Thermodynamic Studies of Cesium and Iodine in Molten Salt Systems: CALPHAD Modeling of the (Na,Cs,Mg,Nd,Pu)(Cl,I) System

A thermodynamic model of the molten salt system (Na,Cs,Mg,Pu,Nd)(Cl,I) has been developed in this work to assess the effect of CsI on the melting and vaporization behavior of the nuclear fuel in molten salt reactors. Investigation using X-ray diffraction (XRD) and differential scanning calorimetry (DSC) of the binary systems NdCl3-NdI3 and MgI2-NdI3, as simulant systems for the analogous Pu system, is presented for the first time. Both systems were found to be binary eutectic systems, with solid solutions NdCl3-3xI3x (hexagonal in space group P63/m) and NdCl3yI3-3y (orthorhombic in space group Cmcm) in the NdCl3-NdI3 system, and Mg1-xNdxI2+x (hexagonal in space group P 3 m1) in the MgI2-NdI3 system. Additionally, the system CsI-MgI2 was scrutinized using DSC, confirming the available experimental data in the literature. Furthermore, the investigation of the reciprocal diagonals in the systems (Na,Nd)(Cl,I), (Cs,Mg)(Cl,I), (Mg,Nd)(Cl,I), and (Cs,Nd)(Cl,I) is presented, allowing the characterization of the quaternary behavior of these salts. Based on the experimental data obtained in this work, a CALPHAD model is presented using the quasi-chemical formalism in the quadruplet approximation for the liquid solution. With the aim of modeling the complete (Na,Cs,Mg,Pu)(Cl,I) system, the binary systems NaCl-CsCl, CsCl-MgCl2, CsCl-NdCl3, NaI-CsI, and CsI-NdI3 were reassessed based on data from the literature. Furthermore, a CALPHAD model of the PuCl3 and PuI3 systems is also presented using Nd as a simulant for Pu in molten halide salts. With the developed thermodynamic models, calculations were finally performed to assess the fission product retention of Cs and I in a molten chloride environment. As opposed to their behavior in molten fluorides, the fission products are well retained in the fuel matrix up to concentrations of at least 5 mol%.