An Energy Department master plan for future production of nuclear weapons calls for revitalizing the existing network of factories and reactors so the military can make new bombs through at least the middle of the next century, according to department officials and government documents.

Despite improving U.S.-Soviet relations and the prospect of deep, negotiated cuts in superpower arsenals, the Department of Energy has based its research and spending plans on "a requirement for nuclear weapons as far as we can see," a senior department official said yesterday.

While the size of the U.S. nuclear arsenal is expected to decline from its current level of about 21,000 weapons over the next 60 years, department officials say they are anticipating a continued need to manufacture many new weapons with improved, safer designs that would replace those being retired or withdrawn as obsolete.

"Complex 21, my vision of a fully modernized {nuclear weapons} complex, is planned to be in operation about 2015 and to support the nation's strategic deterrent until the middle of the century," Energy Secretary James D. Watkins said in a letter to the Senate Armed Services Committee that provided the most detailed explanation yet of the department's long-term goals.

Watkins told the committee that under one option being considered, "Complex 21" -- for the 21st century -- may consist of one or two super bomb-building sites that would consolidate activities now spread across 12 states. This action would create what activists are calling "a nuclear weapons theme park," which some officials say could be operated in a state such as South Carolina where public support for the bomb-building enterprise and its attendant infusion of federal dollars remains high.

But after having officially scrapped a modernization plan prepared at the end of the Reagan administration, Watkins said he expects that the final shape of "Complex 21" will not become clear until late 1993 or early 1994.

His timetable partly reflects a January decision under threat of a lawsuit to conduct a full-scale assessment of the environmental impacts of modernizing the vast, troubled nuclear weapons manufacturing complex. That decision requires the department to conduct extensive public hearings on options that also include upgrading all existing weapons facilities or relocating some of the most hazardous facilities.

It also reflects what one official described as Watkins's desire to get a clearer picture of future U.S. defense spending, which likely will not stabilize for several years. The official said it will provide time for the department's experts to digest a study of the long-term requirements for nuclear weapons that the Joint Chiefs of Staff has promised to complete next year.

In addition, the timetable has the added benefit of postponing until after the 1990 elections the start of public hearings on an issue that has aroused substantial fury in localities where residents have been exposed to radioactive emissions or environmental contamination, such as the Rocky Flats plutonium processing facility near Denver, or the plutonium production complex at Hanford, Wash.

Last week, Hanford residents were angered by the conclusion of a government-funded study that they had been repeatedly exposed to high doses of radioiodine, possibly endangering their health. Radioactive emissions at a nuclear weapons plant near Fernald, Ohio, have caused the government to establish a health screening program there. Citizens at these and other DOE sites have mounted efforts to have the plants closed.

Under this timetable, the costly, final "Complex 21" design also will not have to be presented to Congress and local residents for approval until after the 1992 presidential election, a factor that administration sources have cited approvingly in private comments about Watkins's plan.

"Complex 21 will be designed to comply with all applicable environmental, safety and health requirements, and will efficiently manufacture nuclear weapons at a rate consistent with national security needs," Watkins told the committee.

The project's planned 25-year period of construction reflects what agency officials say is the massive scale of improvements needed after a decade of neglect. And while no formal cost estimates have been prepared, one agency official said that building "Complex 21" from existing facilities would require at least $15 billion. Roughly $55 million would be spent to manage this construction through fiscal 1993, climbing to $77 million in fiscal 1994, and to hundreds of millions of dollars annually thereafter.

Officials have said the cost of cleaning up the extensive environmental damage at nuclear weapons facilities is expected to be as much as 10 times greater than the cost of Complex 21, prompting some environmental and anti-nuclear activists to say that the new production plans should have a much lower federal priority.

Bob Schaeffer, national spokesman for the Military Production Network that monitors environmental compliance at the weapons facilities, criticized Watkins's statement that existing facilities to be used until Complex 21 is ready will "comply as much as possible" with environmental and safety laws. Watkins has said repeatedly that compliance with such laws is his highest priority, ahead of weapons production.

Officials said the Energy Department has been using six different scenarios for future nuclear weapons requirements in their preliminary planning, beginning conservatively at the current level of 21,000 weapons and heading downward toward a "minimum" nuclear force of close to 3,000 weapons. The numbers are based on various classified assumptions about the types of nuclear weapons that will make up the U.S. arsenal through the year 2050.

"We're not going to require {a nuclear weapons enterprise} as large, as complex, and far flung as the one we have today," an official said. While emphasizing that no decisions have been made, he said that consolidation of the existing 17 facilities at one or two sites would sharply cut the cost of transporting weapons materials, increase security and allow less-fettered operation in a "more hospitable" climate where citizens are "comfortable" with bomb-building.

"It would have to be {a site} where we have enough buffer land" to protect the populace from any accidents, said Rep. David E. Skaggs (D-Colo.), whose district includes Rocky Flats. Since Watkins has said that the plutonium processing operations now carried out at Rocky Flats will not be transferred to a facility at Los Alamos, N.M., and that the future role of the Hanford, Wash., site is waste cleanup, not production, that would leave Savannah River, S.C., and the Idaho National Engineering Laboratory as the most likely choices.

"The current distribution of facilities has a lot less to do with physics than politics," the DOE official said, alluding to decisions by powerful members of Congress in the 1940s and 1950s to place the lucrative operations in their home districts. He indicated that the agency prefers to close some of the facilities producing non-nuclear bomb components, such as electronic switches, and buy the equipment from private firms.

He and others said that while many aspects of the "Complex 21" plan may be subject to change based on unforeseen world events, the Energy Department is assuming that it will build one or more new reactors to produce tritium, a vital radioactive component of nuclear weapons that must be constantly replenished. The design of the new reactor is to be decided by late 1991.

Some environmentalists have expressed frustration that construction of some new weapons-related facilities is proceeding even before the "Complex 21" design is complete, under so-called "transition activities" that will cost more than $1 billion in the next fiscal year. The Energy Department is seeking $571 million to rebuild a plutonium processing facility at Rocky Flats. But last week the Senate Armed Services Committee voted against this request because some members want the money shifted to environmental repairs at Rocky Flats. The Energy Department will decide soon whether to restart the Plutonium-Uranium Extraction facility (PUREX) at Hanford to extract plutonium from 2,100 metric tons of radioactive nuclear fuel.

The environmentalists say that according to the law the process should be reversed: environmental impact statement first, then decisions on closure, construction or consolidation of facilities.

"The fact is that the decisions they are making now about the new production reactor, about the modification of existing facilities, those are decisions that are going to affect Complex 21," said Brian Costner, director of the Energy Reserch Foundation, a group that monitors Savannah River. "Here's the DOE making real dramatic decisions at a time when we have no idea how many nuclear weapons we'll need in the next 50 years."