It will be months before the decision must be made, but in the end there is unlikely to be much left of reactor No.2 at Three Mile Island and its giant containment except one of the world's most costly piles of scrap.

The nuclear core is destroyed, a certain $50 million to $100 million loss, according to the experts. As for the remainder of the sophisticated machinery, Metropolitan Edison will have to decide whether it is worth destroying TMI-2 to save it.

Nuclear Regulatory Commission specialists predict that a cleanup of the magnitude facing reactor No.2 will require stripping away everything, right down to the cement walls of the reactor containment itself. The contaminated innards of the containment building must be burried at one of three sites around the nation where nuclear waste disposal is allowed.

An NRC study completed last year estimated it would cost $42.1 million to dismantle and destroy even a non-contaminated teactor after its useful life is over. Cleaning up a highly radioactive plant like No.2 is a far more delicate and costly proposition.

Another possinility is "mothballing" the plant-pulling out still-usable parts like the tubine generator and leaving the rest for future generations to puzzle over or perhaps learn from. Ten major nuclear reactors are mothballed in the United States today.

And finally there is "entombment," a chilling but apt description for sealing off the containment and its radio-active contents beneath a huge concrete shell. Already there are three such nuclear-age sarcophagi-in Hallam, Neb.; Piqua, Ohio and Puerto Rico.

"It's a little like King Tut's tomb," explained a federal nuclear cleaup specialist. "You pour concrete wver the whole thing, walk away and leave it sealed up for a thousand years."