Comparison of in-air and in-water performance of Passive Gamma Emission
Tomography with activated Co rods

With the operations at the geological repository in Finland starting soon, efficient nondestructive assay methods are needed to verify the spent nuclear fuel prior to disposal. Passive Gamma Emission Tomography (PGET) is a method that allows for fuel rod level inspection of the nuclear fuel integrity. Together with the Passive Neutron Albedo Reactivity (PNAR) method, both the gamma activity as well as the reactivity of the spent fuel can be assayed with high confidence. This is essential to make reliable nuclear safeguards conclusions before the fuel becomes inaccessible after the disposal in the geological repository. The PGET method has been developed to be used underwater in spent nuclear fuel storage ponds, but at the spent nuclear fuel encapsulation plant in Finland, there will be the possibility to conduct measurements in a hot cell in air. This has not been tested previously with the device. During June 2022, mockup tests with irradiated cobalt mockup fuel rods were conducted at the Atominstitut in Vienna to investigate the method’s performance in air. Five different configurations of mockup assemblies with activated cobalt rods, steel rods and empty positions were measured with the PGET device both in water and in air. The results show that the device performance is similar in both media. Future investigation topics include simulation studies of the effect of the background radiation originating from the parts of the fuel outside of the imaging field of view, and simulation of the scattering of gamma rays from the surrounding hot cell. Test measurements with real spent nuclear fuel are also required to study some effects that could not be verified with the mockup setup, due to the different attenuation and gamma energies of the cobalt rods compared to the uranium and radioactive elements in the spent fuel.