Difference between default source.energy value and Watt spectrum?

User Support
1

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
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

2

Hi @Thomas and welcome to the community! The openmc.stats.Watt class 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:

energy = openmc.stats.Watt(a=0.988e6, b=2.249e-6)
3

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