Safety problems at aging federal nuclear weapons plants are reaching a critical mass.

Production has stopped at major sites, and Energy Department officials face a firestorm of criticism from Congress, the Pentagon and angry citizens in the 13 states where the complex operates."Tell the truth!" an irate crowd shouted last week as the manager of the Energy Department's Rocky Flats Plant, near Denver, argued that the plant's plutonium reprocessing facility was closed because of a single contamination incident. Investigators from Congress and the Environmental Protection Agency had found "very serious" and repeated safety violations.

"We may sometimes not be good at explaining everything," responded the manager, Earl Whiteman. "But we damn well tell the truth."

Department officials are likely to face a lot more disbelief as they struggle to restart nuclear reactors that produce the plutonium and tritium needed for the nation's nuclear arsenal.

Neither is being produced now and tritium, a radioactive isotope of hydrogen, may be in critically short supply as early as next summer, according to congressional sources.

Federal officials also are likely to face rising anger as they grapple with the daunting task of cleaning up 45 years' worth of radioactive contamination, a job the Energy Department says could cost $110 billion.

If the Manahattan Project, which produced atomic bombs at the end of World War II, was an example of what the United States can do when it focuses its technological genius, then the current mess in nuclear weapons production may epitomize what can happen when such activities are shrouded for decades in secrecy and bureaucracy.

"This crisis has not arisen suddenly but over a long period, and it stems from inadequate attention to maintenance, safety and operating conditions," 31 members of the House Armed Services Committee said in a letter Thursday to Energy Secretary John S. Herrington.

The National Academy of Sciences, in a report requested by Congress, described the complex last year as "a loose-knit system of largely self-regulated contractors" under little control of the Energy Department.

The complex, comprising many potentially dangerous elements, operated under a regime dictated not by safety concerns but by the need to produce nuclear weapons for the arms race, the academy said.

Not only were the reactors and other facilities older than those in the civilian sector, the report said, but they were free of the strict controls imposed by the Nuclear Regulatory Commission on utilities after the near-disaster at Three Mile Island on March 28, 1979.

The Energy Department is the latest in a line of civilian agencies that have built and managed the plants that turn out an estimated 1,800 nuclear weapons a year for the Pentagon under a $7.8 billion budget.

Two other weapons laboratories were added, at Livermore, Calif., and Sandia, N.M., and the government began operating two reactor complexes, the Hanford Nuclear Reservation, near Richland, Wash., in 1944, and the Savannah River Plant, in Aiken, S.C., a decade later.

Other materials are produced at the Feed Materials Production Center, in Fernald, Ohio; the Ashtabula Plant, Ashtubula, Ohio; the Y-12 plant and Oak Ridge Gaseous Diffusion Plant at Oak Ridge, Tenn.; the Idaho National Engineering Laboratory, Idaho Falls, Idaho; the Paducah Gaseous Diffusion Plant, in Paducah, Ky.; and the Portsmouth Gaseous Diffusion Plant in Piketon, Ohio.

Energy Department plans do not call for the construction of a new generation of nuclear weapons reactors until well into the next decade, a $3.2 billion facility at Savannah River and a $3.6 billion facility in Idaho. Budget austerity could cause further delays or cancellation of one site.