Thermal and neutron physics analysis show that above certain concentrations of the isotope Pu-238 hypothetical nuclear explosive devices, made of reactor-grade plutonium, are technically not feasible. Future proliferation-proof fuel cycles are proposed which make use of methods of actinide tansmutation.Reactors operating in the thorium/uranium fuel cyce are loaded with This work was published by Saint Philip Street Press pursuant to a Creative Commons license permitting commercial use. All rights not granted by the work's license are retained by the author or authors.