# Error in the VHTR Example (Running Simulation) **URL:** \ **Category:** User Support\ **Created:** [October 20, 2024, 12:41pm UTC](https://openmc.discourse.group/t/error-in-the-vhtr-example-running-simulation/4902 "2024-10-20T12:41:33Z")\ **Posts on this page:** 2\ **Page:** 1
**Author:** ![Arcsin](https://avatars.discourse-cdn.com/v4/letter/a/59ef9b/32.png) [@Arcsin](https://openmc.discourse.group/u/Arcsin)\ **Post date:** [October 20, 2024, 12:41pm UTC](https://openmc.discourse.group/t/error-in-the-vhtr-example-running-simulation/4902/1 "2024-10-20T12:41:33Z")
Hello! I am new to OpenMC and am working on the examples, I have completed the pincell, hexagonal lattice, and triso fuel examples. I wanted to try out the VHTR reactor example [Jupyter Notebook Viewer](https://nbviewer.org/github/mit-crpg/openmc-reactor-examples/blob/main/VHTR.ipynb) Everything was going well however, when I tried to run the xml exported I encounter this prompt: More than 95% of external source sites sampled were rejected. ![image](https://global.discourse-cdn.com/free1/uploads/openmc/original/2X/0/0fa9c67520edd90cfbacd17687e06dfab4d66598.png) I tried some to change the boundaries by looking at the geom.plot(), and geom.plot(basis=‘xz’) and it didn’t work. As a last resort, I changed the only\_fissionable=True to only\_fissionable=False which made the simulation work. But I’m not sure if this is correct. Any thoughts? Thank you in advance! ---
**Author:** ![Arcsin](https://avatars.discourse-cdn.com/v4/letter/a/59ef9b/32.png) [@Arcsin](https://openmc.discourse.group/u/Arcsin)\ **Post date:** [October 20, 2024, 12:46pm UTC](https://openmc.discourse.group/t/error-in-the-vhtr-example-running-simulation/4902/2 "2024-10-20T12:46:45Z")
Found a related topic after posting [95% External Source Sites Rejected - TRISO Model - User Support - OpenMC](https://openmc.discourse.group/t/95-external-source-sites-rejected-triso-model/4549) Which said that removing the fissionable=true can be done. If there’s a mistake with this method, it would be greatly appreciated. But for now I think its resolved