Verifying Spent Nuclear Fuel With Passive Gamma Emission Tomography Prior To Disposal In A Geological Repository In Finland

Finland is planning to start the disposal of spent nuclear fuel in a geological repository around 2025, and a reliable verification method for the fuel is needed to ensure that all nuclear fuel corresponds to declarations. The development of analysis software for Passive Gamma Emission Tomography (PGET), an IAEA-approved NDA method, is ongoing between STUK, IAEA, the Helsinki Institute of Physics (HIP) and the Department of Mathematics and Statistics at the University of Helsinki. The torus-shaped PGET device consists of two linear arrays of collimated CdZnTe detectors and collects gamma emission data from spent nuclear fuel assemblies in underwater measurements. 2D images of the activity and attenuation of the assembly can be simultaneously reconstructed from the data with an iterative reconstruction method [1]. The method has been verified using measurements conducted between 2017-2020 at the Finnish nuclear power plants, imaging fuel assemblies with varying geometry, burnup, cooling time and initial enrichment [2]. Rod-level anomalies and even intra-rod differences in activity can be seen. Currently the analysis process is being automated, and a graphical user interface is being built to ensure efficient operation. The current classification of fuel rods based on the reconstructed attenuation and activity images separates the rods into present and missing rods, but development is ongoing to include a third category for possibly modified or replaced rods as well. To overcome the challenge of high self-attenuation in the fuel, the reconstruction process needs to be developed further, possibly with simulated data. We have started to use Serpent2 [3] for detailed simulations of the PGET method. The performance of the analysis software will also be optimized and code execution times brought down.