# Warnings: Cannot reflect particle 751 off surface in a lower universe **URL:** \ **Category:** User Support\ **Created:** [December 3, 2018, 2:41am UTC](https://openmc.discourse.group/t/warnings-cannot-reflect-particle-751-off-surface-in-a-lower-universe/377 "2018-12-03T02:41:41Z")\ **Posts on this page:** 2\ **Page:** 1
**Author:** ![Bin\_Deng](https://avatars.discourse-cdn.com/v4/letter/b/d26b3c/32.png) [@Bin\_Deng](https://openmc.discourse.group/u/Bin_Deng)\ **Post date:** [December 3, 2018, 2:41am UTC](https://openmc.discourse.group/t/warnings-cannot-reflect-particle-751-off-surface-in-a-lower-universe/377/1 "2018-12-03T02:41:41Z")
Hello everyone: I met some troubles when I used OpenMC. Following are the messages when I run the “openmc.run()” on the jupyter notebook: `Warnings: Cannot reflect particle 751 off surface in a lower universe Warnings: Cannot reflect particle 251 off surface in a lower universe Warnings: Cannot reflect particle 501 off surface in a lower universe Warnings: Cannot reflect particle 1 off surface in a lower universe Warnings: Cannot reflect particle 752 off surface in a lower universe ...... ERROR:Maximum number of lost particles has been reached.` I don’t know what’s worry about it and I checked my case, I don’t think there was something wrong with it. Does anybody has the similar experience? Any reply will be appreciated! Best regards! Yocy ---
**Author:** ![Bin\_Deng](https://avatars.discourse-cdn.com/v4/letter/b/d26b3c/32.png) [@Bin\_Deng](https://openmc.discourse.group/u/Bin_Deng)\ **Post date:** [December 3, 2018, 3:16am UTC](https://openmc.discourse.group/t/warnings-cannot-reflect-particle-751-off-surface-in-a-lower-universe/377/2 "2018-12-03T03:16:15Z")
Thanks all, I have found the reason. Because of I didn’t give the “region” for the root\_cell 在 2018年12月3日星期一 UTC+8上午10:41:41，yin…@gmail.com写道：