This paper investigates whether a fusion power plant could be designed to be passively proliferation-proof. Even low neutron production rates enable fissile-fuel breeding, so such a fusion reactor must burn neutron-lean fuels. To burn these fuels economically requires a high-power-density fusion concept, and a D-3He field-reversed configuration will be analyzed here. The paper discusses physics and engineering design features that would defeat attempts to modify the reactor to burn the neutron-rich fuels D-T and D-D. These include burning an advanced fusion fuel, utilizing direct energy conversion, minimizing the radius to leave inadequate room for D-T neutron shielding of superconducting magnets, designing a single-module, full-lifetime fusion core requiring no module changeout, and using an organic coolant.