I have a long core design of cylindrical fuel pins that I wanted to segment axially by defining shorter fuel pins then stacking them in a 3D lattice with no gaps between layers as if they were contigous fuel rods. However, when I do this it cuts off the bottom half of each z-layer, leaving an undefined gap (there are supposed to be seven segmentations). When I only do a 2D lattice without changing the universe being used it works fine. Yes, my z pitch matches the height of the fuel pin segment and the lower_left definition is correct. This is the lattice construction I am using where num_segs is the 7 stacks:
core_lattice.universes = [[[f for _ in range(n)] for _ in range(n)] for _ in range(num_segs)]
I just got through the same issue with a HexLattice, so I think I know what is going on here. Everything worked in a 2D lattice, then when I went to 3D I was missing fuel.
I’d want to see your model generation script for sure, but my hunch is that is has to do with the universe definition you are filling into the 3D lattice. The key is the origin (0,0,0) of the universe you define and fill into the lattice gets mapped to the center of the lattice element.
Let’s define the lattice thickness as dz and the height of the assembly as h. I’m guessing you are currently defining a pin universe from 0 to h and filling that into the 3D lattice. What is happening is that the lattice element is chopping your universe from -dz/2 to dz/2, which explains why the bottom half of each lattice element is empty. You need to define a pin universe from -dz/2 to dz/2 and then you will see seven full regions when that is filled into your lattice.