A model-based methodology is proposed to describe fragmentation of polymer particles during the very early stages of heterogeneous olefin Polymerization reactions. The method is based on the analysis of the particle capacity to release the amount of energy that is accumulated in its interior during the polymerization, due to the fast polymer production.. The balance between the rates of energy accumulation and dissipation inside the polymer particle is calculated with the help of a dynamic mathematical model of the prepolymerization reaction. The combination. of the presented fragmentation criteria and of the dynamic model may allow one to design the prepolymerization step more precisely and to predict the morphology of the final polymer particles as a function of the initial catalyst particle diameter, reactor temperature, reactor pressure and concentration of active sites. Numerical examples are presented for gas and slurry propylene polymerizations.