# Problem with OpenMC Simulation Using Cubit Geometry – “More than 95% of external source sites sampled were rejected”

**URL:** <https://openmc.discourse.group/t/problem-with-openmc-simulation-using-cubit-geometry-more-than-95-of-external-source-sites-sampled-were-rejected/6045>\
**Category:** User Support\
**Created:** [October 10, 2025, 8:12pm UTC](https://openmc.discourse.group/t/problem-with-openmc-simulation-using-cubit-geometry-more-than-95-of-external-source-sites-sampled-were-rejected/6045 "2025-10-10T20:12:55Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![fakhar](https://yyz2.discourse-cdn.com/free1/user_avatar/openmc.discourse.group/fakhar/32/4224_2.png) [@fakhar](https://openmc.discourse.group/u/fakhar)\
**Post date:** [October 10, 2025, 8:12pm UTC](https://openmc.discourse.group/t/problem-with-openmc-simulation-using-cubit-geometry-more-than-95-of-external-source-sites-sampled-were-rejected/6045/1 "2025-10-10T20:12:55Z")

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I exported my geometry from Coreform Cubit to use in **OpenMC** , but when I run the simulation, I get repeated errors:  
ERROR: More than 95% of external source sites sampled were rejected.  
Please check your external source’s spatial definition.  
Abort(-1) on node 0 (rank 0 in comm 0): application called MPI\_Abort(MPI\_COMM\_WORLD, -1)  
Caught signal 11 (Segmentation fault)  
export dagmc “model.h5m” overwrite  
Could you please advise what might cause this issue?  
Is there a specific export tolerance or geometry cleanup step I should apply before exporting to ensure proper source sampling in OpenMC?

I’ve attached a screenshot of the terminal output for reference.

 ![11](https://global.discourse-cdn.com/free1/uploads/openmc/original/2X/2/2524368e72cefe5db05fc06146c27ec4329f5c9a.png)

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**Author:** ![makeclean](https://avatars.discourse-cdn.com/v4/letter/m/c5a1d2/32.png) [@makeclean](https://openmc.discourse.group/u/makeclean)\
**Post date:** [October 14, 2025, 9:44am UTC](https://openmc.discourse.group/t/problem-with-openmc-simulation-using-cubit-geometry-more-than-95-of-external-source-sites-sampled-were-rejected/6045/2 "2025-10-14T09:44:05Z")

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Could you share your inputs, ideally the python, xml and h5ms that would be needed to reproduce the problem?