# ERROR: More than 95% of external source sites sampled were rejected. Please check your external source's spatial definition

**URL:** <https://openmc.discourse.group/t/error-more-than-95-of-external-source-sites-sampled-were-rejected-please-check-your-external-sources-spatial-definition/6381>\
**Category:** User Support\
**Created:** [June 27, 2026, 6:24am UTC](https://openmc.discourse.group/t/error-more-than-95-of-external-source-sites-sampled-were-rejected-please-check-your-external-sources-spatial-definition/6381 "2026-06-27T06:24:52Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![s\_rahman\_wave](https://avatars.discourse-cdn.com/v4/letter/s/b5ac83/32.png) [@s\_rahman\_wave](https://openmc.discourse.group/u/s_rahman_wave)\
**Post date:** [June 27, 2026, 6:24am UTC](https://openmc.discourse.group/t/error-more-than-95-of-external-source-sites-sampled-were-rejected-please-check-your-external-sources-spatial-definition/6381/1 "2026-06-27T06:24:52Z")

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[full .py](https://openmc.discourse.group/uploads/short-url/zf9TDrChayc3BPOvg6YkkYBBSRm.py) (25.0 KB)

i dont getting where is the issue,

Packing TRISO particles in fuel zone…  
Placed 64 TRISO particles (PF=0.050248)  
Packing pebbles in active bed (R=90 cm, H=125.881 cm)…

/opt/nuke/lib/python3.12/site-packages/openmc/model/triso.py:1188: UserWarning: Close random pack converged before reaching true sphere radius; some spheres may overlap. Try reducing contraction rate or packing fraction.  
warnings.warn('Close random pack converged before reaching true ’

Placed 17277 pebbles (PF=0.61)  
Geometry exported.

Fuel pebbles : 9848  
Moderator pebbles: 7429  
Total pebbles : 17277  
Active bed : R 0–90.0 cm, Z 258.182–384.063 cm (H = 125.881 cm)

Running OpenMC criticality calculation…

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**Author:** ![munukkj1](https://avatars.discourse-cdn.com/v4/letter/m/90db22/32.png) [@munukkj1](https://openmc.discourse.group/u/munukkj1)\
**Post date:** [July 6, 2026, 10:01am UTC](https://openmc.discourse.group/t/error-more-than-95-of-external-source-sites-sampled-were-rejected-please-check-your-external-sources-spatial-definition/6381/2 "2026-07-06T10:01:16Z")

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Hi!

You can try removing the “only\_fissionable” parameter from your code. There’s probably little fissionable material available, which makes the sampling inefficient. For criticality calculations this does not matter, since the idea is that the source converges to a realistic distribution during the inactive batches. You are running your simulation with inactive batches so this shouldn’t be a problem.

If you haven’t already solved the problem I can try to take a look at your geometry too later.

Best, J