BOYDTON, Va. (AP) _ A year ago, six condemned killers at the Mecklenburg Correctional Center made the largest death row breakout in U.S. history, escaping from a prison that was considered escape-proof.

Since then, two of the escapees have been executed. The others are back on death row, and the prison's warden, the third at the maximum-security institution since the breakout, says it cannot happen again.

''I'm very confident we will not have another escape like (that) one,'' said Toni Bair, who was deputy warden at Utah State Prison before he came to Mecklenburg last January. ''The likelihood of that type of fiasco happening now, I think, is non-existent.''

The escape on May 31, 1984, was the first of a series of crises last year for the Mecklenburg Correcional Center, a sprawling 93-acre institution just outside this small southside Virginia town and about 20 miles from the North Carolina border.

Led by brothers James and Linwood Briley, the inmates bluffed their way out of the prison using a phony bomb and uniforms taken from guards. The manhunt ended 19 days later when the Brileys, the last of the group to remain at large, were recaptured in Philadelphia.

Linwood Briley died in the electric chair Oct. 12, 1984, and James Briley followed him last April 18.

Of the other escapees, Earl Clanton Jr., Willie L. Jones and Derick L. Peterson are awaiting their turns in the electric chair. The U.S. Supreme Court has ordered a restudy of Lem D. Tuggle's death sentence on the grounds he may have been denied his right to an independent psychiatric evaluation.

Less than three months after the escape, a group of inmates seized a prison building floor and took nine employees hostage during a tense 19-hour siege designed to express their growing anger with prison conditions.

Corrections officers were also growing dissatisfied with the prison, and the turnover rate among prison personnel soared alarmingly in the months after the escape.

The increasing tension and decreasing morale was further aggravated by continuing legal tangles with the American Civil Liberties Union, which repeatedly went to court to demand changes in the prison.

The turmoil, however, forced officials to make long overdue changes.

''They have changed for the better dramatically,'' said Alvin Bronstein, director of the American Civil Liberties Union's National Prison Project in Washington, which recently agreed to end its four-year legal battle against Virginia over conditions at the prison. ''It's a whole different atmosphere.''

Unlike previous prison administrators, who sought to use the ACLU as a scapegoat for security breakdowns, Bair has proved a prison can be secure and humane at the same time, Bronstein said.

''They're running a tighter place, but a much more humane place,'' he said. ''There's remarkably less tension among the staff as well as the prisoners. It is a much more relaxed atmosphere.''

Said Bair: ''We're beginning to talk to inmates and treat them as human beings, as adults, and demanding that they respond in the same way.''

One of Bair's first moves as warden was to create an anti-escape task force - a group of 72 officers whose sole job is to make sure the prisoners remain in prison.

''They worry about the perimeter and making sure no one breaches it,'' Bair said. ''They go around trying to find and anticipate what might be a security breach, and they plug that hole.

''(They) actually put themselves in the framework of the inmate and say, 'If I were an inmate in here, how would I get out? What could I do?'''

''We've already plugged up major, major flaws that we discovered when we came on board here,'' Bair said. He cited security reasons in declining to give examples.

Bair, 47, also took a more hands-on approach than his predecessors, going into the prison occasionally to talk to inmates and listen to their suggestions on how to run the prison.

He estimated that some 20 percent of the changes since January have come at the suggestion of inmates. And the prison staff suggested an additional 20 percent.

''We miss a very valuable resource when we do not listen to an inmate or the line staff,'' Bair said.

Bair has also worked to erase the bitter feelings that the escape left in nearby Boydton.

The warden summed up the situation at Mecklenburg today: Inmates are happier with the prison. The ACLU is happier. Corrections officers are happier.

And a happier prison is a safer one, he added.

Does that mean that Mecklenburg is now really escape-proof?

''I don't think man is capable of building an institution that is escape- proof,'' said Bair. ''Prisons are in the business of keeping people in. Inmates are in the business of getting out.

''Sometimes they beat us.''