NDA Verification of Spent Nuclear Fuel Prior to Geological Disposal with
Passive Neutron Albedo Reactivity

Geological disposal of spent nuclear fuel is planned to begin in Finland in a few years. The disposal will start with spent BWR fuel from Olkiluoto 1 and 2 units operated by TVO. Prior to transporting of the fuel to the encapsulation plant and geological repository, all fuel assemblies will be verified in the wet storage facility where they are currently stored. The verification will be done in collaboration with the International Atomic Energy Agency (IAEA), the European Commission (EC) and Radiation and Nuclear Safety Authority of Finland (STUK) using one set of NDA verification instruments with their measurement results shared between all three inspectorates. The presently foreseen NDA methods will be Passive Gamma Emission Tomography (PGET), approved by the IAEA for safeguards inspections in 2017, and Passive Neutron Albedo Reactivity (PNAR). A PNAR instrument has been developed in a STUK-led collaboration for specifically the purpose of the verification of the BWR fuel at Olkiluoto. This paper summarizes the research and development on the PNAR instrument that has been conducted at STUK in preparation for the upcoming geological disposal. Since 2019, annual measurement campaigns have been held at the Olkiluoto spent fuel storage facility to assess the capabilities of the PNAR instrument. Over the course of these campaigns, more than 50 different fuel assemblies have been measured, several of them in multiple campaigns. These measurements have demonstrated the PNAR’s ability to verify the fissile material content of a spent fuel assembly. In addition, the PNAR’s reactivity measurement correlates with the leftover reactivity of the fuel, which can be estimated through simulations when the history of an assembly is known. In the 2022 measurement campaign, an ORIGEN (Oak Ridge Isotope Generation and Depletion) module for PNAR developed by the EC and the Oak Ridge National Laboratory was used to simulate the measurement signals of PNAR for the verified fuel assemblies. Such methods will allow for on-site verification of operator declarations and be part of the PNAR verification process that will be automated in the future.