If the policy planners and politicians in Washington would spend more time at the laboratories where America's nuclear weapons are fashioned, they might begin to grasp a remarkable fact: Neither the Soviet Union nor the United States need worry any longer that the other might come up with some fundamentally new nuclear weapon.

Similarly, if those toiling to legislate a nuclear weapons freeze would also study the facts, they would discover that they might accomplish far more by concentrating on another tactic: seeking severe limits on the size of underground nuclear tests. If such limits were accomplished -- and a treaty has already been negotiated to impose them -- the dangers of nuclear war could indeed be reduced.

What the hardliners and softlines both would discover here at Lawrence Livermore Laboratory and at its old sister lab in Los Alamos, N.M., is that a different kind of freeze is already taking shape -- a technological freeze, a limit imposed by science, not man. Discussions here and in Los Alamos with the scientists who make our bombs point to this conclusion: They have gone about as far as they can go in developing new nuclear weapons.

Yes, research continues on further nuclear advances, but these undertakings are essentially on marginal efforts, such as getting a bigger or more specialized bang from a smaller amount of nuclear material. And, yes, we can still expect new and exotic non-nuclear weapons such as laser and particle beams.

But 38 years to the week after the first two atomic bombs were dropped on Japan, the experts suggest that the basic development cycle of nuclear weaponry is complete.

Roy Woodruff, assistant director of Livermore in charge of strategic weapons development, will tell you that America's newest strategic and tactical hydrogen bombs, now in production, are probably the last nuclear bombs we need to build. He also believes that we may be at the end of designing significantly new missile warheads, adding: "There are no modern systems required with a yield over a megaton." (The bomb dropped on Hiroshima was 0.02 megaton.)

This country's newest missiles are one piece of evidence suggesting there is nothing new to be done. An older basic design is being used for the warheads on both the new MX intercontinental ballistic missile, being developed here at Livermore, and the planned Trident II, submarine-launched ICBM, which Los Alamos will put together. Scientists reckon they cannot significantly improve on the existing design.

William F. Scanlin, who directs the design of Livermore's short-range nuclear systems -- including the controversial neutron artillery shells -- delivers a similar message. The neutron represents "the end of nuclear artillery shells," he remarks. He also agrees with Woodruff that our latest strategic bomb, the B-83, "was the last bomb we ever do."

These men are hardly nuclear disarmers. Woodruff is looking at new defensive nuclear weapons in space, Scanlin at "safer" warheads; both are believers in continued research for new devices. But their personal beliefs cannot alter the fact that America is near the end of the line in basic warhead design, accuracy and types of delivery systems.

There seems little doubt, for example, that we have run out of new ways to base intercontinental nuclear missiles on land and at sea, and space basing of offensive weapons is already prohibited.

As for the size of weapons, we learned years ago that the more accurate we made our delivery systems, the smaller the nuclear warhead yield necessary to destroy any target.

The Reagan administration and its allies, of course, like to stir up fears with talk about the size and power of Soviet weapons. But scientists have long known that making more powerful nuclear weapons is less effective militarily than making more accurate ones.

The experts still accept the 20-year-old formula that says if you make a nuclear warhead twice as accurate, it can have one-eighth the explosive yield and still do the same damage to a target. The Soviet explosion in the 1960s of a 50-megaton hydrogen device (equal to 50 million tons of TNT) was impressive but foolhardy. No target needs that big an explosive, and, according to scientists here, no future U.S. nuclear weapon future need be more powerful than one megaton.

The biggest Soviet ICBM, the SS18, carries 10 separate warheads, each with a yield of more than 600 kilotons. Although it is perhaps one-third larger than the proposed U.S. MX ICBM, the smaller American missile's 10 warheads, each with a yield of about 500 kilotons, will be just as militarily effective because the superior MX guidance system. At explosive yields of those levels, the only difference between the damage done to a target is the size of the dust particles in the radioactive crater.

The increasingly evident limit on technological advances in nuclear weaponry opens the door to the tantalizing possibility of a safer world where what already is known about nuclear devices is all there is, and the unseen need no longer be frightening.

It now is possible for politicians and world leaders, trying to find some means of halting the arms race, to forget about the idea that one side or the other will suddenly come up with the ultimate nuclear bomb.

While senior scientists at Livermore and Los Alomos do not speak in exactly these terms, some of them do see a new possibility: moving toward limiting the size of underground tests to a point where neither superpower could have total faith in the reliability of its deployed and stockpiled arms.

If we do not need a new bomb, a new delivery system, a new artillery shell, after all, why would we need large-scale testing anymore? As for the Soviets, they have long urged a reduction in the permitted size of nuclear explosions. There is no risk in seeing if they will put their bombs where their mouth is.

