The Reagan administration, in a major reversal of the Carter administration's nuclear nonproliferation policy, has decided to permit the export "in certain cases" of sensitive reprocessing technology and equipment that could be used to produce bomb-grade plutonium.

The shift in policy is contained in a classified State Department paper on "Reprocessing and Plutonium-Use Policy." While some of the details--including a decision to give other nations more control over American-supplied fuel for nuclear power plants--leaked out earlier this week, State Department officials who held classified briefings for selected Senate and House members Thursday made no mention of the change in the export of reprocessing technology.

"I think this is an absolutely disastrous thing to do," Rep. Jonathan B. Bingham (D-N.Y.), one of the authors of the Nuclear Nonproliferation Act, said yesterday. "I think this is totally contrary to the intent of the Nonproliferation Act, and Congress will seek out every means to prevent this from happening. If this is done, we can just forget about efforts to control proliferation."

State Department officials have declined to make the policy paper public on the grounds that it forms the basis for negotiations with other countries, but yesterday several officials involved in drafting it confirmed that it alters the policy governing the export of reprocessing equipment.

"This administration does not have as negative attitude toward reprocessing as the previous administration," one State Department official said. "We're not going out encouraging this. But we will look at each case in the future and decide whether it makes sense to do this. In some cases, it may. There may be some opportunities for influencing these programs in a positive way if the U.S. gets involved, particularly in the safeguards and security area."

The new policy, said a State Department official, will apply only to "countries with effective commitments to nonproliferation, where there are advanced nuclear power programs and where such activities do not constitute a proliferation risk and are under effective safeguards and controls." In practice, a source said, this will be defined as Japan and European Atomic Community members.

In fact, sources suggested that one of the factors in the policy shift is the apparent interest of Japan in purchasing American technology for a new $3.2 billion commercial reprocessing plant that it plans to build on the Shimukiia Peninsula in northern Japan.

While France's Saint Gobnain Techniques Nouvelles reportedly has been the leading candidate to carry out the basic design of the new plant, Japanese nuclear sources have recently expressed increased interest in obtaining sensitive equipment for the plant from West Germany and the United States.

"We've made a decision to permit American firms to compete for contracts to construct reprocessing plants," an administration official said. "It's very hard for us to deny American firms the right to participate in what other firms participate in."

Administration sources contended yesterday that Japan, which operates a small pilot nuclear reprocessing plant at Tokaimura, already is capable of separating weapons-grade plutonium from used power plant fuel, so the United States would not be increasing the risk of spreading nuclear weapons by providing reprocessing equipment.

But State Department sources conceded that the new policy inevitably would encourage developing countries like Taiwan and South Korea to renew their efforts to obtain reprocessing technology, which would give them a nuclear weapons capability.

"What this does is again raise the discrimination issue to a fine point," observed Harvard Prof. Joseph Nye, an architect of the Carter presidency's nonproliferation policy. "We certainly thought it was easier to maintain a policy of saying reprocessing does not make economic sense at this point, with it thus wise not to stimulate commerce in this area, than to open up the bidding and say there are good countries and bad countries."

Congressional critics suggested yesterday that the new policy will pose another problem: It could undercut the U.S. effort to persuade countries like France and West Germany to refrain from exporting sensitive nuclear technology to countries that pose a proliferation risk. Under the new policy, those countries could find U.S. firms competing with them in reprocessing equipment exports.

"Here we successfully persuaded other countries to limit their exports of reprocessing equipment because we weren't exporting this at all, and now we are going to be in a position of doing this," Bingham said. "Well, the administration has to submit the agreements of cooperation to do this, and we are going to get a handle on it."

Congressional experts also expressed concern over the other main element of the new policy, which permits countries to negotiate long-term agreements to send American-supplied fuel from their nuclear power plants out to be reprocessed and reused as fuel. Under the previous policy, countries had to seek U.S. agreement each time they wanted the fuel reprocessed.

Sen. Gary Hart (D-Colo.), who along with Rep. Richard L. Ottinger (D-N.Y.) introduced legislation last month to strengthen U.S. nonproliferation laws, said yesterday the new policy on fuel control will enable several nations "to stockpile plutonium, and thus stockpile the raw material for nuclear weapons."

Several Carter nuclear officials said, however, that they had also been moving toward a policy of long-term, rather than case-by-case, agreements for the reprocessing of fuel by certain countries. The policy change was seen as a way of making the United States a more predictable and reliable partner.

While the United States never turned down a foreign request to reprocess spent fuel, sources said, it held up approval in several cases in an effort to gain leverage on other nonproliferation issues.