President Carter's call for a worldwide pause in the use of plutonium nuclear fuel went unheeded yesterday when 40 nations ended three days of talks at the State Department without agreeing to any slowdown.

None of the 40 natins said last night at the end of the talks that they would cancel or defer projects to enrich uranium nuclear fuel, reprocess plutonium out of spent uranium fuel or use plutonium as a nuclear fuel once it had been extracted from spent uranium. President Carter has asked Congress to defer indefinitely plutonium projects in the United States while asking the rest of the world to follow the U.S. example.

State Department officials representing the United States in the talks tried to put a happier face on the closing seesion by insisting they never expected anybody else to agree to a plutonium slowdown.

"They weren't asked to slow anything down," Ambassador-at-Large Gerald Smith said last night at a press conference. "We never expected any agreements, since this is not a negotiation. It was a study designed not to interfere with any plans other nations already have."

Smith described the three days of talks as "serious and businesslike" in which all 40 nation agreed to split up into eight study groups to discuss for the next two years all aspects of the use of nuclear fuel that impinge on the possible spread of nuclear weapons.

"We do not expect these studies to reach solutions to these questions or to reconcile the views of various participants in this conference," Smith said. "These joint technical analyses pose a challenge without precedent byt the fact that they're being attempted bodes well for a succesful evaluation of these issues."

Outside observers viewed even the setting up of these eight study groups as something of a defeat for the United States since six of the eight are to be chaired and run by countries whose views on the plutonium question are in opposition to the White House.

The only two groups not opposed to the U.S. position will deal with "alternate" fuel cycles that do not produce plutonium from spent fuel and worldwide uranium supplies. They are the least controversial of the eight issues to be studied and are to be chaired by the United States and by Canada, which is one of the few countries of the world sympathizing with President Carter's views on plutonium.

Some of the most controversial studies will be chaired by countries most strongly opposed to the White House view on uranium enrichment and plutonium use.

The study group on uranium enrichment will be chaird by France, German and Iran, the first two of which would like to export enrichment plants and the last would like to import them. The study team on the reprocessing of plutonium from spent uranium is chaired by Japan and Great Britain. Japan is one of the world's most vocal advocates of plutonium fuel for power.

A third study group on the fast breeder power plant which produces plutonium and will run on plutonium will be chaired by Belgium, Italy and the Soviet Union. All three countries are at work in some way on fast breeder plants of their own.

Smith said that one bright aspect of the three-day session was that it brought together countries that have not signed the non-proliferation treaty prohibiting the spread of nuclear weapons. It had been reported by observers that some of these countries complained that they were being asked to sign the treaty while being denied technology that could bring them nuclear power.

"A standing concern of many nations is that they're not getting a fair share of nuclear technology," Smith replied. "This is going to be a constant tension in these discussions."

While the conderence was attended by most of the countries supplying and using nuclear energy, the absence of Taiwan and South Africa was conspicuous.

State Department officials insisted every country was welcome but Smith conceded that Taiwan did not receive a formal invitation and South Africa's wassissued only last week.