Correlated fast-neutron-gamma emission tomography for rapid
localisation of special nuclear materials in legacy waste drums

The development of precise and efficient NDA techniques for nuclear materials evaluation is of fundamental importance for nuclear safeguards and security. In particular, the safeguarding of spent nuclear fuel and other types of long-lived radioactive waste potentially containing special nuclear materials (SNM) represents a major global challenge. SNM are difficult to detect, in particular in shielded containments and in the presence of strong radiation fields from other radioactive materials, due to their relatively weak radiation emissions. We report on the development of a scanning system for enhanced non-destructive assay (NDA) of radioactive waste using the novel technique Neutron-Gamma Emission Tomography (NGET). The technique relies on the detection of correlated fast neutrons and gamma rays emitted in spontaneous/induced fission or (a, ng) reactions using organic scintillation detectors. It enables sensitive detection and three-dimensional localisation of SNM without moving components. The scanning system under development is designed for the special category of radioactive waste called “legacy waste”, which has special safeguards, security, and safety concerns due to its mixed, long-lived radioactive components. This application of the NGET technique was awarded the Euratom Innovation Prize 2022. The automatic scanning system, developed in collaboration between KTH, AB Svafo and ELSE Nuclear S.r.l., will serve as the first NDA characterization station for the Swedish legacy waste drum inventory at the Svafo interim storage facility in Studsvik, Sweden. Featuring additionally a collimated HPGe detector for high-resolution gamma-ray emission tomography and 3D gamma-ray densitometry, it will be able to load and perform vertical and azimuthal scanning of large numbers of waste drums with minimal manual intervention.