Every three weeks for the past five years, members of the little-known interagency Subgroup on Nuclear Export Coordination (SNEC) have been meeting at the State Department to review proposed exports by U.S. companies that might help other countries develop nuclear weapons.

During the final year of the Carter administration, one of the applications that came before SNEC was a Control Data Corp. request to sell a powerful Cyber 170/750 computer to South Africa's state-run Council for Scientific and Industrial Research.

Out of concern that the Cyber 170/750's impressive scientific capabilities could be misused in a variety of ways--such as in the design of small nuclear weapons or to give South Africa confidence that a weapon it had built would work without the need for a test--the Carter administration vetoed the export.

But after the Reagan administration came into office, Control Data renewed its effort to sell a Cyber 170/750 to South Africa. A team was sent to Pretoria "to attempt to make some judgments as to whether we were getting a straight story" about the planned non-military use of the Cyber, a member of SNEC said, and South Africa also agreed to periodic access by U.S. personnel to the computer.

"But hell, it's one thing to get these nice assurances and another thing to verify them," a source involved with the decision said. "There's no way to assure that computer won't be used to perform runs for the South African nuclear program."

In April the Reagan administration approved Control Data's export request.

The easing of restrictions on gray-area exports is one of the major differences in the approaches of the Reagan and Carter administrations to the 37-year U.S. goal of nuclear nonproliferation.

Reagan officials defend their more flexible approach, saying the United States must become a more reliable, and thus more influential, supplier of nuclear materials and equipment. And they strongly deny that stopping the spread of nuclear weapons is a lesser priority for this administration than the Carter administration.

"President Reagan doesn't lack the true faith at all," declared Eugene V. Rostow, director of the Arms Control and Disarmament Agency.

Deputy Energy Secretary W. Kenneth Davis, one of Reagan's top advisers on nuclear issues, said, "People have gotten the idea that this administration is somehow less interested in retarding proliferation than previous administrations. That simply is not true. I think we are as much concerned--perhaps even more concerned--about slowing down or retarding it."

Harvard professor Joseph S. Nye, the architect of the Carter nonproliferation policy, agreed that "at a high level of abstraction, the main lines are the same."

"But when you get down to specific details, there are major differences," Nye said. "And I think some of the differences are serious."

Some of the major differences cited by a variety of experts interviewed during the past week include these actions by the Reagan administration:

* The decision to adopt an explictly discriminatory nonproliferation policy, announcing that it will approve nuclear deals for Europe, Japan and Australia that it would not approve for developing countries. "This idea of declaring a new category--the untouchables, who would not get the kind of cooperation we would give to the good guys--worries me," said Rep. Jonathan D. Bingham (D-N.Y.), one of the authors of the 1978 Nuclear Nonproliferation Act. "It will just add to the resentment and add to the problem."

* The new plutonium policy, under which the administration intends to give European nations and Japan long-term approval to reprocess fuel supplied by the United States for power plants and separate out the plutonium. Plutonium can be used to manufacture weapons as well as to fuel power plants. In an effort to prevent plutonium from becoming an article of commerce, both the Ford and Carter administrations treated requests to reprocess fuel on a case-by-case basis.

* The decision to revive the commercial reprocessing plant in Barnwell, S.C., and the accompanying decision to permit export "in certain cases" of reprocessing equipment and technology. Carter had blocked the Barnwell plant in an effort to demonstrate to the world that reprocessing was neither a necessary nor economic step in the atomic power process, and his administration persuaded France to cancel plans to sell a reprocessing plant to Pakistan. The Ford administration earlier aborted a French deal with South Korea.

* The suggestion that it might be necessary to use the plutonium produced by atomic power plants around the United States to manufacture nuclear warheads later in the 1980s. "I think suggesting the use of civilian plutonium for military purposes was most unfortunate," said Charles Van Doren, former assistant director for nonproliferation at the Arms Control and Disarmament Agency. "It would be a terrible example internationally, and would undermine many of the things we have been trying to accomplish."

* The decision to approve the export of uranium enrichment technology to Australia. This technology, the most classified step in the production of material that can be used in nuclear weapons, has never previously been shared with a non-weapons state.

* The decision to resume massive aid to Pakistan, which Carter had suspended after it became clear that Pakistan was secretly building both a uranium enrichment plant and a clandestine reprocessing plant. "This was most unfortunate," said Carter's special nuclear envoy, Ambassador Gerard C. Smith. "It sort of legitimizes reprocessing and enrichment, even though you have no need of it for a power program, as long as you don't stage a nuclear test."

