# Cannot have fissionable material in a fixed source run

**URL:** <https://openmc.discourse.group/t/cannot-have-fissionable-material-in-a-fixed-source-run/136>\
**Category:** User Support\
**Created:** [March 4, 2015, 2:56pm UTC](https://openmc.discourse.group/t/cannot-have-fissionable-material-in-a-fixed-source-run/136 "2015-03-04T14:56:04Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![josh\_sayer](https://avatars.discourse-cdn.com/v4/letter/j/f6c823/32.png) [@josh\_sayer](https://openmc.discourse.group/u/josh_sayer)\
**Post date:** [March 4, 2015, 2:56pm UTC](https://openmc.discourse.group/t/cannot-have-fissionable-material-in-a-fixed-source-run/136/1 "2015-03-04T14:56:04Z")

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**hello,**  
**I want to use uranium 235 in the fixed source, but it gives this message (Can not fissionable material-have in a fixed source run) .**  
**HOW WE CAN SOLVE THIS PROBLEM**  
thank you

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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:** [March 4, 2015, 7:15pm UTC](https://openmc.discourse.group/t/cannot-have-fissionable-material-in-a-fixed-source-run/136/2 "2015-03-04T19:15:19Z")

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This capability was added in version 0.6.2:  
[http://mit-crpg.github.io/openmc/releasenotes/notes\_0.6.2.html](http://mit-crpg.github.io/openmc/releasenotes/notes_0.6.2.html)

Note that currently in fixed source problems, fission is just treated as absorption, i.e. no fission neutrons are created.

Best regards,  
Paul