Need help with modelling one-sixth of a core

Hi. I used OpenMC to model an HTGR core. When I tried to run the depletion analysis of the core, the kernel died. I realized that the calculation was too expensive to perform. So, I tried to simulate one-sixth of the core with rotational periodic boundary condition. I created the entire geometry and put it in the one-sixth portion of the volume. Since the problem remained the same, I think, the invisible materials, are still being depleted. Can anyone please give suggestions about how I can design only the one-sixth portion of the core? Or, how I can make my current model work?
Any advice will be appreciated. Thanks

@Tamim welcome to the community! You can explicitly set a material as being not depletable as follows:

mat = openmc.Material()
mat.depletable = False

Note that any material containing a fissionable nuclide will be depletable by default. In your case, what I would recommend is to ensure that only materials that are used in the 1/6th of your model that is “visible” are marked as depletable.

Thank you for your response. I used the same fuel material in the entire core. If I mark the material as not depletable, won’t it also set the visible part as not depletable? Is there any method I can use to mark only the invisible as not depletable?

That is correct — if you have the same material in the visible and invisible part of the geometry, marking it as not depletable would make it not depletable everywhere. You would need to ensure that you have a different fuel material used in the visible part of the core. If you have a lattice geometry, this would also imply that you need to have different universes used in the visible portion of the lattice.

Can I use any material in the place of fuel in the invisible part or will it have to be a fuel material? Do materials in the invisible part have any effect on the calculation? In my model, some assemblies have half of it in the visible part and the other half is invisible. As a result, I am having trouble using different universes in the visible portion of the lattice. What can I do in this case?
I have attached a picture of my model in case it helps. Thank you.

In principle, you can use any material in the invisible part. It won’t have much effect on the calculation since particles will never reach there, but it can have some side effects (for example, if there are nuclides present in the invisible part that are not present in the visible part, it will still result in their nuclear data being loaded).

I think it’s OK to have some universes that are only partially visible. One thing you’ll need to be careful about in this case is what volume you assign to depletable materials, making sure to only account for the volume of the material that is actually visible and sees a neutron flux.