Source rejection fraction error when using distribmat

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1

Hi there,

This post is closely related to my previous post related to modeling depletion of VVER-440 fuel assembly with burnable absorber .

I found that when modeling fuel pins with Gd it’s recommended to apply distribmat property to the depletion model:

model = openmc.Model(geometry=geom, settings=settings)
model.differentiate_mats(diff_volume_method='divide equally', depletable_only=True)

Trying to do that I encountered the following error:

 Maximum neutron transport energy: 20000000 eV for H1
 Initializing source particles...
 ERROR: Too few source sites satisfied the constraints (minimum source rejection
        fraction = 0.05). Please check your source definition or set a lower
        value of Settings.source_rejection_fraction.
 ERROR: Too few source sites satisfied the constraints (minimum source rejection
        fraction = 0.05). Please check your source definition or set a lower
        value of Settings.source_rejection_fraction.
Abort(-1) on node 0 (rank 0 in comm 0): application called MPI_Abort(MPI_COMM_WORLD, -1) - process 0

for settings.particles = 10_000 and increasing the number of source particles to settings.particles = 100_000 does not help.

Additionally, I set the material of pins which contain Gd to be depletable via the corresponding setter:

gd2o3.depletable = True

and

uo2_gd2o3 = openmc.Material.mix_materials([uo2, gd2o3], [1.0 - val / 100.0, val / 100.0],
                                            'wo', name=r'uo2 enrw% enriched + valw% gd2o3')
uo2_gd2o3.temperature = temperatures_dict['uo2_xxx']
uo2_gd2o3_rho = dens # g/cm3
uo2_gd2o3.set_density('g/cm3', uo2_gd2o3_rho)
uo2_gd2o3.depletable = True

Fuel pins with Gd are divided into a set of concentric cylinders as it’s recommended.
Boundary conditions are specified to be periodic for side surface of the hex. assembly (the entire assembly!) model and reflective for top and bottom surfaces.

Could you please hint me how can I get rid of the error and solve the problem?
If it’s necessary to provide more details about the model, please let me know!

P.S. There are 4 different types of pin in that assembly profile. One of them is filled with uo2 with some enrichment enr, and actually the BA pins material also has the same enrichment (and Gd of course). So, I used the same material instance for the both types of cell. May it cause the error?

Cheers,
Ihor