusion data. Good agreements were obtained from the comprehensive comparisons between the calculated and experimental diffusion coefficients. The interdiffusion behaviour was further well predicted, confirming the reliability of the present mobility parameters.     

Geometric acceleration of complex chemical equilibrium calculations — Performance in two- to five-component systems

Incorporating multicomponent, multiphase, complex chemical equilibrium calculations into process and multiphysics models can provide significant insights into industrial processes that current modelling or measurements cannot. Equilibrium calculations are however, in general, omitted or incorporated in a simplified manner due to their computational expense. Several methods have been developed to accelerate these calculations.A new accelerator algorithm was developed (Roos and Zietsman, 2021) based on phase diagram geometry, the Gibbs phase rule, and the lever rule to include equilibrium calculations into models more efficiently. This framework of established thermochemical theory provides a sound basis for discretisation and interpolation, and allows the accelerator algorithm to work in systems with any number of components. The work presented here aimed to test accelerator performance and demonstrate that it has the capability of achieving noteworthy levels of acceleration while maintaining acceptable accuracy.The accelerator was tested on ten 2-component systems, four 3-component systems, a simplified 4-component ilmenite smelting system, and a simplified 5-component iron- and steelmaking system. As the number of system components increased, so did the computational expense of direct equilibrium calculations. This translated to larger acceleration factors for higher-order systems — from 20 in 2-component systems to 1000 in the 5-component system. In a small number of cases it was observed that the acceleration factor was smaller than one during interpolation. This was attributed to slow searching times for suitable interpolation cells from the database.Phase composition interpolation errors are less than 1 $\times$ 10^-2 mol mol⁻¹. This translates to an interpolated phase composition being accurate to within 99 % of the calculated composition and results in phase fraction errors of 1 $\times$ 10^-2 and less. In a very small number of cases the interpolation errors made on physical and thermochemical properties are as high 10 %. This is because system properties are calculated as a phase fraction weighted sum of phase properties and errors made on system properties can therefore become large due to interpolation errors being made twice. However, the majority of errors made on physical and thermochemical properties are in the order of 1 % and less. The level of accuracy achieved by the accelerator algorithm was acceptable for the chosen discretisation tolerances.