Neutron emission tomography for nuclear fissile materials safeguards

Any nondestructive method for fissile assay in spent nuclear fuel must sample a substantial portion of the total number of reactor fuel elements to account for the radial and axial asymmetry of fissile material distribution that may arise during fuel burnup. Cross-sectional scans of the spatial distribution of the fissile isotopes can be tomographically reconstructed while a spent fuel element is being assayed in a lead slowing-down-time (SDT) spectrometer by employing image reconstruction techniques. The SDT approach employs threshold detectors (fission chambers), which measure the induced prompt fast fission neutrons emitted from the fissile isotopes in spent fuel while being interrogated with a slowing pulse of source neutrons. The objectives of this work were to examine to what extent a cross section representing an array of partially void (missing pins) and intact fuel pins could be reconstructed and how sensitive this method is to diversion, spatial burnup distribution, and gross fissile contents consistency