# 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:** 1\
**Showing post:** 2

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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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