In US, Sakharov's stepson tries to help

''I now consider them disappeared,'' Alexei Semyonov says quietly. Inside a two-story home in this leafy Boston suburb, Mr. Semyonov has been working for the release of his mother, Yelena Bonner, and his stepfather, Andrei Sakharov, Nobel Prize winner and father of the Soviet hydrogen bomb.

Miss Bonner and Mr. Sakharov were detained by Soviet authorities during a hunger strike earlier in the month to pressure the government into giving Miss Bonner permission to seek medical treatment in the West. Their whereabouts are currently unknown.

The Semyonov household is busy doing what it can to help. Alexei sorts through mountains of paper on the dining room table. In another part of the house, doors slam. Phones are ringing. Alexei's wife, Liza Alexeyeva, calms their two-year-old daughter. From the carpet in the foyer, thick black power cords snake into the living room where a BBC film crew is setting up. Walking past the kitchen, one catches a quick, memorable glimpse of Ruf Bonner, the Russian grandmother, sitting motionless with her heavily lined face set firm. A 17-year veteran of Stalin's camps, she has been through such times before.

With the exception of his 84-year-old grandmother, Mr. Semyonov is Sakharov's only relative currently in the United States. The young man has made himself a clearinghouse for information, for human-rights petitions, and has organized help for the Sakharovs within the American scientific community.

Like his stepfather, Alexei is an extremely private, understated man. He is sincere, passionately so, particularly when it comes to his parent's cause.

True to his character, Alexei refuses to dramatize his own situation. ''There is enough drama as it is - where it counts,'' he told the Monitor, referring to his parents' plight. He is sure the Sakharovs are in danger, ''and will remain in danger until we hear otherwise.''

Semyonov also says the Soviets are using his mother's physical problems ''as a way to get back at Sakharov. They (the Soviets) use people close to Sakharov as hostages,'' he says.

Alexei speaks from experience. His own wife, Liza, has known the severe pressures of Soviet officialdom. When Alexei emigrated in 1978, Liza had been promised a visa as well - a promise initially broken, and which prompted the Sakharovs' last hunger strike in December 1981. The authorities yielded on the 17th day of the protest, and Liza joined Alexei in Boston, where he is a math lecturer at Northeastern University.

In many ways, the current situation faced by the Sakharov family began in 1967. That year, along with publishing several articles in official scientific journals, Sakharov composed 12 human-rights essays to be published in Roy Medvedev's samizdat, ''Political Diary.'' The essays, which called for demilitarization and democratization of Soviet society, were so radical that they put Sakharov permanently out of favor. He became estranged from his family, later met the outspoken Yelena Bonner, and eventually made her family, including Alexei, his own.

Semyonov decribes his relationship with his parents as ''very close.'' He says Sakharov's greatest strength is the ''purity of his moral position,'' and his ability to argue with ''convincing honesty'' for intellectual, political, and social freedoms.

''Sakharov is a symbol rather than a leader of human rights in the Soviet Union,'' says Semyonov. He points out further that dissidents in the Soviet Union do not even think in terms of a human rights ''movement.'' Sakharov serves instead as ''a person against whom to measure your own actions.''

The last time he saw his stepfather and mother was in 1978. Until their 1980 exile to Gorky (a word meaning ''bitter'' in Russian) he could talk with them over the phone. After that, he communicated by letter. Now, of course, there is no communication at all.

Despite the fact that the Soviets have shown no sign of backing down, Semyonov feels their resolve in this crisis can be broken. ''They want to create an image of not bowing to any outside pressure,'' he says. ''They would like the West to think there is nothing that can be done.''

When asked how he regards those responsible for deciding what to do with the Sakharovs, Semyonov says, ''I don't think the average Westerner can understand it as a Russian does. (Soviet leaders) are simply part of a totalitarian machine. . . .''

Semyonov is grateful for efforts being made by the Reagan administration, but he also says it ''has not yet used all available channels.'' President Reagan felt it politic at first to not get personally involved, but Semyonov says that now the issue cuts across political boundaries: ''Italian communists, French socialists, Democratic presidential candidates, are all supporting Sakharov - Reagan doesn't have to worry about seeming too anti-Soviet.''

Semyonov urges private citizens to write President Reagan, Soviet head of state Konstantin Chernenko, or UN Secretary-General Javier Perez de Cuellar.