Many industrial processes involve grinding and milling operations to produce powders of well-controlled particle size distribution. Improving these processes requires researchers to consider particle-particle interactions, breakage, aggregation, and in the case of wet milling, hydrodynamics. The purpose of this study is to develop a numerical model, accounting for these mechanisms, with the final goal to guide the design of an optimal process. Computational fluid dynamics is coupled with a population balance model. The hydrodynamics of the multiphase flow is predicted using the multifluid Euler approach, whereas the class method is considered to solve the population balance equation. A theoretical definition of the breakage functions, based on hydrodynamics inside the tank, is proposed. The results show that breakage is highly heterogeneous inside the tank and that particle breakage frequency allows researchers to evaluate milling efficiency.