India has become the first developing country to begin stockpiling separated plutonium at a rate that would enable it to build about 20 atomic bombs a year if it decides to pursue a nuclear weapons program, according to U.S. and international sources.

The process of separating the plutonium from burned-up fuel produced by Indian atomic power plants is taking place at the Tarapur reprocessing facility near Bombay, which went into large-scale operation last November, according to the International Atomic Energy Agency (IAEA).

Previously, the plant only had been used to reprocess small amounts of spent fuel from research reactors.

According to IAEA officials, the separated plutonium is being kept in a storage room there where it will be held until Indian officials decide its use. Indian atomic energy officials said yesterday that the plutonium might fuel an experimental fast-breeder reactor, which itself would produce additional plutonium.

The significance of large-scale reprocessing in India, however, is that it marks the beginning of the era that has long worried nuclear nonproliferation experts--the time when nations without nuclear arsenals start accumulating sizable stockpiles of separated plutonium.

"There simply is no reason connected to their civilian power programs why any of these countries need to begin stockpiling separated plutonium in this decade," a U.S. government official said. "It makes no sense in an atomic power context at all." Until now, virtually all of the plutonium produced in atomic power plants in developing countries has remained part of the highly radioactive spent fuel.

The plutonium stored in this way in spent-fuel pools in several dozen countries is not easily accessible.

If a nation decided to use such plutonium to build nuclear weapons, it first would have to construct a reprocessing plant, which would take at least two years.

But if a country has a stockpile of separated plutonium, the temptation to build nuclear weapons in a time of crisis might increase greatly. A country that already had done weapons-design work might be able to fabricate separated plutonium into bombs in a matter of weeks, or perhaps days.

The fact that India is beginning to stockpile large amounts of separated plutonium is certain to increase apprehension in neighboring Pakistan, where U.S. analysts believe the governnment is attempting to develop a nuclear weapons capability.

India has stated that it does not plan to build nuclear weapons.

But the atomic device exploded by India in 1974 was made from plutonium. So if New Delhi decided in response to developments in Pakistan or for other reasons that it needed a nuclear arsenal, it presumably could turn a stockpile of separated plutonium into weapons in a relatively short time.

The spent fuel being run through the Tarapur reprocessing plant, located about half a mile from the U.S.-built Tarapur atomic power station, is from the Canadian-designed Rajasthan Atomic Power Plant 1, according to IAEA officials.

At the moment, two agency inspectors with a mandate to keep track of the plutonium and sound a warning if any is diverted for military use are observing the entire Tarapur reprocessing operation.

But agency inspectors will not be permitted to observe all of Tarapur's reprocessing runs in the months ahead, since India refuses to place the plant under full international safeguards. Only plutonium subject to international control because of specific agreements between India and foreign governments will be monitored.

The IAEA also has not decided whether to keep the separated plutonium for which it is responsible under "continuous inspection," or simply to check on it "perhaps every second week," officials said.

The Tarapur facility is designed to reprocess 100 tons of spent fuel annually, so if it operated at full capacity--highly unlikely since commercial reprocessing plants frequently encounter problems--it would be able to separate 135 to 150 kilograms of plutonium per year. (A kilogram equals 2.2 pounds.) Six to 8 kilograms of plutonium are needed for a nuclear weapon.

The ability to separate plutonium on this large a scale puts India far ahead of Pakistan, which has built but not yet begun to operate a much smaller reprocessing plant next to the Pakistan Institute for Nuclear Science and Technology on the outskirts of Islamabad.

It also puts India well ahead of countries such as Argentina and Brazil, which have laboratory-scale reprocessing facilities where years might be required to separate enough plutonium for a single nuclear weapon.

But Pakistan is trying to complete a reprocessing plant even larger than Tarapur from a French design; Argentina is building a reprocessing facility that could produce significant quantities of plutonium, and West Germany is helping Brazil design a plant.

The only non-nuclear weapons country other than India operating a commercial-size reprocessing plant and stockpiling large quantities of separated plutonium is Japan, which has a larger and far more advanced civilian atomic energy program than India.

It is estimated that Japan has 500 kilograms of separated plutonium stored at its Tokai reprocessing plant, according to U.S. sources. But Japan is operating a breeder that requires 180 kilograms of plutonium a year and has an atomic power research program that requires 145 more kilograms of plutonium per year.

India, by comparison, has said it ultimately will need 50 kilograms of plutonium to fuel a small experimental breeder reactor being built at Kalpakkam near Madras. Yet its reprocessing plant is capable of turning out much larger quantities of separated plutonium.

India has accumulated 260 to 270 tons of spent fuel at its two Rajasthan atomic reactors, according to U.S. sources. If it reprocesses all of this fuel, which would take at least three years, India would have 350 to 400 kilograms of separated plutonium, they said.

Beyond that, the General Electric-built atomic power plant at Tarapur has produced spent fuel estimated to contain 700 to 750 additional kilograms of plutonium, according to U.S. sources.

India legally is barred from reprocessing the fuel from the Tarapur atomic power station until at least 1993 under its original agreement with the United States and a follow-up agreement with France, which began supplying fuel for the Tarapur reactors last November.

The Tarapur reprocessing plant, however, originally was built to handle spent fuel of the type produced by the Tarapur power reactors as well as from the Rajasthan reactors, which use different technologies, and Indian officials continue to hint that they may reprocess the U.S. fuel at some point.

U.S. analysts say Indian technicians probably could convert the reprocessing plant from handling Rajasthan fuel to handling the different type of fuel from the Tarapur reactors in about two months.

Dr. Hans Gruemm, IAEA's deputy director general for safeguards, said in a telephone interview from Vienna that plutonium being separated at Tarapur is "under continuous inspection" by the IAEA. He said the agency was determining how to keep track of the stored plutonium after the current run.

He said that even when the Tarapur plant reprocesses spent fuel not subject to inspection, which would let India close the plant to agency inspectors, the agency will be entitled to monitor the plutonium under its supervision in the plant's storage room.

"Whether it will be continuous inspection is another question," Gruemm said. "Perhaps it will be every second week, though that is an example. It has not been decided."

He also said he saw no reason why India could not comingle plutonium under international controls and plutonium not subject to inspection in the same storage room. "In principle, it would be possible," he said.