OpenMC Homogenous Cell Test verification

User Support
1

Greetings dear community,

I am testing the homogenous function of “openmc.Material.mix_materials()” and comparing it with the results of manually defined mix material. and the results differ significantly.

The test is:
cube cell of 1.26 cm in length (reflective surfaces) with 2 materials:
H2O
UO2 (4.5% U235)

isotopes used
H1, H2, O16, O17, O18, U238, U234, U235 (4.5%)
all the isotopes are of natural abundance composition except U235

the first test I defined each material separately and then used the openmc function.
the second test i defined one material incorporating the composition of the two materials with updating the density, w%, a% of each isotope respective to the new state of matter as a mixture.

Results are:
OpenMC function: K = 1.43 ,
Manually: K = 1.2

am I missing something? or should I consider extra elements when defining the homogenous mixture in Openmc? or it just doesn’t run properly if I defined it manually?

I would pretty much appreciate the help.

2

Hi Bushnag, welcome to community.
I think you can check the atomic density of each isotopes calculated by openmc mix material and compared it with your calculated (manual) atomic density.
Try using the get_nuclide_atom_densities(), get_mass() or get_mass_density()
https://docs.openmc.org/en/latest/pythonapi/generated/openmc.Material.html#openmc.Material

You can start from something simple at first, maybe just mixing some isotopes instead of nuclide, then nuclide with natural abundance, and then an enriched one.
I think it might came from the difference between what we think openmc did and what actually have been done inside openmc.

3

Thank you for your response. :smile: I will check this and try solving the issue. i’ll do as you suggested.
thanks again!

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