Passive Neutron Albedo Reactivity Measurements Of Spent BWR Fuel

Before geological disposal of spent nuclear fuel in Finland, the integrity of each fuel assembly will be verified. Passive Neutron Albedo Reactivity (PNAR) verification is a part of the national non-destructive verification concept, developed by the Finnish radiation and nuclear safety authority (STUK). PNAR verification utilizes the neutron radiation of the spent fuel itself to measure neutron multiplication in the assembly. Thus, it can be used to verify the presence of fissile material. A PNAR instrument was built and tested for the first time in 2019 in a collaboration led by STUK. In 2020, a second measurement campaign was held at the Olkiluoto spent fuel storage. Several BWR-type assemblies of different designs and operational histories were measured to understand the PNAR’s response to different fuel parameters. Axial measurements of the same assembly reveal an axial multiplication profile in the fuel. From the multiplication profiles, the decrease of fissile content caused by partial length rods can be identified. The uncertainty of a PNAR measurement was studied with repeated measurements. The results show that a typical PNAR measurement, with a total measurement time of six minutes, has a dynamic range of approximately 70 standard deviations. Based on the measurements performed in 2019 and 2020, PNAR can quantify the neutron multiplication of a spent fuel assembly with great precision and assemblies of different histories can be discerned from each other. The multiplication measurement result can be used to verify the assembly declaration and to detect partial defects. The PNAR method is constantly being developed. Our aim is to have the method ready for application in 2025.