The nation's bomb-production nuclear reactors are rapidly becoming too old to be operated safely and may have to be shut down before the government has new reactors in place, a National Research Council panel reported yesterday.

Hours after the panel released its report, E.I. du Pont de Nemours & Co. announced that it intends to relinquish its 37-year-old contract to operate the government's three reactors at the Savannah River Plant in South Carolina. The company said it fears it will be asked to assume liability for the plant and is "not willing to put our shareholders' assets at risk."

The report, prepared by the investigating arm of the National Academies of Sciences and Engineering, contained the sharpest criticism to date of Energy Department management of the four U.S. defense reactors. The reactors, one in Washington state in addition to the three in South Carolina, are the only source of plutonium and tritium for nuclear weapons.

But the panel avoided the question of whether the reactors, which have been under intense scrutiny since a disastrous accident destroyed a Soviet reactor last year, are safe to run now.

"We are not in a position to make that determination," said Chairman Richard Meserve, a physicist and Washington attorney. "The term 'safe' does not lend itself to easy definition."

The report and du Pont's announcement came as twin blows for the Energy Department, which has been struggling to persuade a skeptical Congress and public that the aging reactors can be operated without compromising public safety.

The report "is critical of the department, but the important point to remember is that we invited this criticism," said Undersecretary Joseph Salgado.

The DOE reactors are not subject to the rules that govern commercial power plants, but department officials have maintained that they are designed for "comparable" safety margins.

Asked yesterday whether he was persuaded that the reactors provided "comparable" margins of safety, Salgado responded: "No, I'm not."

Energy Secretary John S. Herrington, who requested the academy study in May 1986, said the department will adopt immediately the panel's recommendation for an independent board to advise DOE on safety issues.

The report criticized DOE safety programs as inadequate and concluded that the department had relied for too long on its contractors to identify safety concerns.

"On occasion, the contractors have pressed for safety upgrades that DOE has then rejected for budgetary reasons," it said.

The report also said there are "uncertainties" about whether the reactors' backup safety systems could handle a severe accident without allowing dangerous amounts of radioactivity to escape.

The report suggested, however, that problems with the defense reactors have gone beyond management concerns. The Savannah River reactors, built in the 1950s, and the 25-year-old N Reactor at the Hanford Nuclear Reservation near Richland, Wash. "are beginning to experience life-limiting material aging caused by irradiation and corrosion," it said.

"We concluded that the remaining useful life of the production reactors is likely to be equal to or shorter than the time needed to authorize, fund, design and build new facilities to produce plutonium and tritium," Meserve said.

DOE has stockpiled plutonium for defense purposes. Tritium, however, decays rapidly and must be replenished to maintain the current nuclear arsenal and build new weapons.

Of particular concern, the report said, is the N Reactor's 1,700-ton graphite core, which is used to slow neutrons during operation. Years of radioactive bombardment has caused the graphite to expand, putting stress on cooling and process tubes. The expansion also is distorting the channels that are used to insert control rods to shut the reactor down.

The N Reactor was shut down last January for $50 million of safety modifications, but the department hopes to restart it by Dec. 1.

The report said that the reactors at Savannah River are vulnerable to stress cracks in welded pipes and reactor tanks. A fourth reactor there was retired last year when efforts failed to repair a cracked tank.

"Unless a repair technology is developed, all the Savannah River reactors may eventually have to be retired from service because of cracking problems," Meserve said.