Under relentless pressure from environmental groups, legislators and its scientific advisers, the Energy Department appears to be faltering in its drive to resume full operation of its weapons manufacturing complex.

Recent developments have raised new questions about the department's ability to meet target dates for three critical projects: resuming plutonium processing at the Rocky Flats, Colo., plant; opening a nuclear waste storage facility in New Mexico known as the Waste Isolation Pilot Project (WIPP), and restarting the nuclear reactors at the Savannah River, S.C., plant.

Failure to restart Rocky Flats and Savannah River would undermine the Energy Department's ability to meet the Defense Department's nuclear weapons production targets later this year, according to DOE officials and members of Congress.

This week, Energy Secretary James D. Watkins's Advisory Committee on Nuclear Facility Safety strongly urged a delay of at least six months in restarting Rocky Flats, and questioned the advisability of operating the facility at all.

Rocky Flats, outside Denver, produces plutonium triggers for nuclear weapons. It has been plagued by environmental and safety problems, including discovery of 62 pounds of potentially lethal plutonium in the plant's ventilation ducts.

EG&G Corp., the contractor that took over operation of Rocky Flats after its plutonium processing line was shut down in December, has proposed to restart by July 1. But the advisory committee told Watkins that "many problems remain in attitudes, in procedures development, in training, in waste management and in cleanup."

The committee, chaired by former nuclear regulatory commissioner John F. Ahearne, called for "wholesale retraining and testing of the Rocky Flats work force."

It said plutonium in the ducts "should be removed. . . This will be a time-consuming chore which could significantly delay the resumption of plutonium processing operations."

The next day another committee, the Defense Nuclear Facilities Safety Board, recommended that the Rocky Flats restart be postponed until the Energy Department has demonstrated that the "fissile material" -- plutonium -- will not explode or otherwise injure workers at the plant.

By law, the Energy Department has 45 days to respond to this board's report. If the board is not satisfied with the response, it can take its findings to President Bush.

Watkins responded by ordering an "operational readiness review" of Rocky Flats. He said that would include technical inspections and public hearings and should take six weeks.

Watkins reiterated that whenever production demands clash with safety concerns, safety will prevail.

But the review order did not address another Rocky Flats problem raised by Ahearne: what to do with the plant's waste.

Ahearne said the plant would have to shut down again less than six months after restart when it reaches waste storage limits imposed by Colorado.

The Energy Department wants to move Rocky Flats waste to storage in salt caverns at WIPP. But before WIPP can open, the department needs a permit from the Environmental Protection Agency, known as a "no-migration variance," and legislation from Congress that would give it land owned by another federal agency.

Sen. Pete V. Domenici (R-N.M.) is opposing the legislation, for economic and environmental reasons. The proposed EPA permit has been taking heavy fire at public hearings in New Mexico.

Watkins wrote EPA Administrator William K. Reilly in December asking quick action on the permit request. In a handwritten postscript, he said, "Bill: This is my top priority." But Reilly responded that "there are certain necessary regulatory steps that we have to complete" and that the variance petition raises "technically and legally complex issues." EPA has never granted a petition of the type sought by DOE.

At a public hearing last month, an environmental group made public an internal EPA memorandum warning that wastes of the type generated at Rocky Flats might explode because of a gas buildup. James D. Werner, the Natural Resources Defense Council engineer who disclosed the memo, said yesterday that "EPA will have very serious problems" in certifying that the wastes would not "migrate" beyond the storage caverns.

Meanwhile, testimony at public hearings about the environmental impact of restarting the Savannah River reactors, the nation's only source of the radioactive gas tritium used to boost explosive power of warheads, made clear that environmental groups will sue on several grounds if the Energy Department proceeds.

The department set January as its target for restarting one of the three reactors shut down for safety reasons since 1988.