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 NdCl_3–3xI_3x (hexagonal in space group P6₃/m) and NdCl_3yI_3–3y (orthorhombic in space group Cmcm) in the NdCl₃–NdI₃ system, and Mg_1–xNd_xI_2+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%.