Molten-Salt Nuclear Reactors (MSR) promise a safer and more e cient operation than existing light water and pressurized water reactors. Some MSR designs use fuel salts, which contain dissolved nuclear fuel. As the MSRs operate, new chemical species are created in the molten salt due to nuclear fission, transmutation, and nuclear decay products. Changing chemical composition leads to changes in key physical properties of the melts such as density, viscosity, etc. In addition, fission products are distributed between all three phases -solid, liquid, or gas. Some fission products dissolve in the melt while some do not. The fission product Cesium (Cs) is of particular concern due to health e↵ects of radioactive Cs if released into the environment. Therefore, the phase diagram of Cs-containing salts in general and the solubility of Cs, in particular, are of immediate interest for MSR developers. It is believed that Cs forms mostly two compounds, soluble CsF and less soluble CsI (iodine is another fission product that should be contained to avoid dangerous health e↵ects). Given the importance of Cs and its potential e↵ects on the salt properties, predictive modeling of its behavior is necessary to ensure safe operation of MSRs. The modeling requires detailed knowledge of the molecular structure and speciation for all salt components. Our measurements and modeling of the molecular structure of Cs in molten LiF-NaK-KF eutectic (FLiNaK) is a fundamental first step in this direction.