A two-year international study of ways to keep nuclear power facilities from being misused to provide material for atomic bombs concludes that there is no hope of preventing a rapid increase in the worldwide availability of "weaponsusable" plutonium.

The results of the study, which will be presented here Monday to more than 50 nations attending the concluding meetings of the International Nuclear Fuel Cycle Evaluation (INFCE), forecast a sevenfold jump in the amount of plutonium produced by nuclear power plants -- to a total of more than 500 tons -- between now and the end of the 1980s.

About 22 pounds of this kind of "commercial-grade" plutonium would be needed to manufacture an atomic weapon, according to the study. The plutonium that will have been turned out by neclear power plants as part of their burned-up fuel by the end of this decade could be enough for more than 45,000 bombs.

While much of this plutonium -- including all that comes out of nuclear power stations in the United States -- will remain part of used reactor fuel elements and continue to be stored, growing amounts of plutonium produced in other countries will be sent to reprocessing plants to be separated from the burned-up fuel.

These reprocessing plants, according to the study, will have separated out 162 tons of "commercial-grade" plutonium by the end of this decade. This plutonium could then be reused as fuel for nuclear power plants. It could also be diverted for fabrication of atomic weapons.

The results of the study, on the whole, are considered a blow to the Carter administration, which organized INFCE in 1977 in hopes of delaying construction of advanced nuclear facilities -- particularly plutonium reprocessing plants and fast-breeder reactors -- while searching for some less-risky alternatives to the plutonium fuel cycle.

The decision to build a plutonium reprocessing plant, the study says, certainly "increases the capability of a nation to construct nuclear weapons if it decides to do so." While it takes about two years to build a commercial-scale reprocessing plant, according to the study, a country that has a stock of reprocessed plutonium could use it to make a nuclear weapon in as little as one to three weeks.

Despite the proliferation risks clearly inherent in the spread of commercial plutonium reprocessing plants, a concern that led President Carter to halt construction of such facilities in the United States in 1977, the study legitimizes the construction of reprocessing plants, particularly for countries with large nuclear programs like Japan and West Germany.

"Reprocessing in itself is important because it is an essential preliminary to many of the possible fuel cycles," the study says.

In fact the study says "substantial and increasing reprocessing capacity would be needed" if a number of countries continue to press ahead with plans to build plutonium fast-breeder reactors -- another area of technology that Carter study found preferable to present-generation nuclear reactors on virtually every score.

The study found no hope of developing an advanced fuel cycle for nuclear reactors that would not produce plutonium -- such as a thorium fuel cycle -- that could be available on a commercial basis until well after the year 2000. The technical studies concluded, moreover, that from a non-proliferation viewpoint, a thorium fuel cycle might have no advantage over plutonium.

"I think the importance of this study for the United States is that we now must recognize the inevitability of plutonium," a top U.S. official who took part said. "It's going to be there. We'd damn well better learn to live with it, and we'd better introduce a system so that the likelihood of diversion is minimized."

Despite the fact that the study is a general endorsement of both reprocessing and the fast-breeder, Carter administration officials maintain that the outcome was not without positive aspects.

During the two-year exercise, forecasts of future world energy demand dropped sharply and many countries revised downward their plans for installing nuclear power.This, in turn, alleviated fears of some countries of a future shortage of enriched uranium fuel for their power plants.

"There was a widespread belief that there was going to be a shortage of uranium enrichment capacity and everybody came around to realizing that this wasn't true," a U.S. official said. "Uranium prices now are considerably lower than when INFCE started."

As a result, some nations that were thinking of building a plutonium reprocessing facility as an assured way of having fuel for their nuclear power plants no longer see reprocessing as necessary on these grounds."

Beyond that, the study concluded that the economic advantage to reprocessing plutonium for use as fuel in current nuclear reactors "is not likely to be large." Reprocessing looks particularly unattractive economically, the report said, for developing countries with small nuclear power programs.

As a result, one U.S. official contended, INFCE has made it virtually impossible for a country that is interested in acquiring an atomic weapons potential to attempt to justify construction of advanced nuclear facilities on economic grounds.

"The INFCE process has made the fuel cycle path to nuclear proliferation less attractive because you can't simply back in," the official said. "I think if a country with one reactor [like Pakistan in 1976] goes ahead now and tries to buy a reprocessing plant and claims that it's a natural part of the development of nuclear power, there is not going to be a national leader in the world that will believe them."

But the study held open the possibility that developing countries -- such as India -- could justify building reprocessing plants on noneconomic grounds. And it said other developing countries such as Such Korea may soon have nuclear programs of a size that will make the desire to have a reprocessing plant "appropriate."

The result, the reports conclude, will certainly be a sharp increase in the amount of reprocessing taking place and the amount of reprocessed plutonium available around the world in the 1980s and 1990s.

"The concern for the future is thus, in the event reprocessing develops, to adopt the best technical safeguards and institutional measures to increase the protection of such material against diversion," the study recommends.

The nations meeting here will be discussing where the nuclear nonproliferation effort should take them during the final meetings this week.