Lithium-based ceramics in the form of pebble beds are used as tritium breeder materials in future fusion reactors . The Drucker–Prager (D-P) model is appropriate for predicting the macroscopic thermomechanical behaviour of granular pebbles. We developed a numerical framework to estimate these model parameters using the Discrete Element Method (DEM). These model parameters controlled the shape and size of the initial yield envelope. DEM was used to perform triaxial compression simulations to extract these model parameters, which can then describe the constitutive relationship of a homogenized finite element model of pebble beds. The impact of varying inter-particle friction and the initial packing fraction on the D-P conical yield surface has also been studied. The results show that the packing fraction dominates the granular behaviour when compared to inter-particle friction for packing fractions exceeding 60%. The effect of friction coefficient was weak for a given packing fraction. Further detailed analysis of the mesoscopic mechanics was conducted by examining various parameters that governed the microstructural similarity of the sample and extent of particle participation. It is concluded that, while the global Drucker-Prager (DP) parameter may be insensitive to µ, the evolution of the morphology during failure and the extent of localization during deformation can vary.