# Keff results significantly different from MCNP

**URL:** <https://openmc.discourse.group/t/keff-results-significantly-different-from-mcnp/528>\
**Category:** Validation and Verification\
**Created:** [December 17, 2019, 1:26am UTC](https://openmc.discourse.group/t/keff-results-significantly-different-from-mcnp/528 "2019-12-17T01:26:38Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![andrea\_saltos\_rodrig](https://avatars.discourse-cdn.com/v4/letter/a/a698b9/32.png) [@andrea\_saltos\_rodrig](https://openmc.discourse.group/u/andrea_saltos_rodrig)\
**Post date:** [December 17, 2019, 1:26am UTC](https://openmc.discourse.group/t/keff-results-significantly-different-from-mcnp/528/1 "2019-12-17T01:26:38Z")

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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](https://openmc.discourse.group/uploads/short-url/vIzXeHK6cpNxG7FaRwm8uW0oDdV.py) (1.38 KB)

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**Author:** ![paulromano](https://yyz2.discourse-cdn.com/free1/user_avatar/openmc.discourse.group/paulromano/32/486_2.png) [@paulromano](https://openmc.discourse.group/u/paulromano)\
**Post date:** [January 5, 2020, 3:12pm UTC](https://openmc.discourse.group/t/keff-results-significantly-different-from-mcnp/528/2 "2020-01-05T15:12:10Z")

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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

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**Author:** ![andrea\_saltos\_rodrig](https://avatars.discourse-cdn.com/v4/letter/a/a698b9/32.png) [@andrea\_saltos\_rodrig](https://openmc.discourse.group/u/andrea_saltos_rodrig)\
**Post date:** [January 5, 2020, 10:30pm UTC](https://openmc.discourse.group/t/keff-results-significantly-different-from-mcnp/528/3 "2020-01-05T22:30:15Z")

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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](https://openmc.discourse.group/uploads/short-url/lF3uV8IS83bDwIFsi6z1ci49kKY) (2.09 KB)

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**Author:** ![paulromano](https://yyz2.discourse-cdn.com/free1/user_avatar/openmc.discourse.group/paulromano/32/486_2.png) [@paulromano](https://openmc.discourse.group/u/paulromano)\
**Post date:** [January 6, 2020, 3:08pm UTC](https://openmc.discourse.group/t/keff-results-significantly-different-from-mcnp/528/4 "2020-01-06T15:08:04Z")

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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