THE ADMINISTRATION is seriously considering a plan to "mine" the used fuel from commercial nuclear reactors for its accumulated plutonium. After separation and purification into its different forms, the plutonium would be used in nuclear warheads. It is a dangerous idea that Congress should place off-limits.

The impetus for the suggestion is a projected shortage of weapons fuel beginning near the end of this decade. The exact numbers are classified, but plans for new weapons systems and for replacing old uranium weapons with new and lighter plutonium warheads could require the production of approximately 17,000 new warheads before 1990. Increasing the capacity of existing plutonium production facilities might still not meet the projected need. So planners are eyeing the 70,000 kilograms of plutonium contained in the used reactor fuel that is sitting around the country.

From the government's point of view, the plan has several attractions. It would solve the projected plutonium shortage--possibly more quickly and at less cost than building new production facilities. It could provide the crucial boost to the administration's fading hopes of getting a commercial reprocessing industry off the ground. And it would provide at least a partial answer to the seemingly unsolvable problem of how to dispose of spent reactor fuel.

But turning used reactor fuel into bombs would also be doing just what the United States has for years argued must not be done. It would erase the distinction--upon which the international trade in "peaceful" nuclear technologies is premised--between atoms for peace and atoms for war: that is, that there is one set of materials and technologies needed for nuclear power production and a different set for weapons production.

Technically, the distinction is a false one. Reprocessing, once considered essential to the civilian fuel cycle, produces plutonium, and reactor-grade plutonium--while not the best for the purpose--makes a perfectly satisfactory bang. But symbolically and politically, the distinction is the foundation of the international non-proliferation regime--from its beginnings in the Atoms for Peace program to the Non- Proliferation Treaty and the safeguards and inspections of the International Atomic Energy Agency.

Looking back, one can wish that it had happened differently, that the large overlap between nuclear energy and nuclear weapons technology had been recognized from the beginning and built into the arrangements worked out for international nuclear trade. It may be possible in the future--perhaps after the world has had a bad nuclear scare--to overhaul the non-proliferation regime. But right now, the existing system is all there is. The very last thing in this country's security interest would be to take a step that could easily destroy what remains of that system's effectiveness and at the same time cripple America's capacity for leadership in the continuing effort to slow nuclear proliferation.