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

Checklist for Reviewers

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Review Checklist before they begin their review.

[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!

[Jeremyb8707bigbrain]

Hey @smpark7, everything for the most part is in order!

I kept the openmc_xs class still available just in case you wanted to mark it for deprecation so users know that they will need to change their parameters slightly.

I have a few questions, did you want me to include the SPN calculation? I can easily (I hope) include that I was just wondering if you would like me to. I also added TOTXS which is not present in the reference godiva.json file, but I see it in the code, would you like me to keep that or remove it? Last question, for your suggestion of having the mgxs.Library() be compatible with OpenMC's own multigroup mode, it would require an additional tally (Absorption), would you like me to add that and add the OpenMC Function check_library_for_openmc_mgxs() to the process_mgxslib() function to ensure advanced users must also meet that requirement?

I did a few tests with multiple statepoints and summaries, here are the results, again I do not have access to all 8 delayed groups so some areas will be off very slightly.

If you have any other concerns or questions please reach out! I also credit my research partner @rafainn for help with the input file work, very helpful!

Example Python File: (I again dont have the cross sections at 900K or 1200K so I made them myself, which means sadly only 6 delayed groups)

import openmc
import sys
sys.path.insert(1,"/moltres_test/moltres_xs-Refactor-MGXS/python/")
from moltres_xs import openmc_mgxslib

fuel_mat = openmc.Material(name='fuel', material_id=1)
fuel_mat.set_density('atom/b-cm', 4.7984E-02)
fuel_mat.temperature = 900
fuel_mat.add_nuclide("U235", 4.4994E-02, percent_type='ao')
fuel_mat.add_nuclide("U238", 2.4984E-03, percent_type='ao')
fuel_mat.add_nuclide("U234", 4.9184E-04, percent_type='ao')

mats = openmc.Materials([fuel_mat])
mats.cross_sections = "/workspace/moltensaltsim/Material_Data/OpenMC_XMLs/cross_sections_godiva.xml"
mats.export_to_xml()

sph = openmc.Sphere(r=8.7407, boundary_type='vacuum')

fuel_core = openmc.Cell(fill=fuel_mat, region=-sph)

geom = openmc.Geometry([fuel_core])
geom.export_to_xml()

settings = openmc.Settings()
settings.batches = 100
settings.inactive = 20
settings.particles = 50000
settings.temperature = {"multipole": True, "method": "interpolation"}
point = openmc.stats.Point((0, 0, 0))
src = openmc.IndependentSource(space=point) # Source was deprecated and it said that IndependentSource was preferred.
settings.source = src
settings.export_to_xml()

tallies = openmc.Tallies()
mgxs_library = openmc_mgxslib.generate_openmc_tallies_xml([1e-5, 748.5, 5.5308e3, 24.7875e3, 0.4979e6, 2.2313e6, 20e6], 6, [fuel_mat], geom, tallies)

#openmc.run() # Used before the input parsing

Input File:

[TITLE]
  godiva_openmc.json

[MAT]
  1
  fuel_mat

[BRANCH]
  2
  fuel_mat            900 1 1 1 1
  fuel_mat            1200 2 1 1 1

[FILES]
  2
  statepoint.900.h5  openmc godiva_openmc_900.py summary.900.h5
  statepoint.1200.h5  openmc godiva_openmc_1200.py summary.1200.h5

Output:

