Two or more universes are not used as fill universes

User Support
1

Hi all,

I model the core using geometry.xml, and I have many types of fuel assemblies.
Each assembly is a universe. However, I do not install all the assemblies in the core
at once. I change the fuel assemblies by modifying the hex_lattice of the core.
But when running openmc, it prompts 'Two or more universes are not used as
fill universes, so it is not possible to distinguish which one is the root universe.
’ How to solve this problem?

2

Hi Dawn, welcome to the community.
If you are directly editing your geometry from geometry.xml, I think you can add <!-- --> so that the not-used cells universe can be treated as comments.
I think that could help you to select which cells and universe be seen by the openmc.
Have you tried it?

<cell id= used cell definition ..... universe="x"/>
...

<!--
<cell id= not used cell definition ..... universe="y"/>
<cell id= not used cell definition ..... universe="y"/>
<cell id= not used cell definition ..... universe="y"/>
-->

<cell id= used cell definition ..... universe="z"/>
<lattice> fill universes definition using x and z universes </lattice>
...