SINCE THE closing months of the Ford administration, the United States has invested an enormous amount of time, energy and political leadership in trying to halt exports of sensitive nuclear technologies--those that provide direct access to nuclear weapons fuel. Foremost among these is reprocessing, the procedure by which used nuclear fuel is separated into wastes and plutonium. Now, newly disclosed documents reveal, the Reagan administration has executed a complete about-face: instead of taking the lead in opposing such exports, the United States will rejoin this suicidal enterprise.

The effort to foreclose the trade in sensitive technologies dates from the Indian nuclear test in 1974. By that time, several such deals had been signed or were under consideration. The Indian explosion shocked this country and some of the other nuclear exporting nations into the realization that for minor commercial gain they were mortgaging their long- term security, changing the world from one with five nuclear nations to one with possibly dozens. The difficulties of reversing policy in midstream and the irritations of greedy sellers and disappointed would-be buyers seemed minor by comparison.

In order to demonstrate by example that there are safer and economically better ways of disposing of spent reactor fuel, the United States announced in 1977 that it would not reprocess for the indefinite future. It renounced exports of sensitive technologies. It leaned heavily on allies who had already agreed to such exports to cancel them. And in 1978 Congress passed the Nuclear Non-Proliferation Act establishing sanctions--the cutoff of all U.S. nuclear exports-- against any nation that subsequently supplied reprocessing technology to a non-nuclear-weapons state.

The new Reagan policy puts the United States in the ludicrous, if not illegal, position of having a law on its books requiring sanctions against other nations for doing what it is proposing to do itself. The reasons for the change in policy have not been fully explained. However, an administration official has made it pretty plain that the motivation was the same one that has accounted for so many past mistakes--greed. French and German firms are competing for contracts to build a reprocessing plant in Japan, he said, and "It's very hard for us to deny American firms the right to participate in what other firms participate in."

The new policy does not stop with Japan. Reprocessing may be exported to any country with "effective commitments to nonproliferation, where there are advanced nuclear power programs and where such activities do not constitute a proliferation risk." In practice, that means the State Department will have to separate nuclear client states into good guys and bad guys, inviting certain diplomatic friction or an export-to-anyone-regret-later policy. It is easy to imagine situations in which the State Department would find it next to impossible to say no to a requesting country no matter how obvious the proliferation risk. What would it say, for example, to requests from Taiwan, Pakistan, South Korea or (absent the Falklands crisis) Argentina?

When this country first launched its drive to halt the export of reprocessing plants, cynics in Europe insisted that the real motivation was to freeze the nuclear trade so as to leave the United States in a position of permanent commercial advantage. The new Reagan policy deserves to be reversed on the merits alone, but its broader implications include the fact that by confirming that view, the United States can kiss goodbye to any hope of exercising future leadership in the continuing effort to contain the spread of nuclear weapons.