Loretta Cornelius once wanted to be a nun, and even today speaks of "a peace and tranquility that I have inside." Which may help explain the calm with which she fired Donald J. Devine, her former boss at the Office of Personnel Management.

Calm also prevailed during her recent testimony before a Senate committee when she said Devine had asked her to lie. Her statement was at least partially responsible for Devine's decision to withdraw his nomination for reappointment as OPM director. After Cornelius' appearance on the Hill, Devine said he believed two Republicans would join the six Democrats on the Senate's civil service subcommittee to block him. One Devine supporter called her testimony "an act of high political treachery."

Cornelius, 49, now acting director of OPM, is a diminutive woman with eyes that appear to miss little. Her voice is soft, with more than a hint of a Kentucky accent.

The reaction of Devine's supporters does not surprise her, she says, but it "bothers" her.

"The platform that is used" by her conservative opposition, Cornelius says, "is that America is back -- we're back to honesty, integrity and truth, and those are the things I have applied -- and I think it's that application that I am guilty of in their eyes.

"I'm a conservative. I think of myself as a conservative person -- in the true sense of the word. I'm not into radical things."

She says she never sought her job at the agency that establishes federal personnel policies, and that she insisted from the beginning that her role not be a public one.

"I never wanted to see my name in print."

As Cornelius sums up the events that brought her so much attention, she appears to be one of those for whom everything is clear.

Devine's four-year term expired March 25. "He has known all along he was going to have trouble being reconfirmed ," says Cornelius. "Any time you make changes, I don't think you're going to be popular." Devine, 48, a former professor of government and politics at the University of Maryland, helped shape such Reagan personnel policies as cutting back the federal work force and proposing to reduce pay and curtail health benefits.

" Devine asked me, if he didn't get confirmed right away would I take him on as a consultant," Cornelius says. As deputy director she automatically became acting director during the interim period.

"He mentioned it weeks before his term was up and then it was not mentioned again. Then, just before his term was due to expire, he came in with two sets of personnel action papers for me to sign . . . hiring him. There were two titles -- one was executive director and the other was executive assistant. I told him I would have a problem with the executive director title. He wasn't angry. Don and I never exchanged any angry words during the whole process.

"On the last day of his term, he wrote himself a delegation of authority to himself as executive assistant, and did not tell me about it."

In his delegation, Devine gave himself full power to continue running the agency.

"But when I heard about it on April 29, he told me there was a delegation," Cornelius continues. "He asked me if I couldn't say I knew about it before. I said, 'No.' He said, 'This could be very damaging to me.' I just wouldn't lie. I told him, 'I pray to God no one ever asks me a direct question. I'll have to give a direct answer.'

"He told me that he had meant to share it with me and had forgotten. I told him that I felt he had deliberately not shared it with me and I was greatly concerned. I also felt it was illegal.

"And that's when I wrote the memo to Don and carried it down to his office -- abolishing his position. I didn't need an executive assistant that operated in that manner. I felt I had no choice. I felt it was something I had to do."

Devine has acknowledged that he "forgot" to give Cornelius a copy of the document delegating authority to himself, but said Cornelius "should have known." He also said Cornelius "misunderstood" their April 29 discussion. The General Accounting Office said on June 5 that Devine "illegally circumvented" federal law by attempting to retain power when his term expired. Devine said the GAO opinion was wrong because he did not act as director during the period.

When Cornelius presented Devine with the memo that dismissed him, she says, he "gave a kind of laugh . . . and said, 'You can't do this . . . we have to talk about this,' and I said, 'No, I've thought about it most of the night and this is the decision I have come to, and I can do it.' "

Asked yesterday about Cornelius' account, Devine said, "I said all I have to say at the hearing. I just don't have any comment."

How does it feel to fire your boss?

"I like Don as a person," Cornelius says with an almost breathless enthusiasm. "And I liked working with him. There were times when we disagreed, but we had an agreement in the beginning that we would never disagree in public. I thought about him, I thought about his family, but I also thought, 'I have to look myself in the mirror every day.'

"It was the idea of a piece of paper being generated that transferred all of his authority. I had the responsibility. I had the legal responsibility. By law I had to become the acting director when his term expired. And you know, if it weren't by law that I was here now, I might not be here," she adds with a laugh.

"But I had that responsibility and I wanted to be sure I knew what that was when we started having these disagreements on some of the policy packages. He was making recommendations I disagreed with and I'd send them back. That's what precipitated the discovery of the whole thing."

Cornelius continues, "I wanted to know what I was going to be held accounted for. I feel very strongly about performance and accountability. I think people should be given a lot of authority and be held accountable for their acts. That goes back to my whole style of management.

"I do not want to control. I don't view this as a powerful position. I never have."

She might have chosen the convent, and you can picture her as a mother superior. But you can also see her as representing the successful professional woman of our time.

She is the mother of three grown children. She took 10 years to complete her undergraduate degree at Bowie State College, attending school between babies and part-time jobs. She has experienced divorce, single parenthood and remarriage -- to Francesco A. Calabrese, a businessman, who also has three children.

She grew up on a farm in Kentucky with four sisters and a brother. Her father had less than 100 acres of his own, but worked other farms. He never graduated from high school. Her mother, college educated, worked as a circuit teacher, traveling from one school to another.

Cornelius remembers her father for a clear sense of "right and wrong." And for advanced thinking. "My father liberated me when I was 12 years old," she says. "He wanted to give us every opportunity to be on our own -- to be independent." By that age, she says, she was already driving farm equipment.

