An attempt to model the nucleation of fragment boundaries during cold plastic deformation of fc.c. metals is presented. The paper focuses on intrinsic nucleation in the grain interior due to elementary processes on the dislocation level. Since orientation fragmentation seems to be linked to slip banding and the underlying mechanisms should be the same, the model is based on the elementary process of double cross-slip. Simulations were carried out for Cu, Ni and Al. Fragment boundary spacings and misorientations could be predicted in reasonable agreement with experiment for Cu. For Ni, comparable results were obtained, when a stacking fault energy at the lower end of the range of literature data was chosen. The resulting rate equation for the generation of partial disclinations as carriers of orientation fragmentation can be implemented into an earlier model for the coupled substructure and texture development during cold plastic deformation.