{
  "fuel_mat": {
      "1200": {
          "BETA_EFF": [
              0.0013479328874237927,
              0.006967492815569392,
              0.006662378531992382,
              0.014980452683184708,
              0.006185780669768732,
              0.0025887417681020298
          ],
          "CHI_D": [
              0.004809526109315934,
              0.38523354014508626,
              0.5815938982198151,
              0.02705814752389195,
              0.001287967194123611,
              1.6920807767085327e-05
          ],
          "CHI_P": [
              0.35204067796316474,
              0.5198257797155467,
              0.12648480548441765,
              0.0014890020136668451,
              0.00015066478799014937,
              8.817408027798893e-06
          ],
          "CHI_T": [
              0.34979280230108517,
              0.5190903735269962,
              0.1293235561075548,
              0.0016233017887243154,
              0.00016070460416726674,
              9.01067875604564e-06
          ],
          "DECAY_CONSTANT": [
              0.013336176998594657,
              0.0327254618353869,
              0.12082517735113926,
              0.3032033227763606,
              0.8513219627189227,
              2.8585311502702324
          ],
          "DIFFCOEF": [
              1.6182274998401185,
              1.3642728170826015,
              0.8320405266913466,
              0.47149354286848477,
              0.37859622913474406,
              0.24662342775124388
          ],
          "FISSE": [
              193.4961292932816,
              193.4201252841644,
              193.40357471688122,
              193.4053366134499,
              193.40537155138682,
              193.40490127356847
          ],
          "FISSXS": [
              0.05681875825825768,
              0.05459421736631732,
              0.05989063772559673,
              0.11294273814778086,
              0.21003655994907477,
              0.5462213108029339
          ],
          "GTRANSFXS": [
              0.22297533231163758,
              0.06473700684651243,
              0.025494746342081185,
              0.00012158922962522632,
              1.7835373598058595e-05,
              1.7149397690440947e-06,
              0.0,
              0.24890051397716456,
              0.037612649690256794,
              0.00016779278544005614,
              1.3686567897544017e-05,
              1.1342459583600008e-06,
              0.0,
              0.0,
              0.4182846249650026,
              0.0005928935677351931,
              2.807852088832213e-05,
              6.239671308516024e-07,
              0.0,
              0.0,
              0.0,
              0.5611416177989257,
              0.0004900411079539684,
              0.0,
              0.0,
              0.0,
              0.0,
              0.0,
              0.5978057420724365,
              0.00029062019546545275,
              0.0,
              0.0,
              0.0,
              0.0,
              0.0,
              0.596995317950723
          ],
          "NSF": [
              0.1640887563798374,
              0.13935790334891593,
              0.1475426911619574,
              0.27305712757940975,
              0.5094169312840311,
              1.3270890801602042
          ],
          "RECIPVEL": [
              3.907802458936875e-10,
              7.172622496270246e-10,
              1.5763527517103987e-09,
              5.78055071193088e-09,
              1.2647622630278322e-08,
              3.541697580573283e-08
          ],
          "REMXS": [
              0.14636069574186267,
              0.03258761538964927,
              0.010312236818636855,
              0.1549538266064593,
              0.2923357653247477,
              0.7945967414514843
          ],
          "TOTXS": [
              0.36933602805350024,
              0.3462251362133263,
              0.49170425781597743,
              0.7169777199881855,
              0.8906911489675221,
              1.391886152750531
          ]
      },
      "900": {
          "BETA_EFF": [
              0.0013477713361262679,
              0.0069667815244903765,
              0.006661711171558475,
              0.014978986450907898,
              0.006185223565016153,
              0.0025885180877550065
          ],
          "CHI_D": [
              0.004816108092933141,
              0.38402391400673824,
              0.5856802013789721,
              0.02386435661906883,
              0.0015985756139615993,
              1.6844288325974322e-05
          ],
          "CHI_P": [
              0.3519712083405351,
              0.5196815853363649,
              0.12668408955805868,
              0.0014885401949253701,
              0.00016500443737073363,
              9.572132698102343e-06
          ],
          "CHI_T": [
              0.34973414403563124,
              0.518934845488141,
              0.1295304380302617,
              0.001614356008810987,
              0.0001764556951631939,
              9.76074194393403e-06
          ],
          "DECAY_CONSTANT": [
              0.013336178367893413,
              0.03272544257848398,
              0.12082523103059183,
              0.30320384982572396,
              0.8513242055280581,
              2.8585382538647544
          ],
          "DIFFCOEF": [
              1.6173834566424388,
              1.3649771588306194,
              0.832532085680547,
              0.4703894981397496,
              0.37537053465658565,
              0.2466560322416023
          ],
          "FISSE": [
              193.49611687720707,
              193.42016116893532,
              193.4035711786227,
              193.40533535036857,
              193.40537484654888,
              193.40528883089382
          ],
          "FISSXS": [
              0.056824395601165534,
              0.054599320358540586,
              0.059874071674284796,
              0.1133143820087574,
              0.20895953468659575,
              0.5505401846941079
          ],
          "GTRANSFXS": [
              0.2228272763898503,
              0.06487676994616874,
              0.0255169429779392,
              0.00012301955320388736,
              1.713364250750522e-05,
              2.741382801200833e-06,
              0.0,
              0.24901044262918903,
              0.03747253621834822,
              0.0001626484510271523,
              1.550834053147492e-05,
              8.321548577864581e-07,
              0.0,
              0.0,
              0.41817719524685815,
              0.0005780702327768786,
              2.6275919671676323e-05,
              6.256171350399119e-07,
              0.0,
              0.0,
              0.0,
              0.5614697598556467,
              0.00047028079639904497,
              0.0,
              0.0,
              0.0,
              0.0,
              0.0,
              0.5955691163233789,
              0.0,
              0.0,
              0.0,
              0.0,
              0.0,
              0.0,
              0.5651209691677761
          ],
          "NSF": [
              0.16409209454954332,
              0.1393737082256517,
              0.14750442373233597,
              0.2739559869142277,
              0.5068196900715216,
              1.3383945829450208
          ],
          "RECIPVEL": [
              3.9087078730945797e-10,
              7.170785092953576e-10,
              1.5751041684526634e-09,
              5.792145962218262e-09,
              1.2766578207104842e-08,
              3.893844194513065e-08
          ],
          "REMXS": [
              0.1465241546944975,
              0.03230783902029771,
              0.010384167333425842,
              0.15546259631531867,
              0.28971530681046687,
              0.8187096717946637
          ],
          "TOTXS": [
              0.3693514310843478,
              0.3461950515956555,
              0.4915508417765714,
              0.7177960944079733,
              0.8858136218329555,
              1.3838348401172338
          ]
      },
      "temp": [
          900,
          1200
      ]
  }
}