Her background was fundamentalist Protestant. From noon on Saturday until Sunday evening, no work was done. And, in her late teens, to the surprise of her parents, she sought religious instruction and converted to Catholicism. "I had been attending mass every morning for some time and they had no idea," she says.

One attraction of the Catholic Church was its cloistered orders. "I wanted to be able to make a contribution in an extremely peaceful way," she says now of that period in her life. Eventually, she backed away from a decision to become a nun, took a business course at a nearby college, married a schoolteacher and started a family.

A series of moves took them to Northern Virginia, at which time Cornelius pursued her college degree. In 1967 she took a part-time job with Planning Research Corp. of McLean and began to move up the corporate ladder; in 1978 she became vice president for administration at PRC, supervising several hundred employes. By 1981 she had earned two master's degrees. Last week she received a doctorate in public affairs from the University of Southern California's branch in Washington.

In 1981 she was asked to interview for the position of deputy director at OPM.

She never saw herself as a token. "I'm more than qualified," she says. "Don Devine didn't have the management experience."

Cornelius and Calabrese (she was divorced from her first husband in 1974) spend most of their time in an apartment in the Virginia suburbs. Their real home -- her husband calls it a "mini-estate," she calls it "a little ol' farmhouse" -- is near Warrenton. They seldom get there now. She travels almost constantly for OPM -- well over 200,000 miles so far, she estimates.

"She's ruthless with her time," says Calabrese, and she agrees: "I simply will not permit people to waste it," she says.

"I think when you grow up in a family of six children you have to have extraordinary concentration," says Cornelius. "There are not that many rooms in a farmhouse to get away from people -- and I did all my schoolwork after I had babies and you have to have that power of concentration. But it all comes down to a ruthless discipline of time."

Cornelius finds it difficult to understand the reactions of others to her actions. She says she is not a political person. Her political activities, she says, have been limited to fund-raising for Virginia Republican candidates and membership in the Greater McLean Republican Women's Club.

She knew, though, that she would not escape lightly from firing Devine. "The day I walked down and delivered the memo, I knew the next day I might be packing my desk and moving out of here."

But in the next breath she says, "I never really thought about what would happen after the hearing.

"I believe that what I did, anyone would have done in the same circumstances, because, as far as I was concerned, it was clear to me what I had to do -- I had no choice; therefore, I don't think it's such a big deal. And I don't understand what people have made out of it, either for it or against it. It's just something that had to happen."

Did she think her testimony was likely to end Devine's chance of being reconfirmed? "I already knew by then that he didn't have the votes . . . and he knew that too."

She laughs at reports that she was called to the White House last week to account for her actions. "I wasn't called to the White House. I had a meeting in the Old Executive Office Building and you have to go in through the Southwest Gate to get in there. It was a meeting on the Blue Cross-Blue Shield refund . . . "

Cornelius is reported to have left her office late that night wearing sunglasses and a grim look. "The reason I had my sunglasses on was because I had three briefcases and I usually have a coat that has a pocket and I didn't have a pocket. I had my keys in one hand and a briefcase on either shoulder and a briefcase in one hand. My arms were full. I just put my glasses on instead of putting them on my head . . . and the next thing I . . . isn't this ridiculous? I mean it's absurd. Can you believe people waste their time on this garbage?"

Rumors that she had been fired circulated quickly after Devine withdrew his nomination. She arrived the morning her "ouster" had been announced by The Washington Times to find her general counsel speechless at her cheery demeanor. "Haven't you read the papers?" she says he asked.

Cornelius has no doubt that there is now a concerted effort by the right to get rid of her. "The focus on me is this one act. The phrase I hear is that I must pay for what I did to Don Devine and then it is followed by the phrase, 'Well, you really didn't do it, he did it to himself . . .'

"If I were faced with the same situation tomorrow, I would have no choice but to make the same decisions and take the same action."

There are also supporters, Cornelius says. Flowers arrive daily in her office but many of the cards carry no name. Calls from friends and supporters from within the administration, she says, have been frequent. Others have been mentioned for the post of OPM director, but Sen. Ted Stevens (R-Alaska), chairman of the committee that oversees OPM, has said that he would have "problems with anyone selected . . . if they said they'd fire Cornelius ."

Asked if her actions did not stem in part from simple ambition, Cornelius denies that she ever even sought or desired a government position.

"Credit is not something I have ever aspired to," she says.

She says she is more relaxed than she's been in a long time. "I watched a movie yesterday with my daughter -- I felt guilty that I wasn't working 24 hours a day. But this is not my whole life. This is just another phase.

"Everything that happens to us is a learning experience," she says.

Does she worry that she might be out of a job? "I don't believe there are people out there that would not hire me because I fired someone or because I told the truth. In most of the environments I have lived in, those are attributes."

Her relations with the White House, she says, remain cordial. "They said, 'Do what you need to do.' " Now, she says, her concerns are to stabilize the agency, to run it smoothly.

In the process, a number of heads have fallen. Her opponents call it a purge. Cornelius says, "I have not fired a lot of people. I asked for the resignations of Don's personal staff over a week after he withdrew his nomination because they were his personal staff and their jobs were responsible directly to him and I did not have jobs for them. The only other person who has been dismissed has been the public affairs person . . ."

She doesn't see this as unusual. "In my environment, personal staff automatically resign. If I were on someone's personal staff and they left, I would automatically tender my resignation. I just think it's a common courtesy . . ."

What would she do if she were asked to take on the job of director?

"I will serve wherever the president requests," she says with finality.