# Keff values vs expect values

**URL:** <https://openmc.discourse.group/t/keff-values-vs-expect-values/1955>\
**Category:** User Support\
**Created:** [April 20, 2022, 2:15pm UTC](https://openmc.discourse.group/t/keff-values-vs-expect-values/1955 "2022-04-20T14:15:53Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![zhaozelong](https://yyz2.discourse-cdn.com/free1/user_avatar/openmc.discourse.group/zhaozelong/32/1385_2.png) [@zhaozelong](https://openmc.discourse.group/u/zhaozelong)\
**Post date:** [April 20, 2022, 2:15pm UTC](https://openmc.discourse.group/t/keff-values-vs-expect-values/1955/1 "2022-04-20T14:15:53Z")

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Hi，here is my python script for modeling the SNAP-10A core with openmc0.13.0 and ENDF/B-VIII.0 library，but I get a strange keff value of 0.02， the expect keff should be about 1.00，where is the problem？

Thanks！  
[8490a.py](https://openmc.discourse.group/uploads/short-url/lgmWkvI4idvnljvWFyZc7ijFaT0.py) (16.2 KB)  
[8490a\_input\_MCNP.xml](https://openmc.discourse.group/uploads/short-url/xxu8hv4pvrcTzqmLxFyi1UzsA0m.xml) (6.8 KB)  
[8490a\_out.xml](https://openmc.discourse.group/uploads/short-url/gHgYPQrmHtMcm8bEbryOKIZiB6q.xml) (2.2 MB)

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**Author:** ![pshriwise](https://yyz2.discourse-cdn.com/free1/user_avatar/openmc.discourse.group/pshriwise/32/3863_2.png) [@pshriwise](https://openmc.discourse.group/u/pshriwise)\
**Post date:** [April 21, 2022, 1:59am UTC](https://openmc.discourse.group/t/keff-values-vs-expect-values/1955/2 "2022-04-21T01:59:30Z")

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Hi @zhaozelong,

I pulled down the model and gave it a try. When I ran OpenMc with the `-g` flag (`openmc -g`), it indicated that cells 100 and 1 were overlapping. You’ll want to resolve that issue in the geometry definition and try again.

Hope this helps,

-Patrick

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**Author:** ![zhaozelong](https://yyz2.discourse-cdn.com/free1/user_avatar/openmc.discourse.group/zhaozelong/32/1385_2.png) [@zhaozelong](https://openmc.discourse.group/u/zhaozelong)\
**Post date:** [April 21, 2022, 4:45am UTC](https://openmc.discourse.group/t/keff-values-vs-expect-values/1955/3 "2022-04-21T04:45:00Z")

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That’s a very helpful message，I will try to modified and fixed it. Besides， I add the MCNP input and output file，the results of MCNP is 1.0052. Hope this is helpful.