Experimental Neutron Signature Measurements of U-233 Plates
at the Device Assembly Facility

The thorium fuel cycle is emerging as an attractive alternative to conventional nuclear fuel cycles, as it does not require the enrichment of uranium for long-term sustainability. The operating principle of this fuel cycle is the irradiation of 232Th to produce 233U, which is fissile and sustains the fission chain reaction. 233U poses novel challenges for nuclear safeguards, as it is associated with a uniquely extreme gamma-ray environment from 232U contamination which limits the feasibility of gamma ray-based assay, as well as more conservative accountability requirements than for 235U set by the International Atomic Energy Agency (IAEA). Consequently, instrumentation used for safeguarding 235U in traditional fuel cycles may be unsuitable. It is essential that the nondestructive signatures of 233U be characterized so that nuclear safeguards can be applied to thorium fuel cycle facilities as they come on-line. In this work, a set of 233U3O8 plates, containing 844 g 233U, was measured at the Device Assembly Facility. A highpressure 4He gaseous scintillation detector, which is insensitive to gamma rays, was used to perform the first passive fast neutron spectral signature measurement of 233U3O8, and was used in conjunction with a pulsed deuterium-tritium neutron generator to demonstrate the first differential die-away signature of this material. Furthermore, an array of 3He detectors was used in conjunction with the same neutron generator to measure the delayed neutron time profile of 233U, which is characteristic of the isotope. These measurements provide a benchmark for future nondestructive assay instrumentation development, and demonstrate a set of key neutron signatures which may inform future nuclear safeguards for the thorium fuel cycle.