A group of scientists asked by Congress to study nuclear weapons safety reported yesterday that most U.S. weapons need design modifications to diminish the risk of accidental detonations that could disperse highly toxic materials or cause a nuclear blast.

The scientists said one weapon in particular, the Trident II missile now being deployed on U.S. strategic submarines, should be subjected to an immediate "national policy review" due to concerns that "a nuclear yield" might result from an unexpected handling accident.

In a report to the House Armed Services Committee, the three nuclear physicists said "the majority of the weapons in the current stockpile will have to be modified" to meet government safety criteria, and recommended "basic changes" in procedures for safety monitoring and new weapons development.

Panel director Sidney D. Drell, a deputy director of the Stanford Linear Accelerator Center, and member Charles H. Townes, a Nobel laureate in physics at the University of California-Berkeley, told the committee that the hazards of existing weapons did not warrant immediate public alarm.

"I don't think this is a panic situation at all, but rather, with a changing world situation -- the decreasing pressures of the Cold War -- we can and should be turning our attention to long-term safety problems," Townes said.

The panel, which also included Defense Science Board director John S. Foster Jr., a former head of the Lawrence Livermore National Laboratory for nuclear weapons development, conducted a six-month review of the nation's most secret nuclear weapons design information. Their report provides what several officials described as the first detailed, independent assessment of the safety of the U.S. stockpile in the 45 years since the first atomic test north of Alamogordo, N.M.

The inquiry grew out of the discovery of safety problems in nuclear-tipped artillery shells, which were modified by the Pentagon in early 1989 to block any accidental detonations, and in some nuclear-tipped short-range missiles, which were withdrawn from some U.S. strategic bombers on war "alert" in July.

"Where some concerns have been resolved in an exemplary fashion . . . others have remained for far longer than necessary or desirable," the report said. It attributed past inattention to weapons safety to "the chilling environment of the Cold War," in which the top government priority was, in effect, to get the maximum nuclear bang from the most efficient weapon.

"A new balance must be struck between the desired military characteristics {of nuclear weapons} and requirements for enhanced safety," the panel said. It urged Defense Secretary Richard B. Cheney and Energy Secretary James D. Watkins, who share responsibility for the nuclear arsenal, to issue a joint directive laying out a strategy "for redressing safety concerns in a timely manner by a combination of {weapons} retirements, improvements, and development of new weapons systems."

It also said the two principal U.S. nuclear weapons laboratories should begin competing to develop new super-safe weapons designs, which would only later be evaluated for military value. This approach would reverse the existing process of nuclear weapons design, which begins with a military requirement and incorporates safety measures where practical.

"We have a situation today where one can argue endlessly about . . . this particular accident or that kind of accident," Foster said. He said such debates would be halted by a new design that removes any possibility for such an event.

In open testimony, which was followed by a classified discussion with legislators, the panel members did not say whether the Trident II missile should be immediately withdrawn from service for modification or replacement. But they noted that the $52 billion deployment program remains in an early stage, and that environmental problems have indefinitely shut down an associated warhead manufacturing plant.

"These circumstances present the country with a tough choice: Should we continue with production and deployment . . . or use the lull . . . to redesign the missile with a safety-optimized design?" the report asked.

Watkins said last May that Trident had been improperly designed because explosive materials surrounding the warhead's plutonium core are vulnerable to detonation in a handling accident. But he did not say what should be done about it.

The Trident missile also uses a powerful propellant that could be accidentally detonated more easily than other propellants and lacks a special shield that could help protect the warhead core from fire, the scientific panel said. These features raise added concerns that a detonation might lead to "plutonium dispersal or possibly a nuclear yield," it said.

An explosion involving just one Trident warhead would contaminate a downwind area of roughly 38 square miles with cancer-causing plutonium dust, and cost more than $500 million to clean up, the panel added.

One option involves changing the propellant and reducing the number of warheads atop each missile from eight to seven so that modern safety features can be installed. "It's not at all clear to me there would be any military disadvantage" if these changes were made, Townes said. Another option involves adopting safer handling procedures for loading the missiles onto submarines.

