WASHINGTON (AP) _ A former engineer at the Rocky Flats nuclear weapons factory contends the plant operator knew for several years of possible accumulations of radioactive plutonium in the ventilation system but failed to investigate.

James Stone, a utility design engineer at the site near Denver from 1980 to 1986, said in a telephone interview Monday that the operator, Rockwell International Corp., ignored the repeated warnings he voiced as early as 1984.

''They just said, forget it, we're not going to do it,'' Stone said.

Stone had no firm evidence of plutonium accumulations in air ducts, but his suspicions were proven correct by a team of independent investigators who last Friday told the Energy Department they found 11 pounds of plutonium in air ducts linked to ''gloveboxes,'' or work chambers, in which plutonium is manually processed.

The presence of plutonium in the ducts is considered a hazard because, under certain circumstances, it could lead to an uncontrolled nuclear reaction. Such a reaction could, in extreme cases, release lethal amounts of radioactivity.

The investigators said the plutonium they found did not present an immediate health danger because it was not being released to the environment. But they criticized Rockwell for not having investigated earlier on the basis of Stone's reports.

Rocky Flats is owned by the Energy Department and makes plutonium triggers for nuclear warheads.

Rockwell had contended prior to the investigation by experts from Scientech Inc., an engineering and management company based in Idaho Falls, Idaho, that there were no accumulations of plutonium in any ductwork at Rocky Flats.

Ed Heintz, a Rockwell spokesman, said Monday he could not comment on Stone's allegations because they might be linked to an FBI investigation of possible violations of environmental laws at Rocky Flats. He said the company was not prepared to comment on the Scientech findings because it had not studied the final report, which is highly critical of Rockwell's safety procedures.

The FBI began investigating Rocky Flats last June but no indictments have been handed down so far.

The independent investigators were sent to the weapons plant in July by Energy Secretary James D. Watkins, who is attempting to resolve uncertainties about operational safety and waste management at Rocky Flats and other weapons plants.

Stone said he realized the probability of plutonium deposits in the ventilation system in 1984 while he was helping plan a renovation of Building 881, a former site of uranium and plutonium processing that now is used as a laboratory and computer center.

Stone said Rockwell did not act on his advice that the hundreds of feet of stainless steel ducts be sandblasted or cleaned in some other way to remove any plutonium buildup.

''It wasn't part of the original scope of their work so they weren't going to do it,'' he said.

Roger Mattson, a vice president of Scientech and leader of the Rocky Flats investigation, said in a telephone interview that Rockwell shut down several of the gloveboxes after the discovery in August and began taking steps to correct the problem.

Mattson said ''it's still an open question'' as to whether plutonium deposits have accumulated in the ductwork at Building 881, where Stone had been working.

Stone contended in an August 1986 lawsuit against Rockwell that he had been fired five months earlier in retaliation for having raised numerous health and safety issues, including the risk posed by the plutonium deposits. Stone's lawyer, Hartley Alley, said the suit is scheduled to go to trial later this month.