AS ESTIMATES of the Soviet Union's political half-life grow ever shorter, Western analysts have expressed concern about the security of Soviet nuclear weapons and nuclear installations. The Soviets' response is a dual strategy of both easing these Western worries and trying to exploit them.

On the one hand, Soviet officials are calling attention to their own vulnerabilities, highlighting the dangers posed to the West by the disintegration of their country. Recent remarks by Prime Minister Nikolai Ryzhkov during an interview with an Austrian television station indicate that he is playing this game.

Ryzhkov said that international stability will be endangered if the Soviet Union fractures into a great number of states, "each having their own armies and maybe even nuclear arms." He warned that "the disintegration of our country would have terrible consequences for all. I regard this danger as very great. If I were a foreign politician I would also be worried."

Soviet Chief of Staff Mikhail Moiseyev was quoted by The Washington Post last month as saying that warheads had been removed from troubled areas in the U.S.S.R., although the Soviet Defense Ministry subsequently denied that redeployment had taken place.

The Kremlin also has the obvious objective of reassuring the West that an unauthorized nuclear attack will not emanate from the Soviet arsenal. To this end, the Soviet press agency Novosti last spring issued an extraordinarily frank and detailed statement about Soviet measures to maintain nuclear security. In accordance with the Soviet objective of keeping the West worried, it concedes that threats to the security of Soviet nuclear warheads do exist and asserts there is a "probability of sporadic attempts" to seize Soviet nuclear weapons by "fanatical groups" that are part of "various informal movements." But the statement goes on to assert that any such attempts will be unsuccessful because of the "high reliability" of the Soviet system for the storage and transportation of its nuclear arms.

Novosti's statement came in the wake of reports that a nuclear weapons storage depot near the Azerbaijani capital of Baku came under attack from armed nationalists in January. Yevgeny Velikhov, vice president of the Soviet Academy of Sciences, said in the newspaper Moscow News that the presence of nuclear-weapons storage depots near Baku was one of the considerations that led Moscow to introduce troops into the area when violence erupted there last winter.

The statement contained an explicit reference to unrest in the Caucasus, suggesting that the Baku incident is indeed one of the factors that have triggered heightened Kremlin concerns about the security of its nuclear weapons.

Offering reassurance about the possibility of nuclear weapons falling into the wrong hands, the Novosti statement said that the U.S.S.R. maintains "a broadly developed system for the production, storage and maintenance of the military readiness of its nuclear stockpile" and that "technical security is provided by an entire range of strict measures for the prevention of their unauthorized application, including coded electronic and mechanical blocking mechanisms that can only be removed by the highest political leadership of the country (concretely, the president of the U.S.S.R. in his capacity as supreme commander)."In addition to physical safeguards that make unsanctioned detonation impossible, Novosti says Soviet nuclear-storage areas are highly secure from unauthorized entry. They are engineered and equipped with a defensive system that could only be challenged by a "military operation" conducted on a "high tactical level." To believe that such an attack could be successful, says the Novosti statement, one must "go too far in one's appraisal of the development of the internal political situation in the U.S.S.R."

Comforting details about Soviet nuclear weapons security have also been offered recently by the mass circulation weekly Argumenty i Fakty. To retain clearances and authorization to work with nuclear weapons, Soviet officers and soldiers must undergo an annual examination conducted by an "expert commission" in which "moral qualities" and "psychophysical condition" are tested. The commission employs a "special methodology" designed by military psychologists that places a "definite psychological burden" on all candidates for nuclear weapons access. Every year 4 to 6 percent of the personnel who undergo the examination "do not withstand this trial" and are denied their clearances. This information, notes the newspaper, is not secret, but "it has never before been published."

Because of widespread anxiety over nuclear power in the wake of the Chernobyl accident, the U.S.S.R. also faces the danger of "green" terrorism. But the Novosti statement says nothing about safeguards against this type of attack.

In July, a "blockade" was erected around the Khelmnitskiy nuclear power plant by hundreds of pickets affiliated with the Ukrainian national movement Rukh and the environmental organization Green World. Operating personnel could reach their workplace only with "great difficulty" and repair brigades were unable to enter the plant at all. The reactor's chief engineer declared that the safety of the "nuclear object" is "under threat."

The Tatar nuclear-power station near the Volga River has been threatened with terrorism, reports the newspaper Komsomolets Tatarii. "A young man" warned protestors gathering signatures for a petition against the plant that peaceful means will accomplish nothing. "You will need terror . . . there are weapons and people."

Remarks by the chief of the KGB in the Ukraine, N.M. Golushko, suggest that such threats are taken seriously. "One cannot ignore the real danger of nuclear terrorism, attacks on nuclear plants and crimes committed using various radioactive materials," he says. The Ukrainian KGB boss notes that his agency is increasingly well prepared for difficulties in this area. "The majority of our employees have professional engineering training," says Golushko, although "some of them had to be replaced in the Chernobyl region primarily on account of medical symptoms."

Crimes involving radioactive materials are another of Moscow's worries. On Oct. 3, a court heard the first case ever concerning "the illegal acquisition, storage and use of radioactive materials" in the U.S.S.R. According to a Soviet radio broadcast, a Mr. Abrizhynikov hid a cache containing two highly radioactive sources in a forest near Moscow. Investigators were unable to determine where he had obtained the materials or for what purpose he was storing them, but Mr. Abrizhynikov was sentenced to four years' imprisonment.

Yet another area of concern is sabotage aboard Soviet nuclear-powered vessels, which have suffered recently from a string of calamities. A Soviet nuclear-safety official has warned that the Murmansk nuclear-fleet base is inadequately guarded. He proposes that control over its security should be shifted to the Ministry of Internal Affairs, saying that this is "one more problem associated with safety." But he did not explain the nature of the threat that requires a higher security standard.

According to a former safety officer of the Leninsky Komsomol, the Soviets' first nuclear submarine, there is a "disastrous" shortage of technical equipment, and reactor core material is therefore routinely removed from reactors by sailors using sledgehammers. The technical neglect, he says, has exacerbated the safety problem to "an extreme extent." As a result of the disarray, "conflicts are brewing" between the officers of technical support ships, rear services and supervising agencies that may have an "unpredictable effect" on safety.

Soviet television recently reported that the U.S.S.R.'s nuclear-fleet sailors staged a two-hour warning strike at the Murmansk port on May 14 to protest inadequate pay and declining safety on civilian nuclear-powered vessels.

Gabriel Schoenfeld is the editor of Soviet Prospects, published by the Center for Strategic and International Studies, where he is a senior fellow.