Functional Expansion Tallies in Vacuum Regions

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For some reason I’m having trouble generating functional expansion tallies (with the SpatialLegendreFilter) over a region with a null or vacuum material. I have no problem getting the spatial flux distribution with a mesh tally, but for some reason the SpatialLegendreFilter in this cell gives all zeros (even the zeroth order moment which is the total flux, and, as confirmed with a lone flux tally, is not zero). For some reason this problem was alleviated when I filled the previously vacuum cell with a material… which seems strange. Perhaps the SpatialLegendreFilter is only defined for non-vacuum regions? If this is the case, this is mentioned nowhere (at least not that I could find) in the documentation.

If that is the case, is there a benign way to introduce some dummy material to fill my “vacuum” cell with purely for the purpose of computing these spatial moments? Thanks for any and all help!

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I’m not sure, but wonder if FETs are only defined with collision based tallies, which requires some material. Most tallies in vacuum regions rely on track length estimators. If this is the case, then it may not be possible to do FETs in vacuum regions.

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I think you’re right, but am unsure why the developers chose this approach. In principle, at least, the flux can be tallied with a track length estimator, and hence its moments can be recovered by directly computing the relevant integrals of say a mesh tally of the flux. Although that strategy is not efficient per se, it’s at least conceptually shows how moment tallies could be obtained from track length estimators in a vacuum. In the meantime, this is the approach I’ve chosen as a workaround.