Thirty-five years after the birth of the atomic bomb, its parent facility, Los Alamos Scientific Laboratory is turning its emphasis away from exploration of new nuclear weapons systems and toward studying how to make current and future stockpiles last longer.

It marks an important step in nuclear weapons research, because in the last eight years Los Alamos has designed all the new strategic warheads, including those for the Minuteman III and the proposed MX land-based ICBMs, the Trident submarine-launched ballistic missile and the air-launched cruise missile.

This "change in focus," says Dr. Donald M. Kerr, who a year ago became the fourth director of Los Alamos, recognizes that "constraints" -- such as reduced testing and fewer weapons as a result of disarmament agreements -- will require that future nuclear systems last 20 years, twice their current longevity.

The change also reflects the view of many top American scientists that there are no major frontiers left unexplored in the nuclear weapons field that would give one current any long-term military advantage over another.

"I don't think there is another breakthrough possible," said Dr. Hans Bethe, a Nobel laureate in physics and a participant in development of the atomic and hydrogen bombs. "I think we have gone plenty far already."

A survey of government officials, military officers and scientists here for a brief observance, 35 years after the first testing of the bomb, disclosed that:

Kerr and other scientists agree that continued weapons research will bring only slight gains in "effects" technology -- making explosive yields somewhat higher and the devices safer from accidental explosions and unauthorized use.

Not all scientists agree that the breakthroughs are over. Dr. Edward Teller and others want to pursue exploration of transuranic materials -- elements created in tiny amounts by nuclear fission that might offer higher explosive yields with smaller amounts of material.

"Some other scientists believe that, with a major effort, a pure fusion weapon -- with no fallout and a yield of blast, heat and radiation -- could be developed.

Critics say that these approaches are not guaranteed and they would not provide militarily useful results. "Why do we need golfball-sized nuclear bombs?" was how one Los Alamos scientist dismissed the idea of transuranic development.

There is growing sentiment within the scientific and military communities -- and emphasized here -- toward deploying a limited antiballistic missile system for protection of land-based intercontinental ballistic missiles such as the proposed MX. The 1972 arms limitation treaty with the Soviet Union limited each country to one ABM site. Kerr and others want to see this subject publicly debated before the treaty comes up for renewal in 1982.

Old-timers here who witnessed above-ground nuclear explosions believe that the new generation of public officials and weapons scientists has misleading, antiseptic views of how devastating the weapons are.

Kerr's predecessor here, Dr. Harold Agnew, who flew on the Hiroshima mission in 1945, for years has suggested that a demonstration of nuclear weapons be staged every few years for chiefs of state. As he said the other day with a smile, he wants them to observe a nuclear shot "in their underwear."

"The thing that impressed me," Agnew said of a 1954 H-bomb test he witnessed in the Pacific, "was the heat 20 to 30 miles away."

Dr. Norris Bradbury, who replaced Dr. Robert Oppenheimer in 1945 as head of Los Alamos, said recently that "every high school senior should look at the pictures of Hiroshima and Nagasaki."

Kerr, who has never witnessed a test shot, said his generation of scientists has some understanding of the weapons but that political leaders appear to "lack urgency" in moving to control them.

Berthe said he was "worried that some people talk about waging and winning nuclear war. They are thinking of paper studies." Nuclear war is "beautiful on paper," he said, but not in reality.

"There can only be losers," he said. "The combatant countries would have their social fabric destroyed and very likely there would be anarchy."

The years after Hiroshima were not the way some scientists had expected.

"We were afraid there would be nuclear war within 10 years," Bethe said. "There hasn't been for 35, and that is a great tribute to statesmen."

In discussing new areas for research, several scientists pointed out that many temporary gains in U.S. weaponry could backfire if adopted by the Soviets.

Bethe said that in 1967 the Johnson administration was warned that a proposal to put more than one bomb on each Minuteman missile could become disadvantegeous if the Soviets followed suit with their missiles. A government study, Bethe said, pointed out that the Minutemen could hold up to three new warheads, while its much larger Soviet counterpart, with further development, could hold more than 10.

The United States went ahead with the system -- called MIRVs (multiple independently targetable reentry vehicles). The Soviets did the same, and moved into the advantage in land-based missile warheads, as predicted.

An example of how treaties can affect weapons programs is found in the 1974 agreement that limits the United States and the Soviet Union to underground nuclear tests of 150 kilotons or less. Although the Senate has not ratified the agreement, both countries essentially have abided by it.

For the United States, this has meant that the new warhead for the proposed MX missile must be of a design already tested, because the desired yield is more than twice the 150-kiloton limit. It also means that warheads now in stockpile with yields exceeding 150 kilotons cannot be fully tested for reliability.

Kerr and other scientists discredit the idea that limited-yield tests can be scaled upward to project what would happen at a much higher yield.

Kerr said that the modernization of the U.S. strategic and tactical nuclear stockpile over the next 10 years will provide breathing room for the change in approach that he foresees for the laboratory.

Most of what is known about the new weapons in production, he said, comes from "computer calculations." More knowledge through testing is needed about "first principles of physics," he said, to make the weapons last longer in the stockpile.