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Gamma-ray images of VVER-440 and SVEA-96 spent nuclear fuel assemblies were reconstructed using the filtered back-projection algorithm from measurements with a passive gamma emission tomography prototype instrument at Finnish nuclear power plants. Image quality evaluation criteria based on line profiles through the reconstructed image are used to evaluate image quality for spent fuel assemblies with different cooling times, and thus different mixtures of gamma-ray emitting isotopes. Image characteristics at the locations of water channels and central fuel pins are compared in 2 gamma-ray energy windows, 600-700 keV, and >700 keV, for cooling times up to 10 years for SVEA-96 fuel and 24.5 years for VVER-440 fuel. For SVEA-96 fuel, images in the >700 keV gamma-ray energy window present better water-to-fuel contrast for all investigated cooling times. For VVER-440, images in the >700 keV gammaray energy window have higher water-to-fuel contrast up to and including a cooling time of 18.5 years, whereas the water-tofuel contrast of the images taken in the two gamma-ray energy windows are equivalent for a cooling time of 24.5 years. Images reconstructed from higher energy gamma rays such as those in the >700 keV present better water-to-fuel contrast in fuel cooled for up to 20 years and thus have the most potential for missing fuel pin detection.

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