Nuclear experts today likened the cleanup operation facing the stricken Three Mile Island power plant to scrubbing a highly radioactive ring out of a giant bathtub and said it would cost at least $40 million.

"First you drain out the dirty water, and then you scrub it down," said Robert Bernero, the Nuclear Regulatory Commission official who will head the federal end of the cleanup if that is the route chosen by Metropolitan Edison Co., operator of the plant.

The experts said cleaning up the plant would cost as much or more as dismantling it. They said however, that no decision on what will be done is likely to be made until officials here have been able to survey the conditions more closely.

Federal officials have raised the possibility that the plant may never reopen.

Both Bernero and David Jones, vice president of Chem Cnuclear Systems, a Bellevue, Wash., firm that specializes in nuclear waste disposal, said they believed that ultimately the plant would be cleaned up and run again.

"I'd be very surprised if it wasn't reopened," said Jones, whose company already is involved in the initial stages of a nuclear cleanup. "Given the energy situation in this country, I think a cleanup will be financially worthwhile."

Bernero, meeting with reporters here today, outlined three major steps that must be taken to clean up the plant and estimated it will take at least a year or two before the nuclear generator can become operational.

The major jobs involve removing some 80,000 gallons of radioactively contaminated water from the reactor's primary cooling system, getting the gas and contaminated material out of the two-million-cubic-foot reactor containment building and draining at least 200,000 gallons of radioactive water from the floor of the containment structure.

A major problem that officials face immediately is the tremendous amount of radioactivity inside the containment facility. Harold Denton, chief of the NRC Office of Nuclear Reactor Regulations, noted at a news conference here today that while the containment is not leaking radiation into the outside atmosphere, a measuring device inside is still registering contamination levels of around 30,000 rems in the dome of the building. Exposure to such radioactivity would kill a human instantly.

Denton and other nuclear experts said it will be at least a month-and probably longer-before the radioactive xenon, krypton and iodine inside the containment cool down to allow anyone to enter even in protective clothing.

The cleanup problem will be further complicated by the radioactivity of the material. There are only three places in the United States where radioactive material can be buried, Jones said. They are Barnwell, S.C., Beatty, Nev., and Richland, Wash.

The Washington state site is the only one where fission fuel components are allowed to be buried and much of the containment contamination is believed to be such material.

In addition the size of the job may also prove troublesome. Normally a nuclear fuel plant generates about 35,000 cubic feet of disposable material each year. But some experts believe that up to 10 times that amount may have to be removed from the Three Mile Island plant.

Bernero said the major cleanup problems would probably be handled this way:

Coolant fluid. Ion exchangers can be hooked to the reactor's primary cooling system and the fluid run through them. In the exchangers material resembling plastic beads attach themselves to the radioactive material, drawing it out of the water.

About 300 cubic feet of water can be cleaned in two hours. The water can also be treated with polymetric chemicals which harden to the consistency of chalk so it can be removed as a solid.

Containment contaminants. This is the "radioactive bathtub ring" which can be cleaned by water sprayed from nozzles in the dome of the containment. The water will wash down radioactive material plated to the walls.

The gases in the containment will eventually decontaminate themselves, Bernero said. It takes about 25 days for xenon, the most prevalent gas, to become decontaminated. Longer lasting gases can be chemically absorbed or cooled to a liquefied state and separated, experts said.

Water on the floor. The containment has built-in pumps in its sump to move this water-which may also contain radioactive debris from the fuel-to an auxiliary system outside the containment. There it can be deionized and recycled back to the containment in a decontaminated state for use in spraying down the walls.

Once the contaminants are removed and the reactor is cooled, the top of the reactor vessel can be unbolted and taken off, Bernero said. The undamaged fuel rods can be lifted out the way they are normally changed, he said. The undamaged rods will probably have to be sheathed in protective metal "cans" before removal.