The photochemical isotope separation schemes have renewed the detailed investigations of gas phase photo physics and photochemistry of UF₆. One of the schemes is based on massive multiphoton excitation of UF₆ using one or more suitable IR laser frequency in 16 µm region in its ground electronic state. In the present work we have modelled such Infrared Multiphoton Dissociation (IRMPD) on a rate equation formalism. Using RRKM theory, the multiphoton dissociation rate constants for UF6 with 16µm laser have been estimated. It is shown that about 50 laser photons, which are 15 photons excess over the dissociation threshold of 3 eV, are required for dissociation of each UF6 molecule. Using the present model, the fluence dependence of dissociation yield has been evaluated and it is found that MPD of UF₆ occurs with threshold fluence of I-1.2 J cm². Isotopic selectivity in this model is described as the difference in the rate of laser energy bsorption for the two isotopic species. Considering various time scales involving excitation, collision al deactivation and life time of energized 235-UF₆ molecule, the extent of selectivity loss has been estimated.