Hi @nnikolop
Try doing
tally_data.write_hdf5("fluka_tally_22_nflux.h5m")
If that works, you can then convert to vtk on the command line, using the mbconvert command in the moab/bin directory
mbconvert fluka_tally_22_nflux.h5m fluka_tally_22_nflux.vtk
Let me know
Thanks
Andy