Material composition problem: set density will change the keff

User Support
1

Hi, all.
I am new to OpenMC. I am compute the depletion of an ADS reactor. When I am defining the fuel (UO2 with enrichment 19.75%wt), I found that the keff will change a lot depending by whether I set the density or not.

The code and output are shown by the following:

CODE
Fuel1.add_element(‘U’,1.0,enrichment=19.75, enrichment_type=‘wo’)
Fuel1.add_element（‘U’，1.0，富集=19.75， enrichment_type=‘wo’）
Fuel1.add_nuclide(‘O16’,2.0)
Fuel1.add_nuclide（‘O16’，2.0）
Fuel1.set_density(‘g/cc’,10.3)
Fuel1.set_density（‘g/cc’，10.3）
OUTPUT
times, keff_values
[0. 0.49828884 0.99657769 1.49486653 1.99315537 2.49144422
2.98973306 3.4880219 3.98631075 4.48459959 4.98288843] [[0.66624459 0.01595817]
[0.66128416 0.01650991]
[0.64560125 0.01037565]
[0.65625062 0.02210639]
[0.63653018 0.00953156]
[0.65091137 0.00894526]
[0.64361347 0.01101912]
[0.63798934 0.01286221]
[0.611911 0.00759741]
[0.62561687 0.01704075]
[0.63742919 0.00570189]]

If I cancel the density setting, the keff is about 1.4.

times, keff_values
[0. 0.49828884 0.99657769 1.49486653 1.99315537 2.49144422
2.98973306 3.4880219 3.98631075 4.48459959 4.98288843] [[1.44597721 0.01460487]
[1.48501885 0.01394671]
[1.44553072 0.03997172]
[1.45520222 0.01787604]
[1.48518239 0.00925756]
[1.46586324 0.00577243]
[1.47380762 0.0156218 ]
[1.46359057 0.00436498]
[1.48135794 0.01335844]
[1.4676961 0.00766751]
[1.45886235 0.00438374]]

However, the paper says keff should be around 0.85, so whether I set the density, the keff is wrong.

By the way, the keff is not decreasing, it’s ok?

:cry: I don’t understand why. Does anybody know it?

Thanks.

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2

Hi @chentiqi ;

I think that in your case, if you don’t set the density there is a default value to be taken by OpenMC: 5. Material Compositions — OpenMC Documentation

Set the density of the material

Parameters

  • units ({‘g/cm3’ , ‘g/cc’ , ‘kg/m3’ , ‘atom/b-cm’ , ‘atom/cm3’ , ‘sum’ , ‘macro’}) – Physical units of density.
  • density (float, optional) – Value of the density. Must be specified unless units is given as ‘sum’.

this means that the default value of your material is the sum of two units (which is the default “ao” here) = 3, and in count this is a huge value of atoms per b.cm

I hope that this will help you to solve this issue

Regards

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3

:ok_hand:Thank you very much! I will check the density for other material.

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