Cleanup of the damaged Three Mile Island nuclear power plant will take at least four more years but it can be done with a "negligible" radiation release to the environment, the Nuclear Regulatory Commission staff reported yesterday.

Issuing an inch-thick draft of an environmental impact statement on the cleanup plans, the NRC's special TMI program office said employes doing the lcleanup work would get the highest radiation dosage, but that it would still be well below safe limits. The estimated 2,500 workers needed could expect a possible three additional birth defects among their children and another 1.6 cancers, the report said.

The report was immediately challenged by a coalition of six antinuclear groups called the TMI Public Interest Resource Center which noted that the draft does not provide for any public hearing on the cleanup process. Research coordinator Steven Sholly asked the NRC to extend the 45-day public comment period now provided before a final impact statement is written.

The NRC staff considered leaving the Middletown, Pa., reactor only partially cleaned up but rejected the idea on grounds it "either would not eliminate the potential risks or would convert part of the site into a long-term or permanent waste repository."

The reactor's core, damaging during the March 1979 accident, must eventually be removed to safe storage to eliminate the remote chance it could reheat, and that is "the paramount objective" of the cleanup, the study said.

Cleanup will produce another 300,000 gallons of contaminated water to add to the 800,000 gallons now awaiting disposition in the reactor building and in an auxiliary building, the study continued.

Outlining the report to the NRC, TMI program chief Bernard J. Snyder said that deciding what to do with all that water will be "the most difficult decision" facing the agency. Area residents firmly oppose even a gradual, dilluted release of the water into the Susquehanna River, which is the method originally proposed by TMI's owner, the Metropolitan Edision Co.

"There are very viable alternatives to the release of this water and we're going to explore those very carefully," Snyder told the commission.

The draft statement said truck shipments of low-level wastes such as wiping rags, protective clothing and filter parts could number as may as 1,700 and might have to go as far as a disposal site in Hanford, Wash. People standing three feet from one of the trucks for three minutes would have 1.7 chances in 10 million of getting cancer from the tiny amount of radiation, the study said. The main impact would be on traffic.

Some continuing psychological stress can be expected among some residents near TMI, so the the cleanup should be finished as fast as possible, the report said. It promised to provide financial estimates in the final statement, expected before Christmas. p

NRC Commissioner Joseph Hendrie noted that the report lacked any cleanup cost estimate. But, he added, "I don't know if it costs $1 billion or $2 billion that it makes much of a damn. You have to clean it up."

In a related development, the Union of Concerned Scientists and five other antinuclear groups yesterday filed suit against the NRC to halt implementation of a new policy on citizen participation in power plant licensing proceedings. The antinuclear group told the U.S. Court of Appeals in Washington that the policy allows the utility seeking a license to fight safety requirements as too restrictive but does not allow citizen groups to fight them as too lax. CAPTION: Picture, Hendrie: "I don't know if it costs $1 billion or $2 billion . . . you have to clean it up." UPI