  When monitoring a reactor site for nuclear nonproliferation purposes, the
presence of an unknown or hidden nuclear reactor could be obscured by the
activities of a known reactor of much greater power nearby. Thus when
monitoring reactor activities by the observation of antineutrino emissions, one
must discriminate known background reactor fluxes from possible unknown reactor
signals under investigation. To quantify this discrimination, we find the
confidence to reject the (null) hypothesis of a single proximal reactor, by
exploiting directional antineutrino signals in the presence of a second,
unknown reactor. In particular, we simulate the inverse beta decay (IBD)
response of a detector filled with a 1 kT fiducial mass of Gadolinium-doped
liquid scintillator in mineral oil. We base the detector geometry on that of
WATCHMAN, an upcoming antineutrino monitoring experiment soon to be deployed at
the Boulby mine in the United Kingdom whose design and deployment will be
detailed in a forthcoming white paper. From this simulation, we construct an
analytical model of the IBD event distribution for the case of one $4\mathrm{\
GWt}\pm2\%$ reactor 25 km away from the detector site, and for an additional,
unknown, 35 MWt reactor 3 to 5 km away. The effects of natural-background
rejection cuts are approximated. Applying the model, we predict $3\sigma$
confidence to detect the presence of an unknown reactor within five weeks, at
standoffs of 3 km or nearer. For more distant unknown reactors, the $3\sigma$
detection time increases significantly. However, the relative significance of
directional sensitivity also increases, providing up to an eight week speedup
to detect an unknown reactor at 5 km away. Therefore, directionally sensitive
antineutrino monitoring can accelerate the mid-field detection of unknown
reactors whose operation might otherwise be masked by more powerful reactors in
the vicinity.
