JACKSON, Wyo. -- It started with a short letter to a local newspaper here four months ago, in which a woman from Ketchum, Idaho, warned Jackson residents that the Federal Government intended to build a nuclear incinerator 90 miles upwind from Jackson and Yellowstone National Park.

At the time, almost no one here knew what she was talking about.

Now, almost no one here does not, and the shift has led to the latest battle between the Department of Energy and a Western community determined to stop construction of an incinerator that some scientists believe will spew radioactive and toxic materials into the air.

The conflict has a familiar echo in the West as the United States works to dispose of nuclear and hazardous waste that built up during 50 years of cold war weapons research. Federal laws require that the material be treated and disposed of. Yet, few communities have been willing to play host to such operations.

Two environmental groups, one of them some Jackson residents who organized to fight the project, plan to file suit on Friday in Federal District Court in Cheyenne, contending that Federal laws have been ignored in allowing plans for the incinerator to proceed.

The groups -- Keep Yellowstone Nuclear Free and the Environmental Defense Institute, a watchdog organization in Idaho -- contend that state officials in Idaho and the Federal authorities forged ahead without properly informing Wyoming residents, without reviewing the full range of health consequences and without considering a less threatening means of eliminating the waste.

They also accuse the Energy Department, in planning the incinerator, of ignoring "past accidents, mishaps and regulatory lapses, which have plagued its nuclear waste program and which call into question its unsupported assurances about the safety of the incinerator project."

The groups, which are represented by Gerry L. Spence, a nationally known lawyer and Wyoming resident, in effect say that the fix is in and that Wyoming, the nation's least populous state, with fewer than 500,000 people, can do little to stop the project.

"I'm concerned about the air I breathe, but ideally, this is not just a matter of what's happening here," said Mary Mitchell, vice president of the ad hoc Jackson group, which has been leading the opposition to the incinerator. "I won't really be happy until they have replaced what I see as antiquated technology with alternative ways of dealing with nuclear waste."

In recent years, community opposition helped thwart proposals to operate similar incinerators at the Rocky Flats plant outside Denver, Los Alamos, N.M., and the Lawrence Livermore National Laboratory in Livermore, Calif At Livermore, opponents of an incinerator were aided by the laboratory's own scientists, who analyzed the proposal and concluded in 1990 that burning nuclear waste violated "the cardinal principle of radioactive waste treatment; namely, containing radioactivity rather than spreading it."

But even successful opposition elsewhere has not stalled plans by the Federal Government to build an incinerator at the Idaho National Engineering and Environmental Laboratory, a facility the size of Rhode Island in southern Idaho that would burn much of the most hazardous nuclear and toxic waste material remaining in the United States. The material is now stored and buried at the site.

The Department of Energy has hired a contractor, British Nuclear Fuels Ltd., to build the incinerator. Construction on the $1.2 billion project is scheduled to begin once the Idaho Division of Environmental Quality issues permits that reflect its satisfaction that state air quality regulations will be met and that disposal operations will not affect the environment.

Steve Allred, the division administrator, said he would rule on the permit applications by the end of the year.

The threat of a lawsuit has at least made the Federal Government listen. The chief defendant in the suit, Energy Secretary Bill Richardson, said in an interview that construction of the incinerator would proceed only if the department is assured that operations would not jeopardize health and the environment.

Richardson said he had offered to meet in Washington with Spence and his clients to hear their concerns, and Spence said he welcomed the invitation, but he added that the plaintiffs would drop their suit only if the Energy Department abandoned plans for the incinerator.

Until then, the plaintiffs have vowed to press on, based on their fears that wind patterns would carry hazardous and radioactive emissions over parts of Wyoming that include Jackson and the two national parks just north of Jackson, Grand Teton and Yellowstone.

They have collected testimony from scientists who contend that despite all assurances from officials in Idaho and Washington that a new incinerator would operate safely, some toxic and radioactive particles would elude even sophisticated filter systems and contaminate downwind areas.

To bolster their efforts and cover legal costs, they have raised more than $300,000, with pledges of $200,000 more, reflecting a rare collaboration in this resort town of 6,000 people, where many rich and famous people have built second homes. In addition to small contributions from hundreds of year-round residents, members of the Yellowstone group said they had received $50,000 each from Harrison Ford, the actor, and James D. Wolfensohn, president of the World Bank, who have homes in the Jackson area.

For now, the two plaintiff groups are on the mission almost alone. Leading Wyoming officials, including Gov. Jim Gehringer and Senator Craig Thomas, both Republicans, have asked the Clinton Administration to explain the need for an incinerator, rather than a containment strategy. But neither the National Park Service nor politically influential groups in Idaho like the Potato Growers of Idaho that could be affected by an accident or inefficient operation have objected.

Michael V. Finley, superintendent of Yellowstone National Park, said members of his staff were reviewing impact studies on the park and discussing pertinent issues with officials from the Idaho laboratory.

Mike Duff, executive director of the potato growers group, which ÿrepresent hundreds of farmers living near the proposed incinerator site, said its members were "comfortable with the activities" at the laboratory.

Still, Jackson residents are worried.