Grain fragmentation is simulated by means of a three-dimensional discrete element approach called bonded-cell method (BCM). In this method, grains and potential fragments may have any polyhedral shape and size, capturing the geometrical complexity of brittle grain failure. As an application of this method, we present the uniaxial compaction of samples composed of several grains and we analyse the load-density relations, the grain size evolution, and the failure mechanism within the grains. This numerical approach permitted us to analyse the e↵ect of the grains internal strength on the macroscopic compaction behaviour and to study the evolution of the grain size distribution towards a power-law distribution as several experiments have shown in literature. Finally, we present a brief micro-mechanical analysis on the failure modes within the grains, letting us know the kind of stresses that prompts grain fragmentation.