Year
2024
Abstract
The Integrated Support Center for Nuclear Nonproliferation and Nuclear Security (ISCN) of the Japan Atomic Energy Agency (JAEA) initiated a project to develop Delayed Gamma-ray Spectroscopy (DGS) in Japan Fiscal Year 2015. The goal of the project is to develop the DGS technique in order to supplement the verification of mixed nuclear materials, especially 235U, 239Pu, and 241Pu in irradiated fuel. Experiments performed in collaboration with the European Commission Joint Research Centre (EC/JRC) have guided the joint development of our analysis and instrumentation. Significantly, it has been determined that the instrumentation necessary to obtain DGS signatures are challenged by the sample itself and the subsequent requirements to observe the gamma rays. Preliminary experimental development focused on observing analytical differences in standard U and Pu nuclear samples using the EC/JRC PUNITA instrument that used a deuterium-tritium neutron generator. Additional studies were performed to evaluate compact instrumentation for small samples like reprocessing plant irradiated-fuel dilutions, notably through the JAEA/ISCN Delayed Gamma-ray Test Spectrometer (DGTS). In 2023, the JAEA/ISCN designed and fabricated the Fission Signature Assay Instrument (FSAI) incorporating lessons learned from previous instrumentation development (see Figure). Similar to PUNITA, FSAI uses a neutron generator that can be turned off, but utilizes deuterium-deuterium neutrons that have 2.5-MeV energies, similar to the 252Cf used in DGTS, that are also more-easily moderated and not radioactive like tritium. Uniquely, FSAI utilizes a novel rotational transfer system that can bring the irradiated sample face to the gamma-ray detector, but concurrently contributes to the irradiation moderator and neutron shielding. Early characterization of FSAI started in October 2023, focusing on the neutron flux in the sample space using activation foils irradiated by 252Cf. Since the final installation in December 2023, efforts are underway to characterize the sample flux from the neutron generator using foil activation while associating the signals reaching the 4He source monitor and 3He prompt-neutron detectors. Preliminary 235U interrogations have been performed as the foundation of the improvement of the JAEA/ISCN DGS Monte Carlo and DGS safeguards verification evaluation. This work will briefly describe past efforts of the JAEA/ISCN-EC/JRC DGS project leading to the development of the FSAI design. We will then provide an overview of the instrument itself, along with the individual components and how they provide an improvement for the DGS technique. Finally, we will present a preliminary summary of the instrument characterization and preliminary 235U interrogation studies, along with a description of the final development goals to utilize the DGS technique for safeguards verification and nuclear material accountancy.