Livermore's Woodruff, for example, is a firm advocate of the first step needed if we are to move in this direction: ratification of the 1974 threshold test ban treaty, the agreement signed by then-President Richard Nixon limiting both Washington and Moscow to underground weapons tests of devices with yields of 150 kilotons (0.15 of a megaton) or less.

Although both countries have agreed to abide by the treaty's limits, neither has ratified the treaty. Ratification is needed to clear the way for further test-size agreements.

The problem is that key Pentagon officials believe some Soviet tests conducted since the threshold treaty was signed have exceeded the existing 150-kiloton ceiling. Last year, when Sen. Charles H. Percy (R-Ill.), chairman of the Senate Foreign Relations Committee, tried to get President Reagan's support for ratification, the Pentagon led the opposition.

In May, Livermore scientists had the opportunity to talk to the new deputy director of the Arms Control and Disarmament Agency, David F. Emery, a former Republican congressman from Maine and an electrical engineer by profession, who visited the lab while awaiting confirmation to his post.

Emery was shown classified data the lab has developed on Soviet underground tests. The evidence, according to Woodruff, showed that there was no proof of Soviet cheating on the unratified treaty. Beyond that, Woodruff argues, even if the Soviets had done what Pentagon officials claimed, the small variance above the 150-kiloton limit "would not give them any military advantage in weapons building."

At Emery's initiative, an inter-agency group is now restudying the threshold treaty issue, and the White House is expected to reconsider in October its opposition to ratification. Arms control advocates within the administration believe the president will change his mind (not because he believes in the issue, but as part of a major administration arms control offensive this fall aimed at aiding deployment of e medium-range missiles in Europe). Woodruff, moreover, says he already has briefed some Reagan administration officials on the Livermore view that the treaty should be ratified, and he plans to be among those available for testimony on Capitol Hill if additional hearings are held.

His support of the threshold limitation fits comfortably into the concept of the coming technological "freeze." If the United States has no plans or need to build bigger weapons, he says, why not also close out that option to the Soviets through a treaty obligation, whose increased weight would force them to stay in the 150-kiloton range for underground tests.

Moreover, he remarks, a formal limit to 150 kilotons would prevent building larger-yield warheads, because scientists are not convinced you can "scale up" devices tested at lower yields.

Scaling up means testing a device at, say, 10 kilotons and then designing a weapon with 10 times the test sample. Theoretically, the result would be a 100-kiloton bomb, but scientists at the nuclear labs don't believe you can safely assume that the bigger version would work as well as the small one, particularly with a weapon whose use has such extraordinary implications.

"Scaling up" is one of the many Washington myths in nuclear weaponry. This particular myth was used by liberals as an argument against ratification of the threshold test ban during the Carter administration; they claimed that the 150-kiloton limit would have no effect because of the ability to "scale up." They were pushing for a comprehensive test ban treaty that would stop all underground tests. In the end they got neither.

If the 150-kiloton treaty were ratified, the next logical step would be for both countries to drive the level down to 50 kilotons, or even 5 kilotons. That surely would put a cap on any significant new nuclear weapons development and provide a further push toward the inevitable technical freeze.

Monitoring a 5-kiloton threshold test ban treaty would be easier than verifying a comprehensive agreement that bars each side from any nuclear tests. Moreover, any variation discovered would have little significance, because such low-level tests would be too small to help develop new weapons concepts.

After a period of years in which no large tests were permitted, both superpowers would have to begin wondering about the reliability of their respective arsenals. Of course, most of their thousands of bombs would probably still work, but with each passing year, doubts would have to grow. Eventually, leaders on both sides would have significant scientific grounds for doubting the reliability of a large part of their nuclear forces. At that point neither side could rationally contemplate a first strike -- and we would have a considerably safer world.

It is time for government leaders, politicians and the public that has concerned itself with halting the arms race to take a serious look at pushing ratification of the threshold treaty as an easy first step. The agreement is already on the Senate calendar and could be called up for a vote when Congress returns in September.

Vocal public and congressional pressure at that time would surely force a favorable response from the White House, which needs to put its best arms control face forward as the Pershing II and ground-launched cruise missile deployments begin in December in Western Europe.

At the same time, the administration, which is uncomfortable with the 150-kiloton level (because it thinks the Russians are cheating on it) could be pushed to drive the threshold down lower.

It would be interesting to see how the numbers negotiations, at the U.S.-Soviet arms control talks in Geneva, would be affected by movement in the areas governed by the technological freeze.

After all, if you are not certain your weapons work, what difference does it make to serious people how many your enemy has?