Since the first observation of antineutrinos from beta decay of the fission products inside a nuclear reactor in 1956, the design and operating experience of antineutrino detectors near reactors has increased to the point where monitoring the reactor's power level and progression through its burnup cycle has become possible. With the expected increase in world nuclear energy capacity, including the dissemination of reactor technologies to non-nuclear states, the need for safeguards measures which are able to provide continuous, near-real-time information about the state of the core, including its isotopic composition, in a tamper- and spoof-resistant manner is evident. Near-field (~20 m from the core) antineutrino detectors are able to fulfill this demand without perturbing normal reactor operation, without requiring instrumentation which penetrates the reactor vessel, and without displacing other plant structures. Two sodium-cooled long-life fast reactors that are characteristic of next-generation reactors which are attractive for installation in non-nuclear states, one large and one small power rating, have been modeled throughout their reference burnup cycles using MCC-3 and DIF3D/REBUS. Various diversions of fissile material from the core designed to obtain weapons-usable material for the purpose of nuclear proliferation were studied as perturbed core states. The difference in detector event rates between the reference and perturbed states was used to determine the probability that a particular diversionary activity would be apparent before the material could be converted into a weapon. These data indicate which types of diversion antineutrino safeguards are particularly strong against and how the technology might be implemented in current and future international policies concerning nuclear proliferation.