International inspectors may have a new tool in the form of an antineutrino detector, that could help them peer inside a working nuclear reactor. Scientists recently demonstrated that the operational status and thermal power of reactors can be quickly and precisely monitored over hour-to month-time scales, using a cubic-meter-scale antineutrino detector. The detector could be used to determine the operational amount of plutonium or uranium necessary to run the reactor and place a direct constraint on the amount of fissile material the reactor creates throughout its lifecycle.