Cooperative monitoring from outside the fence: the promise and practical limitations of remote
antineutrino-based monitoring and discovery of nuclear reactors

We discuss the utility of and obstacles to remote antineutrino-based monitoring, ‘outside the fence’ of a reactor complex. From about 0.1-10 kilometer standoff distances, information about the thermal power and fissile inventory of civil reactors , down to a minimum power of about 500 Megawatts thermal (MWt) can be gained with detectors ranging in size from about ten tons at 100 meters, to about one kiloton at the 10 kilometer extreme of this radial band. From 10-100 kilometers, an excess antineutrino event rate above a robustly predicted background can be detected, and would be an indication of an anomaly, with known statistical significance, consistent with a small undeclared reactor, down to a minimum thermal power of 50 Megawatts (MWt). This latter application would require detectors in the few kiloton to 500 kiloton range, with the largest sizes needed at the extreme standoff limit of 100 km. The above estimates assume current state-of-the-art detectors, or reasonable extrapolations therefrom. We examine costs and practical limitations for deployment, including the need for underground burial of the detector (to shield against backgrounds), the expense of construction and operation, and the possible social or policy impacts of such deployments. We conclude that remote antineutrino-based monitoring has potential utility for cooperative monitoring regimes and confidence building activities, especially when non-intrusiveness, wide areal coverage, persistence, quantitative information about reactor operations or existence, and scientific engagement with the host country are desired by negotiators. The nonproliferation community will benefit from a demonstration of the capability, to gain insight into operational considerations and real-world utility of this new approach to cooperative monitoring of nuclear reactors.