A high-level Soviet defector has explained the decade-old mystery of the disappearance of Nicholas G. Shadrin, an American double agent who disappeared while meeting with KGB agents in Vienna Dec. 20, 1975.

Shadrin, then 47, was accidentally and fatally chloroformed while struggling in the back seat of a sedan with Soviet agents trying to spirit him out of Austria and away from his Central Intelligence Agency protectors.

This account was relayed yesterday from government officials to the lawyer for Ewa Shadrin, widow of the agent. In an interview, Shadrin said yesterday that two FBI agents went to her McLean house just before 2 p.m. Sunday and told her they had confirmed beyond doubt that her husband is dead.

"I was sort of prepared for something like this," she said, "but at the same time it is very disturbing. I would like to know more about what happened, and what they did with the body. One consolation is that he really didn't suffer. I have worried so many times about that."

After the FBI agents left her house, Shadrin said, she relived events of the night 10 years ago when she passed the time waiting for her husband at the Vienna opera house. "I was . . . reliving those moments, they were killing Nikki while I was in the opera," she said.

Shadrin's disappearance while he reportedly was in the care of CIA officers has been a controversial episode in the murky history of U.S.-Soviet espionage.

Concern for his safety led President Gerald R. Ford in December 1976 to make an unusual personal appeal to then-Soviet leader Leonid Brezhnev to investigate the case and reunite Shadrin with his wife, even if it meant they had to live secretly to conceal such extraordinary Soviet cooperation.

Using private diplomatic channels, Brezhnev told Ford that Shadrin had not appeared for the fateful meeting with KGB agents.

The role of the three U.S. intelligence agencies for whom Shadrin was working has been examined in thousands of inches of press accounts and in a book-length study of Shadrin's life as a spy and his mysterious disappearance. News of his death, first reported by NBC News Monday night, is the latest revelation to leak from the top-secret debriefing of Soviet defector Vitaly Yurchenko, a senior officer of the KGB, the Soviet secret police, who defected last July on a visit to Rome.

Yurchenko, being debriefed at an undisclosed location near here, served in several senior KGB posts over the last two decades, including those of deputy chief for North American spy operations, chief of worldwide counterintelligence operations and from 1975-80 as a political officer in the Soviet Embassy here.

His first known contribution to U.S. intelligence was to identify Edward L. Howard, a former CIA officer trained for duty in Moscow, as a Soviet spy who provided details about U.S. information-gathering techniques in Moscow.

Yurchenko also has reportedly told debriefers that, based on Howard's information, the Soviets were able last June to arrest a Soviet aviation scientist who had been providing the CIA with data about Soviet research to conceal planes and missiles from U.S. radar.

Shadrin and her attorney said yesterday that they would like to interview Yurchenko to seek answers to questions remaining in the case.

Richard D. Copaken, who has represented Shadrin in her longstanding effort to pry information from the FBI and the CIA about the disappearance, said the FBI has not responded to his request.

Copekan said he is disturbed that FBI officials did not notify Shadrin of her husband's death until it appeared imminent that the news would be reported on television.

"I think this country owes Shadrin a great deal . . . and at least owes him enough to be truthful to his widow," Copekan said.

Shadrin was born Nikolai F. Artamonov and defected to the West in 1959 as a young Soviet naval officer. He became a consultant to the Defense Intelligence Agency and, in 1966, after being contacted by KGB agents in this country, went to work for FBI counterintelligence and the CIA as a double agent.

His trip to Vienna in 1975 was one of several contacts intended to persuade the KGB that he was working for his native country as a spy in the United States.