Strange values of Keff With c_U_in_UO2

User Support
1

Hello,

I am running a model with LEU UO2 fuel element. When I add thermal scattering for uranium in the UO2, I am getting strange values of Keff which I can’t understand. This is the output that I receive:

With c_U_in_UO2 :

============================> RESULTS <============================

k-effective (Collision) = 1.26846 +/- 0.00045

k-effective (Track-length) = 1.26892 +/- 0.00063

k-effective (Absorption) = 0.98081 +/- 0.00033

Combined k-effective = 0.78211 +/- 0.01401

Leakage Fraction = 0.00000 +/- 0.00000

Without c_U_in_UO2 :

============================> RESULTS <============================

k-effective (Collision) = 1.00567 +/- 0.00035

k-effective (Track-length) = 1.00589 +/- 0.00052

k-effective (Absorption) = 1.00623 +/- 0.00032

Combined k-effective = 1.00613 +/- 0.00032

Leakage Fraction = 0.00000 +/- 0.00000

Looking forward to discussing it. Thanks.

2

Hi Afaf,

These results certainly look strange. Can you provide a little more information about your setup? What version of OpenMC are you running, and where did you obtain the nuclear data library that is being used for the calculation?

For what it’s worth, I’m not aware of many people who actually use the S(a,b) table for U in UO2 even for systems with a thermal spectrum.

Best regards,
Paul

3

Hi Paul,

I use version 0.10.0.

This is the picture of my setup :

I have recompiled OpenMC (with MPI) and HDF5.

Thanks a lot.

Version_OpenMC.png
Version_OpenMC.png863×687 58.5 KB

Afaf

4

Are you using multiple S(a,b) in any materials? There is a known bug in version 0.10.0 when using multiple S(a,b) tables. This has since been fixed and will be in our next release.

Best,
Paul

Version_OpenMC.png
Version_OpenMC.png863×687 58.5 KB

5

Yes , in the same model , I use c_Be , c_H_in_H2O , c_O_in_UO2 and c_Al27 .