Hi Matt,
I think you’ve found a problem with our regular mesh tally routine for tracklength tallies. I say this because if I either a) change the mesh dimensions slightly or b) change the tally estimator from “tracklength” (default) to “collision”, then the simulation runs successfully.
This is a great minimal working example of the bug and I’ll take a look at a fix very soon, but in the meantime maybe this is enough information to proceed with your work. I’ll make sure to link the fix here once it’s been incorporated into OpenMC.
-Patrick