# k-Eigenvalue Filter

**URL:** <https://openmc.discourse.group/t/k-eigenvalue-filter/525>\
**Category:** User Support\
**Created:** [December 11, 2019, 7:16am UTC](https://openmc.discourse.group/t/k-eigenvalue-filter/525 "2019-12-11T07:16:17Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![Beneatha](https://avatars.discourse-cdn.com/v4/letter/b/c4cdca/32.png) [@Beneatha](https://openmc.discourse.group/u/Beneatha)\
**Post date:** [December 11, 2019, 7:16am UTC](https://openmc.discourse.group/t/k-eigenvalue-filter/525/1 "2019-12-11T07:16:17Z")

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Hi! I am a new OpenMC user. Could someone help me with the information that,

Is it possible to calculate the k-eigenvalue with only prompt neutron fission?

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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, 3:09pm UTC](https://openmc.discourse.group/t/k-eigenvalue-filter/525/2 "2020-01-05T15:09:52Z")

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Right now we don’t have any way of doing this directly. I’ve had numerous people ask about this though, usually in the context of trying to estimate beta-effective. I’ve previously put together [a patch](https://gist.github.com/paulromano/5d4ccfb9f32e96e5c288239640fa4139) which changes the behavior of the code to only use prompt neutrons. To use it, you would need to need to get the OpenMC git repository, apply the diff on the ‘develop’ branch, and compile/reinstall the code.

Best,  
Paul

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**Author:** ![Beneatha](https://avatars.discourse-cdn.com/v4/letter/b/c4cdca/32.png) [@Beneatha](https://openmc.discourse.group/u/Beneatha)\
**Post date:** [January 7, 2020, 7:29am UTC](https://openmc.discourse.group/t/k-eigenvalue-filter/525/3 "2020-01-07T07:29:01Z")

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Thank you sir.

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**Author:** ![anas](https://yyz2.discourse-cdn.com/free1/user_avatar/openmc.discourse.group/anas/32/1194_2.png) [@anas](https://openmc.discourse.group/u/anas)\
**Post date:** [May 1, 2022, 10:44am UTC](https://openmc.discourse.group/t/k-eigenvalue-filter/525/4 "2022-05-01T10:44:27Z")

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Hello paul,

Sorry, i not understand yet how to apply “prompt” method in openMC practically. Could you explain more detail about this?

Thanks,  
Anas