Developing DGS for Reprocessing Plant Nuclear Safeguards: Designing a Compact Instrument

In the field of nuclear material safeguard verifications, mixed high-radioactivity nuclear material in facilities such as reprocessing plants are challenging. Addressing this, the Japan Atomic Energy Agency’s Integrated Support Center for Nuclear Nonproliferation and Nuclear Security is developing the Delayed Gamma-ray Spectroscopy nondestructive assay technique. We successfully completed several experiments in collaboration with the European Commission Joint Research Centre to elaborate the instrumentation requirements of a compact instrument. In the method, neutrons from an external source are thermalized in a moderator before inducing fission in the sample. The optimization of the moderator is crucial to achieving an efficient and compact instrument. Having a well-thermalized neutron flux enhances the delayed gamma-ray signature of the fissile due to their greater thermal fission cross section compared to the fertile. After the irradiation, the gamma-ray peaks above 3 MeV are analyzed to determine the initial composition of the fissile nuclides. The gamma-ray spectrum, and subsequent analysis, is strongly affected by the source type, the sample, and the interrogation pattern. To investigate all these aspects, we performed several experiments using small standard samples of both uranium and plutonium with PUNITA to understand basic principles. The result of these were considered in the JAEA Delayed Gamma-ray Californium Test instrument which was subsequently tested in PERLA. The present work first describes the PUNITA and PERLA experiments and how these were used to validate the various model designs. From these, further modifications to reach our final instrument concept design for a deuterium-deuterium neutron generator source are presented.