Neither the Defense Department nor the panel members were able to estimate the cost of making these or other recommended nuclear weapons modifications.

"My opinion is that the Departments of Defense and Energy and the Navy should make the most serious possible study of modifying the Trident II," said Rep. John M. Spratt Jr. (D-S.C.), chairman of an Armed Services panel on defense nuclear facilities and the initiator of the independent study. "The burden is on them to explain why it cannot be done."

Rep. Jon L. Kyl (R-Ariz.) said "there will be plenty of support in the Congress for the implementation" of the recommendations.

The Departments of Energy and Defense responded to the report with a joint statement saying the government considers "safety as a matter of the utmost importance and give{s} it the highest priority in all matters regarding the nuclear stockpile." But they declined to comment on the specific recommendations pending further study.

A spokesman for the British Ministry of Defense, which is buying Trident missiles but not warheads from the United States, responded with extreme caution last night. "We'll obviously want to consider the implications of the report very carefully," he said. "The United Kingdom attaches great importance to nuclear weapons safety."

The panel said many of the safety defects in the existing stockpile were discovered through recent advances in computer modeling of nuclear weapons explosions. "A major consequence of these results is a realization that unintended nuclear detonations present an even greater risk than previously estimated," the report said.

The panel prepared a classified annex to the 50-page report listing the precise weapons needing repair. But it said publicly the risks associated with short-range missiles designed for strategic bombers were significant enough to require immediate modification or elimination.

Cheney's action last summer removing the missiles from bombers on war "alert" did not go far enough, the panel said, because the weapons could be redeployed on aircraft in the midst of an international crisis. "A time of crisis is when I least want to have" an accidental detonation of such a weapon, Drell said.

The panel said electrical switches invented in 1972 to diminish the likelihood of accidental detonations should be installed in the remaining 48 percent of the U.S. stockpile that lacks them. It said all nuclear bombs deployed on aircraft should use materials resistant to accidental detonation; only 25 percent of the total arsenal currently has this feature.

The group suggested administrative reforms aimed at strengthening civilian oversight over weapons safety issues, including elimination of a requirement that the Energy Department official responsible for chairing a nuclear weapons safety committee be on active military duty. The panel said this and other moves would avert "career-threatening conflicts-of-interest" for officers beholden to the military services that order nuclear weapons.

It also urged formation of "red teams" including expert scientists from a competing weapons laboratory who would search for safety defects in nuclear weapons designs and in handling procedures, before the government started producing a new nuclear weapon.

Washington Post correspondent Glenn Frankel contributed to this report from London.

Here is a summary of the key recommendations made yesterday in a report by the Panel on Nuclear Weapons Safety of the House Armed Services Committee:

Equip the remaining 48 percent of the U.S. nuclear weapons stockpile with modern electrical devices to reduce the risk of accidental detonation.

Install less volatile explosive materials and fire shields in hundreds of aircraft-delivered nuclear bombs lacking the features, to reduce the risk of accidental plutonium dispersal.

Eliminate or modify more than 1,000 nuclear-tipped missiles built for strategic bombers, to reduce the risk of an accidental nuclear detonation or plutonium dispersal.

Review the wisdom of retaining submarine-launched missiles with propellant that can be detonated and nuclear warheads that use volatile explosives and lack fire shields.

Develop advanced nuclear weapons designs that are "as safe as practically achievable."

Establish a system for challenging nuclear weapons designs and handling procedures so that dangers are not overlooked.

Establish an independent advisory panel to oversee weapons safety practices and to review an annual safety report to the president.

Enhance the authority of key Energy and Defense Department officials responsible for nuclear weapons safety and provide for direct reporting to the highest official at each agency.

Eliminate a requirement that key safety posts be filled by active-duty military officers, who may be beholden to the services that deploy nuclear weapons.

Establish a consistent policy on movement of warheads containing volatile explosives, instead of allowing the Defense Department to ship such warheads by air while requiring the Energy Department to ship them only by land.