Americans should not expect the current power struggle in the Kremlin to produce either a strong new leader or significant changes in Soviet policies for at least several years after the demise of Leonid I. Brezhnev, according to the highest-ranking Soviet official ever to defect to the United States.

Arkadi N. Shevchenko, 51, who was undersecretary general of the United Nations in New York when he sought American asylum in April, 1978, also says Americans would be wrong to believe they can force concessions because of the Soviets' current economic and agricultural difficulties. In an interview with The Washington Post, the first extensive one he has given since his defection, Shevchenko said no foreseeable Soviet leadership could afford the risks that would accompany significant departures from current foreign and domestic policies.

"Nobody will succeed Brezhnev," Shevchenko said, adding that no new leader could combine the titles and powers that Brezhnev holds. Such power, he said, can only be accumulated over many years.

Shevchenko has spent many months telling U.S. officials what he knows of Soviet personalities and policies and the workings of the Soviet system, and senior officials say his information has been valuable and often unique. Shevchenko, who said American officials now make no attempt to control what he says or does in public, sought out a Washington Post reporter through a U.S. intelligence official he befriended soon after he defected.

During Shevchenko's 22 years as a Soviet diplomat he met the principal contenders for power in the post-Brezhnev era. Much of Shevchenko's access to the upper reaches of the Soviet establishment came as a result of his close ties to Andrei A. Gromyko, the Soviet foreign minister, whom he served as personal political adviser from 1970 to 1973, before becoming the highest-ranking Soviet at the United Nations.

Unlike some Soviet defectors, Shevchenko does not portray the Soviet leaders as implacable opponents of capitalism who are hell-bent on conquering the West. Instead, he said, they are mostly second-rate men without strong personalities or distinct views who are always interested in accumulating personal power, but not if it means risking the system of rank and privilege that has served them all so well.

This does not mean that there are "moderates" or "liberals" in the Kremlin leadership, Shevchenko said, but rather that the men who run the Soviet Union are generally small-minded and cautious.

"These people are political pygmies," he said.

Shevchenko said Brezhnev often has been physically incapacitated since 1976 or 1977, yielding much day-to-day power to his longtime protege, Konstantin Chernenko. During meetings in which Shevchenko took part in 1977, Brezhnev was obviously suffering from severe memory loss and disorientation, he said.

"I was shocked at his condition," Shevchenko said. Although Brezhnev continued to have moments of effectiveness and lucidity in the years since, he added, this was often "a case of a man who could not think."

Shevchenko has married an American woman and settled in an area not far from the nation's capital. He acknowledged that he is anxious to improve his image in this country, which was badly tarnished by his affair with a Washington call girl that turned into a public scandal.

Shevchenko said he was in terrible shape during the first months after he defected, drinking heavily and behaving badly with the call girl, Judy Chavez, whom he paid thousands of dollars--provided by the CIA--every month. Chavez described this period in sleazy detail in her paperback book, "Defector's Mistress."

Shevchenko's first wife died under mysterious circumstances in Moscow shortly after she was rushed back from New York when security agents discovered her husband had defected. Shevchenko charges that the Soviet security police, the KGB, murdered her. A son and a daughter from this marriage are living in the Soviet Union.

Today Shevchenko has given up drinking, and his American friends say his new wife has helped change his life. A small, stooped man with a handsome Slavic face and flowing white hair, he looks fit and talks with energy. He is working on a book about his experiences, and hopes to write and lecture here on Soviet affairs.

Shevchenko's close relationship with Gromyko gave him an intimate view of the top Soviet leaders that was unusual for a Soviet diplomat. Like other experts and specialists, diplomats are seldom allowed into the inner circle. Still, much of Shevchenko's information is secondhand, based on gossip among the cadre of experts who advise the top leaders.

Shevchenko said the advanced age of the entire ruling group has become an important political factor. He revealed that after Gromyko fainted in a meeting of the ruling Politburo in 1973, the group decided to order all its members to take one-month vacations twice each year.

