# Nbatches, particles when using surf\_source\_read

**URL:** <https://openmc.discourse.group/t/nbatches-particles-when-using-surf-source-read/3425>\
**Category:** User Support\
**Created:** [September 13, 2023, 10:34am UTC](https://openmc.discourse.group/t/nbatches-particles-when-using-surf-source-read/3425 "2023-09-13T10:34:39Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![josem](https://yyz2.discourse-cdn.com/free1/user_avatar/openmc.discourse.group/josem/32/4096_2.png) [@josem](https://openmc.discourse.group/u/josem)\
**Post date:** [September 13, 2023, 10:34am UTC](https://openmc.discourse.group/t/nbatches-particles-when-using-surf-source-read/3425/1 "2023-09-13T10:34:39Z")

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Hi,  
Two very basic question that puzzle me:

1. Why OpenMC uses ‘nbatches’ and ‘particles’ in fixed source mode instead of a single number for the simulated histories (as it is done in MCNP or PHITS)?

2. How do ‘nbatches’ and ‘particles’ work when surf\_source\_read is used to read particles from a previously created surface\_source.h5 file. In this case, source is not used and it seems that tallies now are normalized to the number of source particles in surface\_source.h5 (is it?). How ‘nbatches’ and ‘particles’ numbers are used in this case?

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**Author:** ![simonfouc120](https://yyz2.discourse-cdn.com/free1/user_avatar/openmc.discourse.group/simonfouc120/32/4114_2.png) [@simonfouc120](https://openmc.discourse.group/u/simonfouc120)\
**Post date:** [June 23, 2025, 4:18pm UTC](https://openmc.discourse.group/t/nbatches-particles-when-using-surf-source-read/3425/2 "2025-06-23T16:18:01Z")

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Hello,  
I’m stuck at the same point I think.  
Now I set one batch and N particules with 0 inactive batches (obviously) and It works.  
The problem is : with this settings, I don’t have standard deviation on my tally..  
It’s a problem when It’s a complex geometry and you must know if your tally is under a percent.