Hello,
I am calculating the neutron flux spectrum in OpenMC using an energy filter. My simulation uses about 20 million particles, but the resulting neutron spectrum still exhibits significant oscillations, particularly in the resonance energy region (around 1–100 eV).
I expected the spectrum to be much smoother with this number of particles.
Is this behavior normal? Could it be related to:
- the statistical uncertainty in narrow energy bins,
- the number of energy groups used,
- or the way the flux tally is processed after the simulation?
What is the recommended approach in OpenMC to obtain a smooth and physically meaningful neutron energy spectrum for comparison with published results?
I have attached the resulting spectrum for reference.
Thank you for your help
What is the recommended approach in OpenMC to obtain a smooth and physically meaningful neutron energy spectrum for comparison with published results?
neutron flux per unit lethargy
https://github.com/openmc-dev/openmc-notebooks/blob/develop/flux-spectrum.ipynb
https://www.ornl.gov/file/plotting-flux-spectra-keno/display
If you have enough particles, more energy bins (CCFE-709, UKAEA-1102).
If not enough particles, XMAS, VITAMIN, etc.
I recommend a stair-step, histogram plot style
.
I am already using the XMAS energy group structure, which is why I was surprised to still observe noticeable oscillations in the spectrum despite using about 20 million particles.
I will try plotting the neutron flux per unit lethargy with a stair-step histogram as you suggested.
Thank you for your help.