Producing calculations he had made on a legal pad, Shevchenko said that when Joseph Stalin died in 1953, the average age of the three top men most likely to succeed him was 54. When Nikita Khrushchev was deposed in 1964, the top three candidates to replace him averaged 60 years in age. The top three contestants to replace Brezhnev are 67, 70 and 75.

It took Stalin, Khrushchev and Brezhnev four to five years to consolidate their power after assuming the leadership of the Soviet Communist Party, Shevchenko said.

"In the Soviet Union," he said, "everything happens slowly. Always remember that."

If Brezhnev's replacement also needs four to five years to consolidate power, Shevchenko said, he may not live to complete the process, so it would have to begin all over again in the late '80s.

Shevchenko said it was a mistake to describe what is going on now as a "struggle for power."

" 'Struggle' is for some principle, at least there is some kind of fairness or openness or honesty in a struggle," Shevchenko said. "But here? This is a struggle for principle? Good Lord! They're struggling for personal power, that's it! . . . I would call it a power play--it's a contest to seize power."

Shevchenko said the key group that will chose the next general secretary of the Soviet Communist Party consists of the 10 members of the present Politburo who are residents of Moscow. (The other three serve as regional party leaders.)

Shevchenko agrees with the consensus among western Kremlinologists that the most likely new leader will be one of three men: Chernenko; Yuri Andropov, recently transferred from the chairmanship of the KGB to the secretariat of the Communist Party's Central Committee; and Andrei Kirelenko, a senior party official for many years.

Only these three have enough of the necessary credentials, Shevchenko said, which are long experience in party work and experience in domestic affairs.

Shevchenko, who knows all three men, none of them well, said he rated both Chernenko's and Andropov's chances evenly, with Kirelenko, who has been very sick, significantly behind.

"There's always the possibility that some dark horse will emerge," Shevchenko added, noting that Brezhnev was an unlikely candidate to succeed Khrushchev in 1964. One possible dark horse, he said, was Viktor Grishin, head of the Moscow Communist Party organization and surely ambitious for higher office.

On the more likely successors, Shevchenko had these comments:

ANDROPOV: Though most commentators have concluded that Andropov got a promotion when he was moved from the KGB to the Central Committee two weeks ago, Shevchenko said there was a possibility, perhaps only slight, that he was actually being pushed aside by colleagues who feared his power in charge of the huge secret-police apparatus.

On the other hand, Shevchenko said, Andropov would be unlikely to win promotion to the top spot directly from the KGB chairmanship; he would have to move up from a job such as the new one he has just gained, so perhaps this was indeed a promotion.

And perhaps, he added, Andropov thought he was getting promoted, but really wasn't. "Perhaps he got an ace, but lost a king and a queen," Shevchenko said, speculating that a clever card player might live to rue that exchange. "Now he is a general without an army," Shevchenko said, referring to Andropov's relinquishing command over the KGB.

Shevchenko ridiculed the idea that Andropov was a "closet liberal" who would be a reformist leader if he won the struggle. "On what grounds have people gotten this impression?" Shevchenko asked. Answering his own question, he said Andropov, while head of the KGB, had met personally with some dissident intellectuals in Moscow. "This only shows how shrewd he is, how treacherous," Shevchenko said, adding that it was clever of Andropov personally to take the measure of intellectuals who were one of the KGB's principal concerns.

Shevchenko noted that some Kremlinologists have portrayed Andropov as a reformer because he had tolerated reforms in Hungary, where he served as Soviet ambassador in 1956, when a Soviet invasion crushed the Hungarian revolution. Shevchenko said he had personal friends who worked with Andropov at that time and who reported that Andropov relished his role as the Soviet proconsul who could brutally crush the Hungarian uprising.

There was no question about Andropov's desire to win power, Shevchenko said. The best proof of that was the way he aggressively built up the KGB during 15 years. The security police grew significantly in numbers and importance both at home and abroad, Shevchenko said.

Anyone who considered Andropov a cultivated, liberal man should have met the men he sent to New York to be KGB "resident" there, Shevchenko said.

With the exception of Vladimir Kazakov, deputy chief of the Soviet mission to the United Nations who is the current KGB resident and an intelligent man, Shevchenko said, all the others in Andropov's time were crude, anti-American and antidetente officials who took a much harder line than the party.

