[umd]

Hello all, I am having trouble with creating meshes for two nuclear reactor model cases.

Meshing software: GMSH
OpenFOAM version: 1906
Solver: GeN-Foam (Generalised Nuclear Foam)

I have two simple hexagonal meshes representing a simple reactor core geometry. There are six fuel assemblies surrounding a central control rod assembly, with a ring of reflector assemblies around the outside. The only difference between the two meshes is that the first has a single fuel region, and the second is split into ten axial layers. The fuel heights are the same for both cases.

My issue is that when I create these meshes and convert them into OpenFOAM format using gmshToFoam, the single-fuel case ignores internal faces not on the boundary of the polymesh while the layered-fuel case keeps these internal faces. This means that the single-fuel case only has a wall boundary around the outside of the geometry (which is what I want), but the layered-fuel case has internal walls around the control assembly and the outside of the fuel (which I do not want).

These internal walls cause a problem because they appear to be reflecting neutrons. This is fine when they're on the outside (single-fuel case) as this is what I want the reflector assemblies to do. But when they're internal (layered-fuel case) they stop the neutrons passing through to different assemblies or interacting with the control rod altogether. See the pictures below.

Screenshot 2023-07-22 at 15.54.23.png

Screenshot 2023-07-22 at 15.53.31.png

I have tried defining the internal walls specifically in the layered-fuel case and setting a zeroGradient boundary condition, but this seems to reflect the neutrons rather than allow them to pass through.

Can anyone suggest either:

- A boundary condition I can use for the internal walls to make them transparent to neutrons;

- A way I can make the layered-fuel case ignore internal faces in the same manner as the single-fuel case?