Fig. 1: Schematic of the primary radiation damage process in a volume of heterogeneous material subject to a neutron flux ϕN.
From: Spatial distribution of primary radiation damage in microstructures

Every region of space (voxel 1) yields a PKA flux which travels and creates damage in other regions of the material that lie within a given radius. One such voxel in that range is highlighted (voxel 2), in this case residing across an interface in another phase.