# Difference between default source.energy value and Watt spectrum?

**URL:** <https://openmc.discourse.group/t/difference-between-default-source-energy-value-and-watt-spectrum/3138>\
**Category:** User Support\
**Created:** [June 23, 2023, 11:39am UTC](https://openmc.discourse.group/t/difference-between-default-source-energy-value-and-watt-spectrum/3138 "2023-06-23T11:39:39Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Thomas](https://avatars.discourse-cdn.com/v4/letter/t/e36b37/32.png) [@Thomas](https://openmc.discourse.group/u/Thomas)\
**Post date:** [June 23, 2023, 11:39am UTC](https://openmc.discourse.group/t/difference-between-default-source-energy-value-and-watt-spectrum/3138/1 "2023-06-23T11:39:39Z")

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Hello,

I am doing some testing in order to use OpenMC and I am wondering what’s the differences between the calculations done when i comment or uncomment the last line of this source definition:

> uniform\_dist = openmc.stats.Box(lower\_left, upper\_right, only\_fissionable=True)  
> energy = openmc.stats.Watt(a=0.988, b=2.249) # defaut extracted from  
> [7. Execution Settings — OpenMC Documentation](https://docs.openmc.org/en/stable/usersguide/settings.html)  
> src = openmc.Source(space=uniform\_dist)  
> src.energy=energy

uncommented the run time is around 10 minutes  
commented, the run time is around 51 minutes

Is there any explanation for these differences ?

More important, there is differences between results too !  
When comparing doses for the 2 calculations, the one without watt spectrum is according to the calculations done with eigenvalues whereas with the watt spectrum, values are (at best) 12 times lower.

Thanks for any explanations.  
Thomas

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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:** [June 23, 2023, 12:04pm UTC](https://openmc.discourse.group/t/difference-between-default-source-energy-value-and-watt-spectrum/3138/2 "2023-06-23T12:04:42Z")

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Hi @Thomas and welcome to the community! The [`openmc.stats.Watt` class](https://docs.openmc.org/en/stable/pythonapi/generated/openmc.stats.Watt.html) accepts those two arguments in units of eV and 1/eV respectively. So, if you want to mimic the default, you would really need to write:

```python
energy = openmc.stats.Watt(a=0.988e6, b=2.249e-6)

```

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**Author:** ![Thomas](https://avatars.discourse-cdn.com/v4/letter/t/e36b37/32.png) [@Thomas](https://openmc.discourse.group/u/Thomas)\
**Post date:** [June 23, 2023, 12:17pm UTC](https://openmc.discourse.group/t/difference-between-default-source-energy-value-and-watt-spectrum/3138/3 "2023-06-23T12:17:02Z")

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Thanks for the answer… it’s so obvious !!