Mix_materials not giving the same atomic concentration as expected

User Support
1

Hello, everyone.
This is my code:

Fuel Element [UO2 - 3.6 wt% U-235]

FE_Z36 = openmc.Material(name = “FE-Z36”, temperature = 1073 )
FE_Z36.add_nuclide(‘U235’, 0.036)
FE_Z36.add_nuclide(‘U238’, 0.964)
FE_Z36.add_nuclide(‘O16’, 2.0)
FE_Z36.set_density(‘g/cc’, 10.2)

Gadolina Gd2O3

GD = openmc.Material(name = “Gadolina”, temperature = 1073 )
GD.add_element(‘Gd’, 2.0)
GD.add_element(‘O’, 3.0)
GD.set_density(‘g/cc’, 7.407)

Mixture [3.6wt% U235 and 5wt% Gd2O3]

FUG_Z36G5 = openmc.Material.mix_materials([FE_Z36, GD], [0.96, 0.04], ‘wo’)

when I go to materials.xml file, I find the following ‘ao’:

‘U235’, 0.011409959604026973
‘U238’, 0.3055333627300556
‘O16’, 0.6633886644668167
‘Gd152’, 3.9336026398201764e-05
‘Gd154’, 0.00042876268774039926
‘Gd155’, 0.0029108659534669307
‘Gd156’,0.0040260423018559505
‘Gd157’, 0.003078044065659288
‘Gd158’, 0.00488553447865666
‘Gd160’, 0.004299427685323453

However, this is the atomic concentration given in VVER-1000 benchmark by OECD/NEA:
235U 7.2875E-4
238U 1.9268E-2
16O 4.1854E-2
152Gd 2.5159E-6
154Gd 2.7303E-5
155Gd 1.8541E-4
156Gd 2.5602E-4
157Gd 1.9480E-4
158Gd 3.0715E-4
160Gd 2.6706E-4

Can anyone help me with this?

2

material.add_nuclide and material.add_element accept an argument for percent_type which defaults to ‘ao’ for atom fraction. You might want to change this to ‘wo’ for weight fraction.

For example this line

FE_Z36.add_nuclide(‘U238’, 0.964)

Would hecome

FE_Z36.add_nuclide(‘U238’, 0.964, percent_type=‘wo’)

3

I did try that argument several times but there isn’t any significant change