  Safeguarding the disposal of spent nuclear fuel in a geological repository
needs an effective, efficient, reliable and robust non-destructive assay (NDA)
system to ensure the integrity of the fuel prior to disposal. In the context of
the Finnish geological repository, Passive Gamma Emission Tomography (PGET)
will be a part of such an NDA system. We report here on the results of PGET
measurements at the Finnish nuclear power plants during the years 2017-2020.
Gamma activity profiles are recorded from all angles by rotating the detector
arrays around the fuel assembly that has been inserted into the center of the
torus. Image reconstruction from the resulting tomographic data is defined as a
constrained minimization problem with a data fidelity term and regularization
terms. The activity and attenuation maps, as well as detector sensitivity
corrections, are the variables in the minimization process. The regularization
terms ensure that prior information on the (possible) locations of fuel rods
and their diameter are taken into account. Fuel rod classification, the main
purpose of the PGET method, is based on the difference of the activity of a
fuel rod from its immediate neighbors, taking into account its distance from
the assembly center. The classification is carried out by a support vector
machine. We report on the results for ten different fuel types with burnups
between 5.72 and 55.0 GWd/tU, cooling times between 1.87 and 34.6 years and
initial enrichments between 1.9 and 4.4%. For all fuel assemblies measured,
missing fuel rods, partial fuel rods and water channels were correctly
classified. Burnable absorber fuel rods were classified as fuel rods. On rare
occasions, a fuel rod that is present was falsely classified as missing. We
conclude that the combination of the PGET device and our image reconstruction
method provides a reliable base for fuel rod classification.
