When plumber Pepe J. Mestres moved from New York to South Carolina in 1982, he was looking for little more than steady work. The Savannah River nuclear weapons plant, with its miles of pipes and valves, looked like a safe bet.

But Mestres doesn't work there anymore. He was fired, according to a Department of Energy investigation, because he took too keen an interest in safety on the job.

Now DOE officials are eyeball to eyeball with their Savannah River contractor, E.I. du Pont de Nemours & Co., over whether Mestres must be rehired or provided other "appropriate relief," or whether the department must play its only trump card: a stop-work order that could bar B.F. Shaw Co., a major Du Pont subcontractor and Mestres' former employer, from the weapons facility's 300-acre complex outside Aiken, S.C.

"If Du Pont were to agree to our position and B.F. Shaw did not agree, a stop-work is a possibility," said Cliff Webb, a DOE spokesman at Savannah River.

But if the idea of barring an important subcontractor in the midst of a frenetic construction schedule is distasteful to DOE officials, the prospect that Du Pont also might refuse to comply with their order is even more nettlesome. According to safety officials, the department's only administrative option would be to sever its 30-year relationship with Du Pont at Savannah River.

"There are contractural obligations," said Bill Thornton of DOE's occupational safety office at Savannah River. "There are grounds for a contract being reviewed and terminated. It's never been done for a large contractor."

Du Pont has until today to respond to DOE's Jan. 7 ruling in favor of Mestres, and department officials say they are hopeful that a settlement will be reached short of the unthinkable.

"I can't visualize them not obeying the order," said Donald M. Ross, a DOE occupational safety official here. "We've never had an instance where we haven't gotten our way."

Du Pont officials directed a reporter's phone calls back to the DOE public affairs office.

Some Energy officials acknowledge that they are puzzled by the apparent reluctance of both Du Pont and B.F. Shaw to rehire Mestres, who was described in the DOE ruling as "an average to good employe who was apparently well regarded by supervisors and fellow employes."

Mestres is less puzzled. "There is a point to be proved," he said in an interview last week. "Companies can see that DOE decided in my favor. It's like a victory to us. They can't treat us this way and get away with it."

Mestres lost his job on March 28, 1984, after repeated confrontations with his superiors over what he saw as lax safety practices and the lack of protective equipment. His case reached DOE headquarters last fall, under a 1983 regulatory change that for the first time allowed such appeals, and on Jan. 7 a special appeals board took his side.

The three-member panel rejected arguments that Mestres was fired for poor performance, finding that his "repeated inquiries and discussions of valid safety concerns . . . would appear to be the real reason for his termination."

Mestres, the report said, "tended to be very assertive where his rights or his safety were concerned. . . .B.F. Shaw management was clearly concerned about this and seemed uncertain about how to handle Mr. Mestres and considered him a problem employe."

The investigators found that Mestres did his assigned work, despite his safety concerns. But they added that "B.F. Shaw Co. policy -- 'If you think it is not safe, don't do it' -- rings hollow in the face of a foreman's warning -- 'If you don't do it, that's insubordination.' "

In one case, according to the DOE report, Mestres steam-cleaned bearings without the required protective equipment after a foreman rejected his request for protective gear by saying "go on and do work. Safety is not here tonight."

In another case, Mestres was burned "fairly seriously" after being assigned to overhead cut-and-burn work without being issued proper protective equipment.

In still another case, Mestres' complaints about defective brakes on a forklift were dismissed by a foreman who stated that "it's no pipefitter's responsibility to know whether that machine is unsafe or not. . . . All this is for the driver himself." Questioned about a later incident in which Mestres developed a hernia after manually lifting a 300-pound canister, the same foreman responded that "this man was assigned to a forklift."

According to Mestres, his troubles began when he was transferred out of a "hot" work area -- one involving exposure to radiation -- to a "clean" area. The transfer was ordered because of tests that showed Mestres suffered from polycythemia, an excess of red blood cells.

Exposure to radiation is suspected of causing or exacerbating some forms of polycythemia, although the condition also can have other causes, including living at a high altitude or smoking. In Mestres' case, the condition apparently predated his work at Savannah River -- it showed up on his prehiring health tests.

Mestres said he wasn't informed of it then, and he has yet to learn why he was cleared initially for a radiation zone, or why the clearance was lifted. The DOE report attributes the situation to "a lack of communication," but Mestres notes that the change came shortly after the Atlanta Constitution ran a series on high rates of polycythemia in the Savannah River area and the federal Centers for Disease Control agreed to look into the situation.

Mestres said he believed that some of his troubles stemmed from the fact that he was considered an "outsider" -- a New Yorker of Cuban ancestry -- but he also believes that the transfer may have triggered resentment.

"Around here, they say, 'You don't want to work radiation, then we don't need you. Goodbye,' " he said.

Still, Mestres, who has five children, said that he would rather have his job back than a financial settlement. "A lot of people won't work out there, because they feel no one really knows about radiation," he said. "People who are hard up like me, I got to feed the family."

"Am I concerned about my health? Yeah," he said. "DOE knows I'm concerned about that issue."

His wife, Susan, agrees. "If he does go back, maybe things will change," she said. "Now he knows it's not a matter of 'do it or else.' He knows now he can go to DOE."