An interagency committee studying long-range U.S. nuclear weapons needs has recommended construction of a $3 billion production reactor to keep providing nuclear materials for warheads and bombs beginning in the 1990s, according to participants in the study.

In recommending the new reactor, the committee made it clear that the nuclear arms race will probably continue into the 21st century.

The new production reactor would be the first built for the weapons program in 26 years and would represent a major step toward modernizing the aging nuclear arms-building complex.

Some facilities still used to produce today's nuclear bombs and missile warheads were constructed as part of the Manhattan project in the early 1940s that produced the first atomic bomb.

The study also recommends a $500 million program to upgrade these facilities around the country, each of which produces a key part of each nuclear weapon.

The interagency committee was established six months ago by the undersecretaries of Defense and Energy, both of whom are involved in weapons-building. Defense determines what weapons are needed; Energy runs the research and development of warheads and their production.

The study directed by retired Lt. Gen. A. D. Starbird, attempted to project the Defense Department need for nuclear weapons through the year 2000. Rather than settling on one specific level, it agreed on a range of four possibilities. These extended from a cutback of the current stockpile of some 25,000 tactical and strategic warheads to a steady increase caused by failure to ratify the arms limitation treaty with the Soviet Union.

Currently, the United States has three reactors at Savannah River, S.C., operating at two-thirds of their capacities, turning out weapons-grade plutonium and tritium for the biggest weapons-building program the country has undertaken in 20 years.

Over the next six years the United States plans to turn out: new warheads for its Minutemen III land-based ICBM and Trident I submarine-based strategic missiles; a short-range Lance missile; an air-launched cruise missile; a new tactical nuclear bomb; a new strategic nuclear bomb; the extended-ranged Pershing missile and the ground launched cruise missile, both to be stationed in Europe.

In addition, engineering is being completed for the planned MX ICBM warhead and a new 8-inch nuclear artillery shell.

Beyond filling these approved new weapons needs, the reactors must continue to produce tritium for hydrogen warheads already in the stockpile, since tritium decays and must be replaced in the nuclear package every eight years.

Members of the Starbird panel made their recommendation for the new reactor because they were concerned that the aging Savannah River reactors could not be counted on to continue operating into the 1990s.

"They are not expected to break down," a scientist close to the study said, "but their useful life was only expected to be from 25 to 30 years."

Another production reactor at Hanford, Wash., which could turn out weapons-grade plutonium but now produces fuel-grade plutonium, is expected to be shut down by the end of the decade.

"The need for a new reactor is obvious," a study participant said.

Since it is expected to take at least 10 years to design and build the new reactor, the Starbird group recommended that conceptual design begin immediately.

The recommendations now go to the secretaries of Defense and Energy. If approved, they would go to the White House for review by the Office of Management and Budget. Eventually, the proposals would have to be submitted to Congress for authorization and funding.

In the past, neither the White House nor Congress has been eager to put money into a new production reactor. There has been some discussion -- when it appeared the program needed more plutonium -- of starting up one of two older Savannah River reactors that have been shut down for many years.

"It is better economics to shoot for a newer designed reactor," a leading nuclear scientist said in support of the Starbird study's proposal.

The new reactor would be larger than the reactors now in operation and could turn out more nuclear materials. It therefore would enable the United States to meet any needs created by a sharp increase in weapons proposed for the 1990s and beyond.

Scientists said that a second production reactor eventually will have to be constructed in the middle to late 1990s, when the Savannah River facilities are finally closed down. Thereafter, the two new larger reactors would handle the nuclear materials needs that now are met by three smaller reactors.

One issue bound to arise is where to locate the first new reactor. Since the Hanford reactor is to be shut down by 1990, that Washington site is considered a prime contender, according to one scientist close to the study. Another study participant, however, said Savannah River would most likely get the first facility.