Flux Energy Group Conversion using lethargy-weighted redistribution #3705
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This adds convert_flux_groups() to convert flux spectra between different energy group structures. The source and target flux can both either be arbitrary user input energy group structures or existing named GROUP_STRUCTURES in OpenMC. The spetra conversion is done on an equal flux per unit lethargy basis, similar to FISPACT-II GRPCONVERT, https://fispact.ukaea.uk/wiki/Keyword:GRPCONVERT)
Convert FNX flux from CCFE-709 to UKAEA-1102. Plot compares flux vs. flux. Flux/leth. Zooms in on areas of different energy structure. Mapping to finer bins is evident.
GuySten
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Jan 4, 2026
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GuySten
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GuySten
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In general this is a nice feature!
I think you should drop one of files from this PR as it is not a test file.
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This looks like an example and not a test.
You should remove this file from the PR.
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Description
Often it is necessary for various reasons to have fluxes in different energy structures.
It is not always possible to just directly re-score the flux in a Monte Carlo with the desired spectrum.
e.g.: an experiment provides a given or particular evaluated neutron flux with a given energy spectrum, but later on it is desired to use it with a different application, that perhaps has a slightly different energy structure. Etc.
This new method adds the capability to convert an arbitrary 'source' spectrum to an arbitrary 'target' spectrum, it can use user specified spectra and/or existing OpenMC GROUP_STRUCTURES. It uses the standard approach, that is energy group re-binning with flux-per-lethargy conservation.
https://fispact.ukaea.uk/wiki/Keyword:GRPCONVERT
https://fispact.ukaea.uk/wiki/Spectra_conversion
https://scientific-publications.ukaea.uk/wp-content/uploads/api_manual.pdf#subsection.10.1
A number of tests are included, including conversion of FNS flux from CCFE-709 to UKAEA-1102 with flux, flux/d_leth plots with zoom in of the energy ranges with differing structure.
I have furthermore tested down-sampling (fine -> coarse) and other structures.
Below is a plot of FNS CCFE-709 flux as:
Checklist