Growth of the KGB was justified as a necessary means of controlling Soviet society and preserving the existing system, Shevchenko said, but members of the Soviet elite who were supposed to benefit from this protection grew to fear the security apparatus. Even the wife of Foreign Minister Gromyko would point anxiously at unseen microphones she presumed were planted in the plaster of her own home when a subject came up that she considered too sensitive to discuss, Shevchenko said.

In earlier years, Shevchenko said, Andropov discouraged suggestions that he move from the KGB into the party apparatus, a sign that he realized he had greater power as head of the security forces.

Shevchenko used to go to the same resort sanitorium for senior officials that Andropov frequented in the mountain town of Kislevodsk, he said. From the doctors there he learned that Andropov has suffered a serious heart attack. Unlike other top party officials who frequented this resort and mingled with other guests, Andropov stuck to himself, Shevchenko said, taking meals apart from the others and declining to converse with them.

CHERNENKO: Shevchenko said western experts had been slow to realize how powerful Chernenko became after Brezhnev became seriously ill in the mid-1970s.

Because he was secretary of the Politburo and kept the only record of its proceedings (formal note-taking or transcripts are discouraged), and because he conducted polls of Politburo members on questions that had to be resolved outside its formal meetings, he was strategically placed to wield influence, Shevchenko said.

Some senior officials, including the two senior members of the Politburo after Brezhnev, Mikhail Suslov and Alexei N. Kosygin, resented the power Chernenko accumulated because of Brezhnev's bad health. But they have both died, and Chernenko gets along better with the remaining members of the group, Shevchenko said.

Shevchenko said he spent considerable time with Chernenko when the latter visited the United States twice during Shevchenko's U.N. tenure. Shevchenko found Chernenko to be rude and crude, the sort of high official who could never acknowledge that he might be able to learn something from his underlings.

Other top party officials who visited the United States could acknowledge that there were enviable features of American life that the Soviets could not match, but Chernenko reacted to the United States with conventional, orthodox, "ours-is-better" Soviet reflexes, Shevchenko said.

Shevchenko said he used to think Chernenko was by far the most likely to step in when Brezhnev leaves the scene, but lately he has become uncertain.

KIRELENKO: Before his unidentified but apparently serious illness, Kirelenko was a considerable figure, Shevchenko said. He has extensive experience as overlord of all civilian industry and manager of the party apparatus and was a relatively modest, quiet man who appeared to have quite a good education.

When it comes time to decide on a new leader, Shevchenko said, a basic paradox of Soviet politics will come into play. Ambitious younger men who are not candidates for the top spot this time will be tempted to back the man they expect to be the weaker leader, he said, on the theory that the weaker the next incumbent, the better their own chances for promotion.

"Of course there will be changes" in domestic policies under a new leader, Shevchenko said, if only because economic conditions are so bad. But he cautioned that "they won't go beyond a certain line" and certainly won't lead to fundamental reform of the system.

Such reform, he said, is simply too risky, and no Soviet leader will chance it. There is no easy way to isolate problem areas like agriculture and try to deal with them, Shevchenko said.

What would happen, he asked, if a successful rural reform produced farmers who earned much more money than party bosses? It couldn't be tolerated and therefore wouldn't be attempted, he said. But without such changes, he added, the agricultural situation probably won't improve.

Any basic reforms would threaten the system of centralized control and would amount to voluntarily opening "a Pandora's box," Shevchenko said.

As for foreign policy, Shevchenko predicted that any new Soviet leader would try to improve relations with the United States.

"This is a must, for two simple reasons: First, they need the United States for help--technology, grain, and so on. And second, it's not the right historical moment to have bad relations with the U.S. because they have enough troubles already."

However, no new leader will be willing to give up the Soviet right to take steps like the invasion of Afghanistan or forcing the imposition of martial law in Poland in order to improve relations with Washington. The new leadership could make only "marginal" sacrifices of Soviet interests to improve Soviet-American relations, Shevchenko predicted.