The Soviet Union today rejected President Carter's bid to forego the plutonium economy and its threatened spread of atomic weapons.

A Soviet official, Vladimir Shmidt, told an international nuclear power conference here that "plutonium must be used."

The Soviet Union, he said, will press on with its fast-breeder reactors that create more plutonium than they use up as fuel.

The Soviet Union is believed to have one fast-breeder operating commercially and a family of smaller ones for experiments.

His statement came as no surprise to U.S. officials here, although they said it was the first time that Moscow had responded in public to the President's example.

Last month Carter announced that to United States would indefinitely postpone one commercial development of its own fast-breeder and seek a fuel other than plutonium. He also warned of the dangers from a plutonium age.

Plutonium, an element created in nuclear reactors, is one of the fissionable or splittable materials from which an atomic bomb can be made easily. Fast-breeders are attractive to nations concerned about the limited supply of uranium, the vital element in conventional nuclear plants.

Carter fears that the spread of fast-breeders will endow nations with a bomb-making capacity that they now lack.

Sdmidt, who spoke at the International Conference on Nuclear Power and its Fuel Cycle, sponsored by the United Nations' International Atomic Energy Agency, is a ranking official in Moscow's Union Institute of Inorganic Materials.

He bluntly dismissed the proliferation problem, saying: "Problems of proliferation do not concern us so much" because "We are a socialist society."

Shmidt indicated that the Soviets would use plutonium because uranium is limited, and he made it clear that they would not slacken their work on fast-breeders.

In private, Soviet officials were less hard-line, pointing out that Carter's renunciation is only a month old and that Moscow is still digesting its implications.

In any event, they said, the Soviet Union is a centrally planned economy that makes investment decisions years in advance and cannot simply turn off a major program in an instant.

Other delegations here, while less blunt than Shmidt, made it clear that they have no intention of following the Carter lead. Sir John Hill, chairman of Britain's Atomic Energy Authority, threw cold water on U.S. suggestions that another fuel, phorium, could be substituted for plutonium. Too much, he indicated, had already been invested in existing technology to junk it.

French, Belgian and Japanese spokesmen also made it clear that they are devoted to plutonium. As one said, the only thing to do with plutonium is "burn it," as a nuclear fuel.

One lone dissenting voice, Hannes Alfven, a Nobel Prize-winning physicist from Sweden, told them that existing nuclear technology "is obsolete and should be phased out as soon as possible."

It is, he said, "a serious threat to human ecology" because it is inexplicably linked to the production "of enormous quantities of radioactively poisonous substances" and the making of nuclear weapons.

The best scientists, Aleven said, no longer go into the field, so second-raters are now dealing with the complex proglems that nuclear energy poses.

another Nobel Prize-winning physicist, Hans Bethe of Cornell, told them that "exaggeration demagoguery and emotionalism" marke the foes of nuclear power. The proponents, he said, have all the best agruments.