Materials name persist when running depletion simulation

[shimwell]

Description

When running a depletion simulation and then loading up the materials from a depletion_results.h5 file the material names are lost.

This makes depletion with DAGMC workflows a bit tricky as one typically knows the material name and not the ID in DAGMC workflows (this stems from material tags in the DAGMC file).

Anyway it would be super useful for me if we can preserve the material name when writing the depletion_results.h5 file so that I can easily find materials once again.

I have added tests but also manually tested with this script

I have also tried to keep this PR to the bare minimum and not cram in extra fixes or deviations from the objective.

"""Minimal OpenMC depletion example to test material name preservation."""

import openmc
import openmc.deplete
from pathlib import Path
openmc.config['cross_sections'] = Path.home() / 'nuclear_data'/ 'endf-b8.0-hdf5/' / 'cross_sections.xml'
openmc.config['chain_file'] = Path.home() / 'nuclear_data' / 'chain-endf-b8.0.xml'
fuel = openmc.Material(name="My Fuel Material", material_id=1)
fuel.add_nuclide("Fe56", 1)
fuel.set_density("g/cm3", 10.0)
fuel.depletable = True
fuel.volume = 4/3 * 3.14159 * 10**3  # sphere volume
materials = openmc.Materials()
sphere = openmc.Sphere(r=10, boundary_type="vacuum")
cell = openmc.Cell(fill=fuel, region=-sphere)
geometry = openmc.Geometry([cell])
settings = openmc.Settings()
settings.batches = 10
settings.run_mode  = 'fixed source'
settings.particles = 1000
settings.source = openmc.Source(
    space=openmc.stats.Point((0, 0, 0)),
    energy=openmc.stats.Discrete([14.0e6], [1.0])
)
model = openmc.Model(geometry, materials, settings)
model.deplete(
    method="predictor", 
    operator_kwargs={
        "normalization_mode": "source-rate",
        "chain_file": openmc.config['chain_file'],
        "reduce_chain_level": 5,
    },
    timesteps=[1,1,1,1],
    source_rates=[1e20,0,0,0],
)
results = openmc.deplete.Results("depletion_results.h5")
step = results[-1]
material = step.get_material('1')
assert material.name =="My Fuel Material"
print(material.name)

Fixes # (issue)

Checklist

• I have followed the style guidelines for Python source files (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

[shimwell]

Thanks for the improvements @GuySten

I also notice that when a user makes a material and sets density units to g/cm³ the returned materials come in units of sum regardless of the inital input density. I would be keen to try to keep the density units consistent with the original material. Naturally I could do this on a separate PR but it is somewhat releated to this and I'm keen to hear if it would be a popular change?

[GuySten]

I think preserving density units should be in a separate PR.

Separately I do not think that it is very useful compared to the usefulness of preserving material names.

[shimwell]

@paulromano if you have time to take a peak does this PR look ok to merge?

[GuySten]

Looks good to me.

[paulromano]

I do see a few things that need changes here so putting a placeholder review for now. Sorry to hold this up but will really try to get to it this week.

[paulromano]

All good now; thanks @shimwell!