grain boundaries or core shell structure

[GreenBird12]

Hi, everyone
can neper generate the geometry like the figure ,where there is boundary thickness between grains. And in the article ,it suggests that The GB phase is the space that is created by shrinking every grain with respect to its Voronoi generating point and subsequently subtracting all of them from the original cube. In the same manner, the shell is created as the space between the original and the shrunk polyhedral grain.
The article is https://doi.org/10.1063/1.4943058
Thanks