Refactoring moltres_xs to be able to use mgxs.Library() functionality.

[Jeremyb8707bigbrain]

Summary of changes

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Required for Merging

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.
  • CI tests pass
  • Local tests pass (including Serpent2 integration tests)

Associated Issues and PRs

• Issue: Refactor moltres_xs.py to use OpenMC helper functions for group constant generation #319

Associated Developers

• Dev: @smpark7

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[Jeremyb8707bigbrain]

Hey everyone,

I ran local tests myself for the godiva.json files, mostly everything lines up like it should.
The main issue is my cross sections that I had myself only are 6 delayed groups, and I really dont know how to get the 8 that godiva.json used.

Another discrepancy that may be noted is the version, I am on OpenMC version 0.15.2 so the numbers are within std. dev, but for the smaller values it had a bit more range.

99% of everything works as it should, and I plan to add support for multiple statepoints and summaries soon. I added the openmc_mgxslib class so even if it is not 100% it can be added (maybe).

Also I forgot to say, this one does not require an input file that specifies energy groups delayed etc, that all comes with a properly configured mgxs.Library(). Also I am working on ordering the files consistently in the.json.

[smpark7]

Hey @Jeremyb8707bigbrain thanks again for working on this PR. I have been very busy, but I have some general feedback:

• Your code should replace the existing class openmc_xs, not create a new class.
• The new version should maintain compatibility with the existing workflow for group constant generation for backward compatibility and user-friendliness. Current workflow:
  • Use openmc_xs.generate_openmc_tallies_xml to configure tallies (currently through individual mgxs tallies, to be migrated to mgxs.Library())
  • Run OpenMC, potentially at multiple reactor temperatures
  • Run $MOLTRES/python/moltres_xs.py input.inp to parse and generate json group constant file.
• It should continue to have the openmc_xs.generate_openmc_tallies_xml function that helps beginner users automatically configure mgxs.Library() such that the generated group constants are compatible with the neutronics solvers in Moltres. Advanced users can choose to ignore this function and configure mgxs.Library() in their own way if they wish.
• It should be compatible with the existing approach of using .inp files for users to specify which material group constants and temperature values they want.
• You'll find how totalxs and nuscattermatrix group constants are extracted in moltres_xs.py for the new SN solver in Moltres.
• You're missing remxs values that are needed by the neutron diffusion solver.
• If possible, the mgxs.Library() configuration settings should be compatible with OpenMC mgxs generation for its own multigroup solver (https://nbviewer.org/github/openmc-dev/openmc-notebooks/blob/main/mg-mode-part-ii.ipynb). This would avoid having duplicate mgxs tallies that would otherwise significantly increase OpenMC simulation time.

[Jeremyb8707bigbrain]

Perfect I can do all of that!
I will put another comment when I would like it to be reviewed, for now I will start working on that feedback!