ISCASSET, Me., May 10 — Power company executives, environmentalists and state government officials fought for most of the 80's and 90's about whether the Maine Yankee nuclear power plant was safe and economical. But once the owners agreed that the plant should close, the debate turned really complicated.

Suddenly, said Ray Shadis, who had fought for years to shut down Maine's only reactor, "there were a lot more things to argue about."

How much radioactive building material could safely be left at the site? Should nonradioactive concrete and concrete structures below the ground be removed? What should happen to the highly radioactive spent fuel, which the federal government is supposed to take, but, for the next few years at least, has no place to put?

Now, more than five years after Maine Yankee split its last atom, the cumbersome process of decontamination and demolition gives a hint of what lies ahead for the 103 power reactors still operating around the country — whether economic problems close them, as happened here, or fear of terrorism shuts them, a threat faced by Indian Point in New York, or whether they run for years to come and retire at a ripe old age.

First comes the argument over how much radioactive material can be left. Some experts have described as excruciatingly tough the Nuclear Regulatory Commission's standard, which says the annual extra dose of radiation of the person most heavily exposed should be no more than 25 millirem. People who do not work with radiation are exposed to about 350 millirem a year, counting cosmic rays, radon gas and radiation from medical procedures and naturally radioactive rocks and minerals.

In the regulatory commission's calculation, the individual is assumed to live 24 hours a day at the site. That is unlikely at many reactor sites that will remain industrial — as will probably be the case here — where workers typically spend eight hours a day. Maine Yankee, one of the first big reactors to be shut, has rail service, town water and sewerage, access to the electric grid, and a river full of water for barge traffic or cooling, all of which contribute to its industrial appeal.

The commission's calculation also assumes the individual is a subsistence farmer who drills a well in the most contaminated spot and uses its water for drinking and irrigation. Coastal Maine has no such farmers, and the water under the site is brackish, company officials say, making it unsuitable for drinking or irrigation.

But after protracted debate, the state decided that the commission's standard was too loose; it imposed a standard of 10 millirem a year. That standard is so low that technicians have difficulty determining whether dirt or concrete has enough radioactivity above natural background that it will contribute to extra exposure. So hundreds of tons of material are being shipped out to other states on the presumption of being slightly radioactive, because shipping is cheaper than testing.

Not all environmentalists are convinced that this is sound.

"Parts of this can be depicted by others, outside the state of Maine, to be pretty selfish," said W. Donald Hudson, who is the president of the Chewonski Foundation, an environmental educational institution a mile from the plant.

Moving the material does not make it any less radioactive, although it may end up somewhere with a lower population density and less rainfall, reducing the likelihood that contaminants will be washed into drinking water.

Mr. Hudson said plants decommissioned in the future might not be able to ship out so much material, because states designated to receive the waste might "put their foot down."

Nationally, only three low-level waste dumps are operating, and one, at Hanford, Wash., accepts material only from the Pacific Northwest. The other dumps are in Barnwell, S.C., and at a desert site about 80 miles west of Salt Lake City, which is expected to receive most of Maine Yankee's contaminated concrete. Thus one certainty of decommissioning is a long trip.

About 65,000 tons of radioactive waste from the plant will require shipment off site. More highly radioactive materials will go to Barnwell. About 50,000 tons of material that is not radioactive will go to an ordinary industrial landfill in Niagara County, N.Y. About 75 trainloads of radiaoactive and nonradioactive waste have already been shipped.

If all goes as scheduled, it will take eight years to demolish the plant, which took four years to build. The construction was easier, because at that point all the material was clean, said Wayne A. Norton, president of the company.

Demolishing the plant and shipping the waste will cost $500 million, more than twice the $231 million the plant cost to build (although that was in 1972, when a dollar bought more concrete than it does today.) The job is 61 percent done and on budget, managers say.

Maine Yankee is a single-unit plant, about two-thirds the size of Indian Point 2 or 3 in New York, which suggests the cost of decommissioning a plant the size of Indian Point could well exceed $1 billion,

Another factor in deciding how thoroughly to clean up the site is radiation exposure to workers performing the decommissioning. The more exhaustive the operation, the more that level will rise. Maine Yankee has a "budget" of no more than 1,150 rem of exposure to all of its workers collectively during the entire cleanup, although the actual exposure will probably be somewhat lower. In contrast, 200 rem to 400 rem was typical for a year in which the plant was operating.

But neither Maine Yankee nor any other power reactor can really be fully decommissioned now because there is no place to put spent fuel. So a major policy issue that remains is how to store the fuel, which is now kept mostly in spent fuel pools around the country.

At Maine Yankee, workers are preparing to put the fuel into 60 giant stainless steel canisters, which will be dried out and filled with an inert gas to prevent rust. Each will be loaded into its own giant concrete cylinder, with holes to allow air circulation. Those will go on concrete pads, surrounded by razor wire, motion detectors and armed guards. The casks are licensed for 20 years by the regulatory commission and guaranteed by the builder for 50 years, but their stay at the site could be a lot longer. An application by the manufacture to license the casks for shipping is pending.

The fuel must be loaded into the canisters under water, because in open air, the radiation it gives off would be lethal. But the plan is that after loading, workers will dismantle the pool, so the site will lose the ability to repackage the wastes if something goes wrong with a canister in a few years.

State officials say the casks may be vulnerable to terrorist attack.