The Carter administration, having lost its two-year struggle to curb the spread of nuclear facilities that produce plutonium, has decided to strongly back establishment of internationally controlled plutonium storage banks where countries would deposit their "excess" stocks of the weapons-grade material.

An International Atomic Energy Agency technical group had been working on the proposal since December 1978 with very little encouragement from the United States. But it now seems to be emerging as one of the main elements of a new Carter administration nuclear nonproliferation policy.

Sources here believe that an International Plutonium Storage agreement could be put into effect by the end of 1981. It would greatly increase the nonproliferation role of the International Atomic Agency by putting its inspectors for the first time in direct physical control of materials that could be used to fabricate nuclear weapons.

The first international storage faclity, sources said, would quite likely be located in France or Britain.

International plutonium storage facilities could represent a major step toward reducing the risk of the spread of nuclear weapons, since they would greatly restrict the availability of plutonium at the most sensitive stage of the nuclear power fuel cycle. This stage occurs after the plutonium has been separated from used atomic power plant fuel by a reprocessing plant, but before it is reused as fuel for a reactor.

The International Nuclear Fuel Cycle Evaluation study that just concluded here reported that more than 36 tons of plutonium have been separated from used fuel to date, with only 15 tons of this plutonium put back into use.

This leaves about 21 tons of plutonium already in storage at various points around the world, and the study predicted that this amount would rise to 58 tons by the end of the 1980s.

Since about 22 pounds of this "commercial-grade" plutonium is sufficient to fabricate a nuclear weapon, there would be enough unused separated plutonium in storage by 1990 for more than 50,000 atomic bombs.

The Carter administration, until now, has been relatively cool to proposals to set up an international plutonium storage administration.

"We didn't like the idea very much because it sort of legitimized the notion that it was okay to separate plutonium, and of course we were against reprocessing," a U.S. official said.

But both the plutonium fuel cycle and reprocessing were legitimized at last week's windup of the 66-nation INFCE study, and U.S. Ambassador Gerard Smith noted as the final plenary that "it was recommended that as one follow-on mechanism, special attention should be given to placing excess plutonium under international oversight."

"The U.S. is prepared to work cooperatively for an effective international plutonium storage scheme," Smith declared.

Foreign sources said they were told privately that President Carter personally approved this statement and that the United States now hoped such an arrangement would be set up as quickly as possible.

A tentative plan for an international plutonium storage procedure as drawn up by the 24 nations taking part in the international working group, looks like this:

A nation that wanted to separate the plutonium from the used fuel turned out by its atomic power plants would register all of its plutonium with the IAEA.

Unless the IAEA -- before the used fuel was reprocessed -- received and approved a country's proposal spelling out exactly how it was going to reuse the separated plutonium, the plutonium would automatically be considered "excess" and would have to be deposited in internationally controlled storage.

The storage banks would probably not be built from scratch in a number of countries, but would be set up alonside existing reprocessing facilities and fuel fabrication plants. Since reprocessing plants turn out plutonium and fabrication plants need it to produce new fuel, these are places where plutonium is normally stored in quantity as part of industrial opeations.

"We are only talking about the prospect of a dozen sites in the foreseeable future," said Michael James, who has been directing the IAEA's plutonium management program.

These would presumably exist initially in the major industrial countries that already have reprocessing or fuel fabrication faciltities -- Britain, France, West Germany, Belgium -- although the possibility exists within a decade that they could also be established in such developing countries as Brazil and Argentina.

These international storage depots would be controlled by an IAEA inspector, who would take custody of the plutonium when it was deposited and not release it except on authorization by the IAEA.

"This would physically make it impossible for plutonium to leave the storage without agreement of the international authority," James said.

The working group has responded to the concerns of some countries, however, who were troubled by the idea that an international agency would have to pass on their requests to withdraw their plutonium, by creating a set of conditions under which release of plutonium would be automatic.

A country under this plan would submit an application for withdrawal of plutonium specifiying how much it needed, in what reactor it would be used, the date on which fabrication into fuel would begin, how long it would take, and when the fuel would be loaded into a reactor.

This statement of facts would be checked and the relevent facilities inspected. As long as the proposed quantity and the release date requested made sense, release would be automatically authorized.

"There would be no power to pass judgment on whether a country really needed the plutonium, James said.

One factor spurring on efforts to set up such storage procedures is an agreement by URENCO, a British, West German and Dutch consortium, to sell low-enriched uranium fuel to Brazil for use in its nuclear power plants at the end of 1981.

The agreement stipulates that for the sale to be completed, the plutonium prouduced when the URENCO fuel is used in the Brazilian reactors must be subject to an international plutonium storage scheme set up by the IAEA.