# 95% of external source sites rejected

**URL:** <https://openmc.discourse.group/t/95-of-external-source-sites-rejected/5535>\
**Category:** User Support\
**Created:** [May 21, 2025, 12:31pm UTC](https://openmc.discourse.group/t/95-of-external-source-sites-rejected/5535 "2025-05-21T12:31:59Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Enigma374](https://avatars.discourse-cdn.com/v4/letter/e/d78d45/32.png) [@Enigma374](https://openmc.discourse.group/u/Enigma374)\
**Post date:** [May 21, 2025, 12:31pm UTC](https://openmc.discourse.group/t/95-of-external-source-sites-rejected/5535/1 "2025-05-21T12:31:59Z")

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Hi there. I’m trying to model a fusion reactor in OpenMC.

I get the following error message:  
ERROR: More than 95% of external source sites sampled were rejected. Please check your external source’s spatial definition.

My source code is as follows. I have 18 spheres whose centres lie on a ring of radius 700 in the xy plane. The spheres are located in void material (fill = None). My universe boundary edges are really far away (a z-cylinder with radius 1500, and 2 planes at z = ± 1500.

 ![image](https://global.discourse-cdn.com/free1/uploads/openmc/original/2X/8/81616fdf6d6c16d3a39ceeeae18ba59f96147f2f.png)

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**Author:** ![ligross](https://yyz2.discourse-cdn.com/free1/user_avatar/openmc.discourse.group/ligross/32/1653_2.png) [@ligross](https://openmc.discourse.group/u/ligross)\
**Post date:** [May 21, 2025, 4:42pm UTC](https://openmc.discourse.group/t/95-of-external-source-sites-rejected/5535/2 "2025-05-21T16:42:40Z")

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This is a very common issue ([see other Discourse posts with the same error message](https://openmc.discourse.group/search?q=95%25%20of%20external)) that likely has to do with the definition of the position of your source. You might have source locations in vacuum regions which can cause the issue

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**Author:** ![Enigma374](https://avatars.discourse-cdn.com/v4/letter/e/d78d45/32.png) [@Enigma374](https://openmc.discourse.group/u/Enigma374)\
**Post date:** [May 22, 2025, 1:18pm UTC](https://openmc.discourse.group/t/95-of-external-source-sites-rejected/5535/3 "2025-05-22T13:18:22Z")

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Thank you. I replaced some void materials with low density hydrogen, and this seems to have solved the issue!