# Cannot reflect particles in lower surfaces

**URL:** <https://openmc.discourse.group/t/cannot-reflect-particles-in-lower-surfaces/2529>\
**Category:** User Support\
**Created:** [December 5, 2022, 4:18pm UTC](https://openmc.discourse.group/t/cannot-reflect-particles-in-lower-surfaces/2529 "2022-12-05T16:18:28Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![Openmc\_beginner](https://avatars.discourse-cdn.com/v4/letter/o/65b543/32.png) [@Openmc\_beginner](https://openmc.discourse.group/u/Openmc_beginner)\
**Post date:** [December 5, 2022, 4:18pm UTC](https://openmc.discourse.group/t/cannot-reflect-particles-in-lower-surfaces/2529/1 "2022-12-05T16:18:28Z")

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Hello everyone,  
I’m new to openmc and trying to build a simulation with Triso particles and graphite matrix. I tried multiple of times to run the simulation but I am receiving an error ‘maximum number of lost particles has been reached’. I try to fix by dealing with the geometry but then the error change to ‘more than 95% of external source…’ and so on  
Anyone has any suggestions?

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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:** [December 7, 2022, 5:18am UTC](https://openmc.discourse.group/t/cannot-reflect-particles-in-lower-surfaces/2529/2 "2022-12-07T05:18:00Z")

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Hi @Openmc_beginner and welcome to the community! You can find some advice on troubleshooting geometry problems in the [user’s guide](https://docs.openmc.org/en/latest/usersguide/troubleshoot.html#error-maximum-number-of-lost-particles-has-been-reached). Take a look through that and then if you’re still having trouble, feel free to ask more questions.