TRISO particle outside of lattice


I have a reactor model that is edited from the VHTR example model available on GitHub.

When increasing the TRISO particle packing factor above 0.30, I run into a warning:

“UserWarning: TRISO particle is partially or completely outside of the lattice.”

It seems to me that the issue is from the sphere packing algorithm placing a coordinate outside of the accepted lattice region.

I have attached the section of code responsible for this below. Any help is appreciated.

kernelsph = openmc.Sphere(r=250e-4) # try 212.5um and 250um
buffsph = openmc.Sphere(r=350e-4)
IPyCsph = openmc.Sphere(r=390e-4)
SiCsph = openmc.Sphere(r=425e-4)
OPyCsph = openmc.Sphere(r=470e-4)
triso_outer_radius = 470e-4
layers = [kernelsph, buffsph, IPyCsph, SiCsph, OPyCsph]
triso_mats = [fuel, buffer_mat, IPyC, SiC, OPyC]
triso_cells =
for i in range(5):
if (i == 0):
triso_cells.append(openmc.Cell(fill=triso_mats[0], region=-layers[0]))
triso_cells.append(openmc.Cell(fill=triso_mats[i], region=+layers[i-1] & -layers[i]))
triso_universe = openmc.Universe(universe_id=1001,cells=triso_cells)

cylsurf = openmc.ZCylinder(r=r_pellet)
maxz_pin = openmc.ZPlane(z0=+pellet_height/2)
minz_pin = openmc.ZPlane(z0=-pellet_height/2)
maxz = openmc.ZPlane(z0=+core_height/2)
minz = openmc.ZPlane(z0=-core_height/2)
maxz_refl = openmc.ZPlane(z0= core_height/2+top_refl_height, boundary_type=‘vacuum’)
minz_refl = openmc.ZPlane(z0=-core_height/2-bot_refl_height, boundary_type=‘vacuum’)
lattice_region = -cylsurf & -maxz_pin & +minz_pin
spheres = openmc.model.pack_spheres(radius=triso_outer_radius, region=lattice_region, pf=0.35)
triso_particles = [openmc.model.TRISO(triso_outer_radius, fill=triso_universe, center=c) for c in spheres]
lattice_cell = openmc.Cell(region=lattice_region)
lower_left, upp_right = lattice_cell.region.bounding_box
shape = (4, 4, 4)
pitch = (upp_right - lower_left)/shape
triso_latt = openmc.model.create_triso_lattice(triso_particles, lower_left, pitch, shape, graphite)
lattice_cell.fill = triso_latt
lattice_universe = openmc.Universe(universe_id=1002,cells=[lattice_cell,lattice_cell_outer])

Hi,i think you can plot this geometry, then you may find some problems,I 've had the same problem before, probably caused by the inconsistency of the lattice and the cell 's outer boundary shapes

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I would agree with the previous suggestion (plotting is always helpful), although staring at your code for a little bit, I don’t see any obvious reason why a TRISO particle would end up outside the lattice since the lattice by definition should cover the region that was used when generating the TRISOs. @averyarnone If you’re able to share a full script, I can try to reproduce the behavior you’re seeing in order to give more specific advice.

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