Operating reactors generate huge numbers of antineutrinos – about 10^26 per day from a typical power reactor. Several thousand of these can be measured per day in a modest sized detector at tens of meters standoff. Per fission, the antineutrinos produced by uranium and plutonium have different total count rates and energy spectra. The antineutrinos therefore carry with them information about the amount and type of fissile material in the reactor core. By counting the antineutrino rate in a broad energy window, and tracking the evolution of this rate in time, the integral amount of plutonium in the core can be estimated at the granularity of a few assemblies, a few tens of kilograms depending on the reactor design. In the simplest implementation, the additional input of the reactor thermal power is required to fix an absolute estimate of the core plutonium content. For more information, please see our publication in the Journal of Applied Physics.