IT WAS a seductive argument that the Carter administration used to justify sending fresh nuclear fuel to India. Without it, the Carter people said, India might begin reprocessing spent fuel and acquire the plutonium to make more nuclear bombs if it chose. So to "maintain a dialogue" and to preserve "leverage," Mr. Carter overrode heavy congressional resistance and kept fuel flowing, notwithstanding India's refusal to accept full safeguards against nuclear proliferation.

So what is happening now? The Indian government has just said it will begin reprocessing spent Canadian-supplied fuel anyway -- to make more fuel, not bombs, it insists. Moreover, it is asserting a right to reprocess American-supplied fuel, notwithstanding the American veto written into the uranium-supply agreement of 1964. The Indians even propose to put a "friendly" end that agreement -- they will not raise a fuss if they are relieved of their nonproliferation obligations.

Unfortunately, India is not the only place where an American attempt to be accommodating on nuclear fuel is coming to grief. There's also Japan. The Carter administration had opposed its building of an experimental reprocessing plant at Tokai-Mura because it would have the potential to produce weapons-grade plutonium. But the plant was built and, for the usual go-along reasons, the administration agreed that Japan could do a little reprocessing of its American-supplied fuel. Then it said Japan could do a little more. Now Japan wants all American restrictions on the plant removed.

There is a great difference between India and Japan. India cheated on its international obligations and exploded a bomb, which is called a peaceful device, in 1974. Japan has provided no similar grounds for suspicions, and its experience as an atomic victim puts it in a special category. With plutonium, which comes from reprocessing spent nuclear a bomb can be built in 10 days.

To set an example; President Ford in 1976 committed the United States to a policy of self-denial in reprocessing. But President Carter found several reasons, all bad, to do less than he might have done to restrict reprocessing by other nations. President Reagan inherits a situation where, in some instances, the horse either has been stolen or is in the process of being stolen. His minimal requirement is to see that other nations, especially purchasers of American fuel, are in no doubt as to whether or not he opposes the spread of nuclear bombs.