FOR decades the superpower nuclear arms race has sped on, with smaller, cleverer, more accurate weapon designs following one after the other.But now that disposal of thousands of Soviet warheads has become a top American concern, scientists face an unexpected challenge: What's the best way to throw the atomic arms race into reverse? Setting aside $400 million to help take apart Soviet nukes, as Congress has done, is one thing. Spending that money on a useful activity anytime soon is another. US warheads are dismantled at the same highly secure Texas facility where they were assembled in the first place. "They are very carefully put together," says a US government official, "and they are very carefully taken apart." All four Soviet republics with long-range strategic nuclear weapons on their soil now say they will be glad to accept US advice on dismantlement. American experts probably will travel to the republics in early January. "Certain steps have to be taken, particularly on this nuclear question, which is vital, and where the United States is uniquely qualified to lead," said Secretary of State James Baker III after his recent meeting with republic leaders. In the US, retiring nuclear warheads is rather routine, something that has been done on a large scale for decades as new types of warheads replaced old. In the former Soviet Union, only Russia has warhead-handling facilities comparable to those in the US. The other republics where nuclear weapons are concentrated - Kazakhstan, Byelorussia, and the Ukraine - have only some of the technical capabilities needed to handle nuclear arms safely throughout their life cycle. The actual unscrewing and dismemberment of US weapons is done at the Pantex warhead factory near Amarillo, Texas. The most sensitive part of this job is removing a weapon's conventional high explosive, which is used to start a nuclear reaction. If the high explosive were to accidentally go off during dismantlement, there wouldn't be a nuclear reaction - circuitry needed for that to happen would have already been severed. But deadly radioactive material could still be widely scattered. So at Pantex, all warhead dismantlement, as well as assembly, is carried out in bunker-like "bays" strong enough to withstand a high-explosive blast. This sort of watchmaker dismantlement is only the final step in the process now being talked about for the former Soviet Union. As far as the US is concerned, the most important nuclear issue is making sure the warheads are quickly made unusable. At issue are two categories of weapons. The first are long-range strategic warheads that are scheduled to be retired under the terms of the new START arms control pact. The second is short-range nuclear weaponry, the estimated 15,000 shells, mines, and other tactical nukes scattered throughout the republics. US and Soviet experts are talking about a four-step process for these weapons. First, they would be inventoried and tagged, for individual identification. US scientists have long worked on tamper-resistant tags for arms treaty verification. One approach involves splashing on a dab of clear plastic that contains bits of silvered Mylar. When the plastic dries it glitters in a random pattern that's impossible to duplicate. The second step would be transportation to one of several dozen deactivation sites. Third would be interim deactivation of some kind: deforming the bomb to ruin its carefully machined tolerances, for instance, or removing the tritium used to boost nuclear-weapon yields. Even before such a deactivation it would most likely be difficult for a terrorist, say, to make use of a stolen Soviet nuclear weapon, say experts. Many weapons, for instance, are designed so that they must first undergo the environmental change associated with weaponry - such as acceleration up a gun tube - before they are armed. The fourth step would be final dismantlement and continued guarding of the dangerous left-over nuclear material. This could well be a long-term process that requires construction of new facilities. One high-ranking Soviet official, Viktor Mikhailov of the Ministry of Atomic Power and Industry, suggested recently that the $400 million in US dismantlement money could best be used to build a storage site for radioactive materials from retired warheads. Such a center could be under joint control with the US, he said.