Pakistani technicians are assembling a clandestine plutonium reprocessing facility near Rawalpindi that U.S. intelligence experts say may enable the country to test its first atomic bomb two years earlier than previously expected.

The small reprocessing plant, whose existence has not been previously disclosed, could give Pakistan enough fissionable material to stage an initial atomic test in the fall of 1981, intelligence experts say.

It may enable Pakistan to stage a symbolically important initial nuclear explosion without waiting for completion of the large uranium enrichment plant under construction at Kahuta, about 25 miles south of the capital, Islamabad. U.S. intelligence experts believe the enrichment plant will not be producing bomb-grade uranium until at least 1983, and more likely not until 1985.

Western experts had thought work on the small reprocessing plant was abandoned years ago, when Pakistan bought a commercial reprocessing plant from France.

But when the French deal was scuttled in 1979, work on the small plant apparently was resumed clandestinely. Sources say that by the time the United States learned of the resumption and alerted European goverments to this development, Pakistan had already obtained the items it needed to build it.

Construction of the clandestine reprocessing facility, in a small building just outside the fence of the Pakistan Institute of Science and Technology near Rawalpindi, points up the determination of the government of President Mohammed Zia ul-Haq to join the club of nuclear weapon states as quickly as possible.

The Carter administration, in a declared effort to curb proliferation of nuclear weapons, has attempted to dissuade Western countries with nuclear capability from exporting their technology. It recently suspended nuclear cooperation with Switzerland, accusing that government of failing to block Swiss firms from selling Pakistan technology that can be used to develop nuclear weapons.

The small reprocessing plant, which intelligence experts say appears designed to provide 22 to 44 pounds of plutonium a year, will give Pakistan only enough fissionable material for "one or two -- at the most three" nuclear bombs annually.

"But fissionable material could be flowing out of the reprocessing plant in 1981," a U.S. government source said. "For Pakistan to have obtained a bomb's worth of plutonium from this facility within a year is probably the worst-case scenario, but we regard it as a prudent target to worry about."

U.S. intelligence experts are also keeping a close watch on a site in northern Pakistan that analysts feel may be the location that has been chosen for an initial underground nuclear test. Sporadic construction has been taking place there on a tunnel for which there appears no other explanation, and analysts note that the site is surrounded by watchtowers.

"Work at the site seems to have stopped again for the moment," a government source said. "But if they do plan to stage a test there, it would not take them long to get it ready."

The clandestine reprocessing plant that experts say is in the final stages of construction is not much more than one-tenth the size of the commercial reprocessing plant that France agreed to sell Pakistan in the mid-1970s. While France, under American pressure, suspended this deal before any sensitive equipment had been shipped to Pakistan, U.S. sources say French firms sold Pakistan key items for use in the clandestine plant before the new project was uncovered.

The discovery that a clandestine plutonium reprocessing facility -- and not the highly publicized uranium enrichment plant -- will give Pakistan a nuclear weapons capability much earlier than had been feared has also focused new attention on that country's only atomic power plant, a Canadian-built heavy water reactor located near Karachi.

In almost eight years of not very successful operation, the Kanupp power station has turned out spent fuel containing an estimated 220 pounds of plutonium -- enough to produce about 15 bombs. Because of the nature of heavy water reactors, the plutonium contained in spent fuel is better suited for use in weapons than the plutonium produced by light water nuclear power plants such as those in the United States.

One possibility worrying Western intelligence experts is that some of the spent fuel from the Kanupp plant, which is stored in a pool near the reactor, has already been diverted for use in the clandestine reprocessing plant.

The storage pool is nominally monitored under the "safeguards" procedures of the International Atomic Energy Agency -- even though Pakistan is not a signatory of the Nuclear Nonproliferation Treaty -- as part of the deal under which Canada provided the Karachi reactor.

But agency's "safeguards" consist mainly of photographic surveillance on the storage pool and auditing of records with only occasional on-site inspection -- a process that even the agency concedes provides no guarantee against diversion. In an effort to find out whether some material may already have been diverted, the agency has agreed to take a new inventory of the storage pool outside the Karachi reactor.

An even more worrisome path for diversion of fuel, however, was opened with Pakistan's announcement three weeks ago that it is now fabricating its own fuel elements for the Karachi reactor using natural uranium it is obtaining from Niger.

Unlike the light water atomic power reactors in the United States, which have to be completely shut down for a partial refueling, the heavy water CANDU-type reactor operated by Pakistan is designed in a way that permits fuel elements to be inserted and taken out while the plant is in operation.

U.S. experts say that if Pakistan is now manufacturing its own fuel elements, it could fairly easily insert these into the Karachi reactor, leave them in the reactor the relatively short period that is optimum if the aim is to produce plutonium for a weapons program, and then ship them to the clandestine plant for reprocessing.

"I think we can be relatively certain that one way or another, the material that they put through the reprocessing plant will come from the Kanupp reactor," a U.S. source said. "And they're going to be getting pretty good quality plutonium, too."

Ironically, construction of the small reprocessing facility now worrying Western governments was started in the mid-1970s, but was abandoned when France agreed to sell Pakistan the commercial reprocessing plant.

The French firm St. Gobain Techniques Nouvelles (SGN), actually supplied Pakistan with blueprints for the commercial plant -- a scaled-down version of the French commercial plutonium reprocessing plant at La Hague -- and construction of the plant was begun at Chasma deep in the Pakistani desert.

Even though the French government suspended the contract for the Chasma plant, civilian construction work has continued to limp along at the desert site to this day. Pakistan has also continued trying to buy sensitive nuclear equipment for the Chasma plant from France, Switzerland and other countries -- with some modest success. But U.S. intelligence experts say there appears to be no prospect that this plant will be completed in the foreseeable future.

After France suspended the contract for the Chasma plant, it was discovered that Pakistan was quietly buying a variety of components on the world market to build a uranium enrichment plant from plans stolen in the Netherlands. U.S. efforts accordingly shifted to identifying and attempting to block Pakistani purchases of equipment for the enrichment plant.

U.S. analysts believe that despite continued sales of "gray area" equipment to Pakistan by Switzerland, the American-led effort to deny Pakistan key items it needs for the enrichment plant has set back completion of the facility by a couple of years.

But it was only belatedly, according to sources, that the United States discovered that Pakistan had simultaneously resumed work on the clandestine plutonium reprocessing facility that had been abandoned earlier and it was too late to prevent Pakistan from obtaining the items it needed for the small reprocessing plant from French firms.

Initially, U.S. analysts thought the new reprocessing facility was very small -- perhaps not much larger than the laboratory-scale reprocessing facility already in existence inside the Science and Technology Institute -- and estimated that it would be able to produce only a couple of kilograms of plutonium a year.

But based on the capacity of the equipment Pakistan has been ordering, officials now believe the reprocessing plant may be able to produce as much as 44 pounds of plutonium annually.