Reconstructing Activity and Attenuation of a Spent Fuel Assembly from Passive Gamma Emission Tomography (PGET) Measurements

Verification of spent nuclear fuel is a prerequisite for disposal in a geological repository. The verification method needs to be reliable and quick and allow non-destructive investigation of the fuel assemblies to ensure the integrity of the fuel prior to disposal. Recently IAEA has approved Passive Gamma Emission Tomography (PGET) to be used as a tool in rod-level examination of assemblies. The PGET device consists of CZT gamma ray detector banks arranged in a torus that rotates around the fuel assembly in the middle. Gamma activity profiles are recorded from all angles around the assembly in underwater measurements. Recently, we proposed a method for reconstructing an image of the spent fuel assembly from measured 1D gamma ray profiles. The method is based on simultaneous reconstruction of activity and attenuation images with sufficient prior information, formulating the reconstruction as a constrained minimization problem with a data fidelity term and regularization terms. We have now performed tests to ensure this reconstruction method tested earlier on simulated data also works with data from measurements at the Finnish nuclear power plants. We report results that show detection of individual missing fuel rods in a varied range of different fuel assembly types and fuel parameters: 10 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%. Reconstructions are compared to information about known anomalies and water channel positions in the assemblies. The reconstruction method is concluded to be suitable for safeguards verification of BWR fuel in the context of the geological disposal of spent nuclear fuel in Finland. For VVER-440 fuel further work is needed to investigate the ability to detect missing rods near the center of the assembly.