Keff results significantly different from MCNP

Validation and Verification
1

Hi,

I’m doing criticality calculations for a model consisting of a sphere of U-7wt%Mo enriched at 20 at.% surrounded by water (I’m attaching the input file). The resultant keff is 0.52281 when running with OpenMC, and 0.43593 when running with MCNP. Both models use the same cross-section data libraries, number of particles, and number of active and inactive batches. Any suggestions why is there such big difference in the keff? I have tried running for more a critical system by increasing the enrichment, and difference is still large ( ~9%). Increasing the number of particles, and inactive and active batches by a factor of 10 does not make any difference either.

Thanks,

Andrea Saltos

U-7Mo.py (1.38 KB)

2

Hi Andrea,

Generally we get very good agreement between MCNP and OpenMC when using the same model / data. Would you be able to share your MCNP input file as well? I can try running both to see if I see the same behavior, and also check if there’s anything in the input files that pops out at me as being different.

Thanks,
Paul

3

Hi Paul,

Thanks for your response and help. I’m attaching my MCNP input file. I’m using ENDF/B-VII.1. Please let me know if you need anything else.

Best,

Andrea Saltos

input-mcnp-U7Mo (2.09 KB)

4

Hi Andrea,

Your water definition in the MCNP input appears to be wrong; you have 1 atom of H and 2 atoms of O! Turning those around should give you better agreement with OpenMC.

Cheers,
Paul