Warn when a material spans multiple cells in material_wise MGXS - #109
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The "material_wise" convert_to_multigroup method produces one flux-weighted cross section per material. A material that fills cells with different neutron spectra (for example structural steel near the source and deep inside a shield) is therefore collapsed to a single spectrum-averaged cross section, which can be inaccurate. Warn in that case, recommending Model.differentiate_mats() to give each location its own material and hence its own cross section.
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Summary
The
material_wiseconvert_to_multigroupmethod produces one flux-weighted cross section per material. A material that fills cells with different neutron spectra (e.g. structural steel near the source and deep inside a shield) is collapsed to a single spectrum-averaged cross section, which can be inaccurate.This adds a warning when a material fills more than one cell, recommending
Model.differentiate_mats()to give each location its own material (and hence its own locally-weighted cross section). It is a heads-up only — one cross section per material is the intended multigroup behaviour, so nothing in the output changes.Companion to the duplicate-name fix in #108 (which is a genuine bug); this PR flags expected-but-easily-overlooked behaviour. Note the warning fires whenever a material fills multiple cells, which is common.
Tests
Added a unit test asserting the warning fires when one material fills multiple cells.