A nuclear accident similar to the one that destroyed a Soviet reactor in the Ukraine last year cannot be ruled out in U.S. weapons-production reactors, according to a draft of a National Research Council report.

The report said "there exists the possibility" of a runaway reaction similar to the April 1986 accident that turned the Chernobyl reactor into a radioactive cauldron and showered much of the world with fallout. The conclusion contradicts statements from the Department of Energy and the Nuclear Regulatory Commission, both of which contended last year that such an accident was inconceivable in U.S. reactors.

The National Research Council report, commissioned 18 months ago by Energy Secretary John S. Herrington, is scheduled to be released next week.

Panel chairman Richard A. Meserve, a Washington lawyer, declined yesterday to comment on a draft copy obtained by The Washington Post, saying the report was still under review and subject to change.

However, the panel's broad conclusions are likely to intensify congressional concern about the safety of the nation's bomb-production reactors, which came under scrutiny after the Chernobyl accident, and about the future of the nuclear weapons program.

"The production reactors all display symptoms of acute aging that could affect safety and are likely to limit the useful lives of these reactors," the panel concluded in the draft. Because there are no new production facilities on the drawing boards, it said, the nation may be forced within the next decade "to rely upon a small number of reactors that demonstrate serious safety problems" or face shortages of the materials needed to maintain and expand the nuclear weapons program.

Nuclear material for weapons production is produced at four federal reactors: three at the Savannah River Plant in South Carolina and one at the Hanford Nuclear Reservation in Washington state. The Savannah River reactors date from the early 1950s; Hanford's N reactor was built in the early 1960s. The plants are operated by private contractors for the Energy Department.

DOE officials have maintained that the reactors are as safe as commercial nuclear reactors, although they are not subject to the same regulatory requirements. Unlike commercial reactors, for example, the weapons reactors do not have concrete containments to prevent the release of radioactive material in the event of an accident and rely instead on "confinement" systems that are supposed to filter out the most dangerous radioactive particles.

The panel said that DOE does not have enough data to know if its reactors would withstand a severe accident, and it may be too late to conduct the necessary studies. Hanford's N reactor, for example, is expected to be shut down permanently by the mid-1990s because its graphite core is swelling and threatens to break through its steel shell.

A thorough analysis of possible accidents "would require three to five years to complete and thus might be completed too late to provide any benefit," the draft report said.

The panel urged "prompt evaluation" of one possible accident: an uncontrolled reaction called a power excursion similar to that which occurred at Chernobyl. DOE officials have maintained that such an accident was not possible in the United States because of design differences.

According to the draft report, however, DOE has known since 1958 that similar conditions could occur in the Savannah River plants in the event of a severe accident. While the plants are equipped with backup safety systems to thwart a runaway reaction, the draft report suggested that even the backup systems might not be adequate under some conditions.

"At the current state of knowledge, there exists the possibility of an uncontrolled reactivity excursion in the Savannah River reactors," it said.

The draft report also said that there are "significant uncertainties" about the adequacy of confinement systems, which might overload and rupture under the strain of a severe accident.

But the panel warned that efforts to improve safety systems is increasing the complexity of the reactors and may be increasing the risk. An improved confinement system being developed for the Savannah River plants, it said, "may cause a buildup of hydrogen in the event of a severe accident."