Using Antineutrinos to Detect Single-assembly Diversion from a Long-life Fast Reactor with a Once-through Fuel Cycle

Nuclear reactors have been used in neutrino experiments since their first detection decades ago. Recently, there have been proposals to incorporate the monitoring of reactor antineutrino signatures into the IAEA safeguards regime, and significant effort has been invested in developing and producing a small (~1 m3 ), robust detector positioned aboveground which could unobtrusively perform the required surveillance in real-time, for example via inverse beta decay. Using antineutrino detectors for such a purpose adds tangible advantages to the current array of procedures (tamper-evident seals, stamped assemblies, short-notice inspections, etc.) and monitoring technologies: the detector is always-on and very difficult to spoof, and antineutrinos provide a transparent means of monitoring any core regardless of coolant due to their low cross-section. Furthermore, by taking advantage of the differences in antineutrino flux and spectrum produced by the various fissile isotopes, monitoring the isotopic evolution of the core becomes possible during normal operation. By comparing the observed antineutrino event rate to the one which should occur at the corresponding point in the burnup cycle, detection of a diversion of nuclear material is possible. This method of detecting diversions is particularly suited to monitoring long-life fast reactor cores because they incorporate blanket regions in order to reduce burnup reactivity swing, the plutonium they produce in these regions is of weapons-grade purity for a significant duration due to the fast neutron spectrum, and the interval between refueling is often at least a decade, so performing assembly-level inventory during regular refueling is an impractical method of accounting for safeguards purposes. The Ultralong Cycle Fast Reactor concept, a sodium-cooled, 2600 MWth design with a once-through cycle lasting approximately 60 years, is currently under investigation to produce a reference antineutrino signature as a function of burnup. Diversion scenarios include a single assembly replacement with a reactivity-equivalent assembly fueled with low-enriched and natural uranium, multiple assembly swap, and power manipulation to alter the neutron spectrum near assemblies of interest. The antineutrino signature for each of these diversion scenarios will be compared to the reference signature to assess the duration before which monitors can be 95% confident a diversion has occurred.