I did another test with a different material, and same temperature and it worked as it should.

[rafainn]

Compatible until openMC 0.15.3 - since not mentioned explicitly elsewhere.

[smpark7]

Thanks @rafainn for also contributing to this PR. Yes SPN and TOTXS should be included. They're missing in the godiva files because the godiva test case was created before SPN and TOTXS were required for the new SN solver. (You can use this 1-D MSRE test case for an example that generates SPN and TOTXS)

I see now that there are some incompatibilities between Moltres and multigroup OpenMC (OpenMC-MG) regarding required group constants:

• As you mentioned, OpenMC-MG requires absorption XS while Moltres uses removal XS. We originally used Serpent which provided removal XS directly. When we adopted OpenMC, we chose to continue using removal XS.
• Moltres wants ScatterMatrixXS with nu=True but the default is nu = False. This may be why OpenMC gave tally merging errors when I tried to have it generate group constants for both Moltres and OpenMC-MG with mgxs_lib.add_to_tallies_file(tallies, merge=True) after running generate_openmc_tallies_xml(...).

One solution may be to stick with group constants that Moltres requires (as you have done) and let advanced users who want to use the same library for multigroup OpenMC add absorption, nu-scatter matrix, and multiplicity matrix.

I think you need the JEFF cross section library to get 8 delayed groups, but that's not a big deal.

[Jeremyb8707bigbrain]

@smpark7 I got the SPN Stuff down, adding new constants is quite easy with the library method.
Is there anything else you need us to do?

[smpark7]

Thanks! I hope to have time next week to review this in detail.