The Bush administration has estimated that the number of U.S. nuclear weapons will decline by 30 to 85 percent during the next two decades, allowing a substantial shrinkage of the nation's bomb-building complex, senior Energy Department officials said yesterday.

The estimate provides the basis for an expected, multibillion-dollar redesign of the U.S. network of weapons factories and associated nuclear reactors to be announced today by Energy Secretary James D. Watkins, the officials said.

Under the plan, which will take up to three decades to implement, some key bomb factories -- including the plutonium processing facility at Rocky Flats, Colo. -- are expected to be relocated to safer and more hospitable sites. The functions of four immense, aging factories producing various bomb components also are expected to be consolidated at a single site, possibly under private ownership, the officials said.

The plan reflects the end of Cold War nuclear confrontation with the Soviet Union, rising public demand for greater environmental protection and safety, and the realities of a tight budget, the officials said.

The era of "open checkbook for nuclear weapons is over," Undersecretary of Energy John G. Tuck said. "The world is changing. The world has to be looked at in an entirely different context . . . than it has over the past 35 years."

A major, unresolved issue in the new plan is where any new bomb-building factories will be located. A legacy of environmental contamination at current plants has left many citizens and politicians with little enthusiasm for the enterprise, the officials said. But they expressed hope that a state such as South Carolina, Texas or Nevada will donate at least 5,000 acres "free of charge" to the federal government for at least one major new bomb plant.

Watkins, whose choices have been constrained by numerous legal and environmental requirements, plans to announce today a series of studies and analyses that will lead to a final decision by 1994, after completion of President Bush's current term in office.

Any new construction would begin after 1997 and cost between $6.7 billion and $15.2 billion, the officials said.

The plan is a major overhaul of projections by the outgoing Reagan administration in early 1989 that the current U.S. arsenal of approximately 20,000 nuclear weapons would not be significantly reduced. Watkins decided quickly that "this didn't reflect reality," Tuck said, and ordered new projections drawn up by a panel that also included senior officials from the Defense Department, Joint Chiefs of Staff, National Security Council and Office of Management and Budget.

Tuck said the results largely reflect uncertainty about the nature of the Soviet military threat. At the high end of the spectrum, a reduction of only 6,000 U.S. weapons could encompass those set to be eliminated under the forthcoming strategic arms reduction treaty (START) as well as modest additional cuts driven by budget constraints or arms negotiations.

At the low end, a reduction of about 17,000 weapons -- leaving only 3,000 strategic and tactical warheads -- would be possible only if the current arsenal is drastically restructured and its aims revised. In the past, officials said, annual weapons production rates have varied sharply, amounting to only 80 in the late 1970s but rising to a high of 1,400 under President Ronald Reagan.

The shrinking of the nuclear weapons complex -- an industry that sprawls across 12 states and employs more than 100,000 workers -- is already underway. Plutonium production and fuel fabrication at the giant Hanford facility in Washington state have ceased, and Watkins has said that Hanford's future mission will be to clean up the environmental problems left by more than 40 years of production. The uranium processing center at Fernald, Ohio, also has been taken out of production and operates solely to clean up after itself.

Congress refused to approve construction of a new plutonium processing facility at Rocky Flats, and Watkins announced earlier this week that he would seek funding for one new nuclear reactor to produce the gas tritium, not two as previously proposed. Last year he abandoned plans to construct a plutonium separator at the Idaho National Engineering Laboratory.

Tuck and Rear Adm. J. Michael Barr, deputy assistant energy secretary for military applications, said the plan assumes that three basic functions will have to be carried out somewhere for the foreseeable future: uranium processing, currently done at Oak Ridge, Tenn.; recycling plutonium triggers, scheduled to resume later this year at Rocky Flats; and weapons assembly and disassembly, currently conducted at the Pantex facility in Amarillo, Tex.

Environmental laws require public participation in decisions that were made in secret for nearly 50 years, Tuck said. "The decision-makers of the 1940s and 1950s would be turning over in their graves," he said.

A "site evaluation panel" will consider possible locations for new and consolidated facilities. Separate committees will evaluate which functions could be transferred to private enterprise, how weapons could be standardized to save costs and which new technologies might be adapted to making nuclear weapons.