The simulation determined a very important parameter, the so-called coordination number (CN), which is the average number of ions surrounding a given ion. CNs of small ions, Li and Na, in FLiNaK are similar to those in binary salts. But the CNs of K and Cs are much larger in FLiNaK than in binaries. In KF, the CN of K is 5.3 while in FLiNaK it is 7.64. We found that for Cs, the CN ⇡ 10. This number is significantly larger than that deduced from NMR measurements in CsF-LiF mixtures. The discrepancy is likely the e↵ect of admixture of progressively larger Na and K ions. Also, the coordination of Cs in solid and molten CsF is close to 6, 48 while in solid CsLiF 2 , Cs is bonded to 8 F atoms. It will be interesting to compare the CNs of Cs in other important fuel salts such as FLiBe and NaCl, and with dissolved UCl 3 (since Cs as a fission product, it will be present at about 1 mol% level together with large U ions). To the best of our knowledge, these CNs have not been reported.