The cost of cleaning up radioactive and toxic wastes and restoring the environment at the Department of Energy's nuclear weapons plants could average more than $6 billion a year from 1992 through 1996, the department predicted yesterday.

Actual expenditures are likely to be less, but apparently will exceed $20 billion over the five years, according to figures provided by Leo P. Duffy, director of DOE's cleanup.

The money will pay for construction of waste handling facilities, transportation and storage of processed nuclear wastes, excavation of contaminated sites, replacement of obsolete equipment and training for additional personnel, according to the second annual version of the Energy Department's five-year environment and waste management plan. This is the price for 40 years in which DOE and its predecessor agencies produced lethal plutonium and nuclear weapons with little concern for the air, soil and water at the factories.

Asked at a press briefing where the money would come from, Duffy said, "We're going to get it from the taxpayers. The treasury isn't just going to print it." The General Accounting Office has said the total cost of the cleanup between now and 2019 may reach $200 billion, but Duffy refused to discuss any cost projections beyond the plan's five-year horizon.

Duffy said the report is "a planning document, not a budget document." The cost estimates are based on projections from the department's regional offices of what ought to be done, and together the proposed measures exceed DOE's resources and expertise, Duffy said. The real costs are also likely to be less because the nuclear plant managers and their civilian contractors are sending inflated numbers to headquarters to protect themselves against liability in civil lawsuits or criminal cases, Duffy said.

The foreword to the 622-page plan says DOE is committed to complete cleanup and environmental restoration at its weapons production sites by 2019, but does not intend to spend more money in any one year than it can manage effectively. In the current fiscal year, the Energy Department budgeted $2.36 billion for the cleanup, but the House Appropriations Committee added $353 million over DOE's objections that it cannot usefully spend that much money.

The report includes a chart entitled "What Is Not Smart?" that answers in part, "Trying to manage, with too few qualified managers, more work than there are qualified workers to do."

This approach reflects the view expressed many times by Duffy's boss, Energy Secretary James D. Watkins. According to Watkins, the poor state of U.S. science education means a shortage of environmental scientists and nuclear technicians.

Duffy said it would damage the department's already fragile credibility with the public to "throw money" at environmental problems, however urgent, only to "have some scandal break out three years down the road."

About 75 percent of current spending is for tests and studies to determine the full scope of environmental damage and the best ways of dealing with it, Duffy said. But over the next five years, the plan calls for a dizzying array of cleanup projects, some expensive, technically difficult or potentially dangerous, and some dependent on untested technology.

These include replacement of transformers and capacitors leaking toxic PCBs at the Los Alamos, N.M., National Laboratory; construction of a $79 million tower to cool water from a reactor at the Savannah River, S.C., plant; "design and construction of effluent discharge holding systems for high explosives and laboratory facilities" at the Pantex weapons assembly plant in Amarillo, Tex.; and startup of a "supercompactor" to reduce waste volume at Rocky Flats, Colo.

The plan also calls for disposal of 2,100 metric tons of irradiated waste fuel from an inactive reactor at the Hanford, Wash., reservation. This waste contains 2,150 kilograms of plutonium, including about 350 kilograms of weapons-grade plutonium. Duffy said one way to dispose of it would be to restart the Plutonium Uranium Extraction Facility (PUREX) at Hanford. Environmental groups have served notice they will resist this effort in court, arguing it would actually be production of unneeded plutonium masquerading as environmental cleanup.

Duffy acknowledged that the department faces political as well as technical and financial problems in carrying out its cleanup.

"We are going to have hundreds of million of cubic yards of material that has to go somewhere and we don't have any place to put it," he said. Nuclear waste is "a very sensitive subject," he said. "Nobody wants it."