Guard multigroup tally scoring against void regions - #4126
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score_general_mg bound references into data::mg.macro_xs_ using the particle's material index without checking for MATERIAL_VOID, so a tally covering a void cell indexed the table with -1. Scores that read the particle's cached cross sections (flux, total, absorption) happened to survive this, but inverse-velocity reads the material's multigroup data directly and aborted the run. Return early for particles in void and score the three quantities that remain defined there: - flux, which depends only on the track length; - inverse velocity, which depends only on the particle's speed. OpenMC already defines that speed for a multigroup particle in a void: Particle::speed falls back to the approximate group-average default_inverse_velocity_, and the same data is used here; - events, which counts scoring events and reads no material data. Every remaining score the function handles is a reaction rate, which vanishes without a material. This matches continuous-energy mode, where inverse velocity is flux / speed and events is a bare count, both scored in a void like anywhere else. Only a tracklength estimator can reach a void, since the analog and collision estimators score at collisions, which require a material and whose expressions divide by the total cross section. Add unit tests covering all four cases; the inverse-velocity case aborts without this change. The test library supplies no inverse-velocity data, so the material falls back to the same default as the void, and the inverse velocity per unit flux must therefore agree between the two cells to round-off. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_017ci4aodNXq5BPbv3PXZNcp
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score_general_mg bound references into data::mg.macro_xs_ using the particle's material index without checking for MATERIAL_VOID, so a tally covering a void cell indexed the table with -1. Scores that read the particle's cached cross sections (flux, total, absorption) happened to survive this.
Return early for particles in void, scoring the flux for tracklength tallies and leaving every material-dependent score at zero. This matches how the continuous-energy scoring functions already treat MATERIAL_VOID.
Add a unit test covering both cases; the inverse-velocity case aborts without this change.
Please include a summary of the change and which issue is fixed if applicable. Please also include relevant motivation and context.
Fixes #2403
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I have made corresponding changes to the documentation (if applicable)