  Radiography with cosmic ray muon scattering has proven to be a successful
method of imaging nuclear material through heavy shielding. Of particular
interest is monitoring dry storage casks for diversion of plutonium contained
in spent reactor fuel. Using muon tracking detectors that surround a
cylindrical cask, cosmic ray muon scattering can be simultaneously measured
from all azimuthal angles, giving complete tomographic coverage of the cask
interior. This paper describes the first application of filtered back
projection algorithms, typically used in medical imaging, to cosmic ray muon
imaging. The specific application to monitoring spent nuclear fuel in dry
storage casks is investigated via GEANT4 simulations. With a cylindrical muon
tracking detector surrounding a typical spent fuel cask, the cask contents can
be confirmed with high confidence in less than two days exposure. Similar
results can be obtained by moving a smaller detector to view the cask from
multiple angles.
