Final answer:
President Carter imposed a moratorium on the reprocessing of commercial nuclear fuel due to fears of nuclear proliferation and the potential creation of nuclear weapons from reprocessed materials, in the context of heightened Cold War tensions and nuclear safety concerns.
Step-by-step explanation:
The fear that led President Carter to impose a moratorium on the reprocessing of commercial nuclear fuel was principally concerned with nuclear proliferation and the potential development of nuclear weapons from diverted plutonium. The late 1970s was a fraught period for nuclear energy, exacerbated by the 1979 Three Mile Island accident, which highlighted the safety issues inherent in nuclear power and the profound consequences of potential accidents. Additionally, the global political climate was tense, with Cold War anxieties peaking following the Soviet invasion of Afghanistan in 1979, and increasing concerns over nuclear arms control, as exhibited in the SALT II negotiations. President Carter's response to these compounding factors included the imposition of the moratorium to address fears of nuclear weapons proliferation, which could be aggravated by the reprocessing of plutonium from commercial nuclear reactors.