* The decision not to attempt to press countries like Argentina, South Africa, Brazil and India to place all their nuclear facilities under full international safeguards, nor to press supplier nations like France to make full-scope safeguards a criteria for nuclear exports. "This clearly is no longer a high priority," said Nye, "and we lost a real opportunity with the French right after Francois Mitterand was elected president. If we had pushed hard, there were indications Mitterand might have agreed. But the French said as far as they could tell, the Americans had lost interest."

* The decision to suspend indefinitely talks designed to lead to a comprehensive nuclear test ban treaty, pending renegotiation with the Soviet Union of two Ford and Nixon-era treaties limiting some nuclear tests. "I felt the comprehensive test ban treaty was the key to ultimately having a nondiscriminatory nonproliferation policy," said Smith. "For us to now say we will suspend negotiations sends a very bad signal throughout the world."

* The fact that Reagan has made only one major statement on nonproliferation, and decided against raising the issue at either the Versailles or Ottawa economic summits. His first secretary of state, Alexander M. Haig Jr., also was relatively silent on nonproliferation, and there now are far fewer people--some estimate only half as many as in the Carter era--working on nonproliferation. "We made quite a nuisance of ourselves around the world," said Smith. "Stopping making a nuisance of ourselves sends a signal that maybe it's tolerable."

Reagan officials for the most part do not dispute the suggestion that there has been an easing of export policy--particularly for dual-use items. The administration fairly quickly abandoned plans to try to amend the Nuclear Nonproliferation Act, which spells out the statutory criteria for most nuclear exports.

In a May 12 letter to Senate Foreign Relations Committee Chairman Charles Percy (R-Ill.), Commerce Secretary Malcolm Baldrige justified the export to South Africa of 95 grams of helium 3, which could be used to make tritium for thermonuclear weapons, by saying:

"This administration has adopted a more flexible policy with respect to approvals of exports of dual-use commodities and other material and equipment which have nuclear-related uses in areas such as health and safety activities."

But Reagan officials insist that, far from undermining efforts to curb the spread of nuclear weapons, a more flexible export policy will increase America's influence in this area.

"The old methods to which earlier administrations and many people in Congress are still committed rest on illusions--the illusion of American omnipotence and the illusion that the United States Congress has jurisdiction over the policies of many other parts of the world," said Rostow.

"We want to get rid of the nostalgia for an epoch of monopoly that is gone. There is absolutely no use trying through export controls to achieve what can't be achieved," he said.

The Energy Department's Davis contended that the Carter "policy of denial would have had as its ultimate goal more or less excluding us from world councils on nuclear power.

"We used to be the leader in the world, developing safeguards, developing international agencies, developing the whole process," said Davis. "We were the leaders because we were the leaders in the international trade related to nuclear power. We lost a substantial part of our leadership in technology under the previous administration." The Carter policies, he said, "essentially made us unreliable and we lost our credibility as a supplier of enrichment and reprocessing."

"We were trying to set examples by not reprocessing. All we did was stimulate other countries to consider or start to get into reprocessing themselves," Davis said. "Because of the great hiatus, we are having great trouble getting reprocessing back on a sound footing in this country."

Davis and Rostow were both sharply critical of the Nuclear Nonproliferation Act's requirement that the United States cut off fuel supplies for atomic power plants in countries that refuse to place all of their facilities under full international safeguards. While the Carter administration attempted to press India, South Africa and Brazil into complying with the requirement, the Reagan administration resolved these disputes by permitting the three countries to get fuel from Europe.

"By and large, we certainly would like to see full-scope safeguards," Davis said. "But I think we can visualize instances where that might not be a reasonable requirement. We are in a situation where, even despite previous contracts and previous arrangements, we are obliged to try to achieve that regardless of the logic. Other countries are not under that kind of compulsion. I'd have to say that right now, we are sort of the ones that are out of step."

Reagan officials agreed that creating a discriminatory export policy also seemed certain to create tensions with countries such as South Korea and Taiwan, but placed a higher importance on "reestablishing our credibility" as a nuclear supplier with our major allies.

"One of the things we have to accept is that we've got to be very selective. We have to work with the people we trust and be very tough on those we don't," said Davis. "It raises a certain amount of tension. But we want to create that perception--that we are not going to be pushed over. We hope they get the message. We certainly do not want to see nuclear weapons spread to additional countries."