Argentina is about to start building an "experimental" plutonium reprocessing plant, the president of Argentina's Atomic Energy Commission disclosed.

The Argentine decision is certain to dismay the Carter administration, which has sought to curb the spread of reprocessing facilities because the plutonium they produce can be used to fabricate nuclear weapons.

The commission head, Adm. Raul Castro Madero, said in a interview that the Argentine reprocessing plant, which will be on the grounds of the heavily guarded Ezeiza Atomic Center just outside Buenos Aires, probably will be completed in the early 1980s.

This suggests that Argentina, long the Latin American leader in nuclear technology, is likely to have its experimental facility in operation at least five years ahead of Brazil, which is scheduled to start building a laboratory-scale plant with West German aid in 1985.

Moreover, the Carter administration, which unsuccessfully mounted a major effort to halt the West German-Brazil deal, is unlikely to have much success in changing Argentina's plans, since Argentina does not intend to seek outside assistance.

"It will be done entirely nationally," Castro Madero said during the interview in his office at Atomic Energy Comission headquarters.

"It is difficult but we have a very high level of scientists and professionals, so we think we will be able to build our own without any help."

Informed foreign observers take this contention very seriously, since Argentine scientists and engineers built a laboratory-scale reprocessing plant at Ezeiza in the 1960s.

Castro Madero said this earlier facility "reprocessed some milligrams of plutonium" before it was dismantled in the early 1970s. Argentina sources said the fuel that was reprocessed in the first Ezeiza plant came from the RAI research reactor at the Constitutentes Atomic Center, also on the outskirts of Buenos Aires.

While Argentina has steadfastly refused to sign the nuclear nonproliferation treaty contending that it discriminates against states without nuclear weapons, all Argentina's nuclear facilities are under Internation Atomic Energy Agency safeguards.

This is because Argentina's three nuclear research reactors, while designed and built locally, are fueled with enriched uranium supplied by the united States, which provides it only to safeguarded facilities. The Atucha nuclear power plant - which in 1974 became the first and still the only operating nuclear power station in Latin America - also is under safeguards.

"We are de facto full-scope safeguards," Castro Madero said. "When we in the future build a reprocessing plant, that will also be under safeguards from the very moment fuel from either the research reactors or Atucha comes in, because all fuel elements are under safeguards and they carry the safeguards along with them."

Despite this pledge, the Carter administration will certainly refuse to allow even minute quantities of the enriched uranium that the United States supplies for the research reactors to be reprocessed at Ezeiza.

Argentina, however, is working to achieve independence from foreign suppliers by striving to produce domestically all materials needed to operate natural uranium reactors lik Atucha.

A new factory already is under construction at Ezeiza that will fabricate the fuel elements needed to operate Atucha from the natural uranium present in large quantities in Argentina.

"We think that in the second part of next year, we will start fabrication on an industrial scale," Castro Madero said.

Argentina also plans within the next month to start construction of an experimental heavy water plant, which is expected to go into operation in 1980 and produce two or three tons a year.

"Once we achieve all the know-how, we will be in a position to ask for bids for an industrial plant - 250 tons," Castro Madero said.

The United States and other major supplier nations have been trying in recent years to halt also the spread of heavy-water technology, which is regarded as a proliferation problem akin to reprocessing.

While heavy water is needed to operate a natural uranium power plant like Atucha, the ability to produce heavy water would give Argentina the capability of constructing a large unsafeguarded natural uranium research reactor like Israel's Dimona - which yields enough plutonium to produce several nuclear weapons annually.

Castro Madero emphasized during the interview that Argentina sees no "need for nuclear weapons," but is determined to control the fuel cycle for the natural uranium power reactors it intends to continue constructing.

Castro Madero said Argentina believes that reprocessing will be "economical" by the 1980s, and that "all countries in the future will have to reprocess."

"For instance, plutonium will be very important for the Atucha fuel elements," Castro Madero said. "If you put some plutonium there, burnup will increase and the cost of electricity generation will decrease.

"Now, the United States says no reprocessing. They day after, they says yes," Castro Madero said. "For a country like ours, it takes time to develop a new technology. So we are planning to be ready for the 1980s to be in a position to decide by ourselves whether to reprocess or not.

"We think we have the right," declared Castro Madero, "to decide by ourselves to manage the technology."