The transverse flow of cohesive powders in rotary kilns equipped with lifters was studied experimentally and theoretically. A laboratory device was built up in which the flow of uranyl difluoride (UO2F2), uranium sesquioxide (U3O8) and uranium dioxide (UO2) powders was filmed, recorded and analyzed using partly manual image analysis techniques. Experiments were performed both at room temperature and at high temperature. A constitutive law describing the powder discharge was derived, involving a relationship between the volume fraction of powder contained in a lifter and the angular position of this lifter. This law based on geometrical calculations is successfully compared with the experimental results of unloading. (C) 2013 Elsevier B.V. All rights reserved.