No word properly describes the stomach knots and the concern, as local folks put it, that go with today's macabre preoccupation: living a nuclear countdown and not knowing where to run.

Atomic power generation has come to Pennyslvania in a big-league way, and now one of the state's five commercial nuclear reactors, just down the river at Three Mile Island, has the whole world watching.

The possibility of catastrophe is clear. A reactor core meltdown at the betroubled plant could lead to the release of lethal radiation over the surrounding countryside. Or, the place could blow up.

That is only part of the problem. Radiation is invisible and tasteless. One may evacuate, as many have done, to nearby Hershey or Harrisburg or Carlisle, or farther to Baltimore or Washington, and still not know if that is far enough.

A map on the wall of the Dauphin County Civil Defense headquarters is illustrative. It shows concentric circles rippling out from Three Mile Island, much as a Holiday Inn map informs a traveler of distant lodging.

The Pennsylvania state map of nuclear reactors is a series of overlapping concentric circles, which suggest there is somewhere to run from windborne radiation fallout, but it is not close by.

The people of Middletown and other communities in the three countries around the Metropolitan Edison Co. plant have begun to grasp the full meaning of those concentric circles.

The clerk at the newstand here, where out-of-town papers are a hot commodity, explained: "My brother called from Baltimore and told me to drive on down while this is going on. I told him, 'No, I wouldn't be any safer there.'"

The three Mile Island plant was dedicated last year by Deputy Energy Secretary John F. O'Leary, one of the administration's most vowed nuclear power advocates, who called it a "success."

Some breathed easier after Gov. Richard Thornburgh said Friday night that an evacuation of the area would not take place since conditions at the plant had not worsened.

Today the countdown-watching, waiting and worrying - continued here. Pregnant women and preschool children who left Friday stayed away. Others had bags packed, ready to go if Thornburgh gave the order.

About 440 residents of two nursing homes left Middletown by bus and ambulance today as a precautionary step. But in the case of those two institutions, Frey Village and the Odd Fellows Home, it was because not enough employes showed up for the first shift.

While the language of nuclear technology and conflicting official statements have confused them, some in this borough of 11,000 react to their grim wait with a fatalistic sort of gallows humor.

At Karl Kupp's diner, where scrumptious homemade pies are served, the patrons joke about hair falling into their soup, about radiation burgers, about sterility and glowing in the dark after drinking Kupp's coffee.

"I'm tired of all this," Kupp's son said. "I'm going home and put on the stereo good and loud."

Monica Drayer of nearby Royalton could put the arrival of the nuclear age in a special kind of perspective. As a schoolgirl in England during World War II, she would hide under a large round dining room table when the Germans bombed Liverpool.

That was security enough. The dining room table in Royalton is no help today. Her three small grandchildren left with their parents yesterday, to spend the duration in the mountains 70 miles north of here.

"I am concerned, but not afraid," Drayer said. "I feel our governor would not allow us to stay if the danger worsens. If there are any deaths from this, I feel it would be the end of the nuclear age in this country. I don't feel in my heart they could let this happen."

Her bags are ready and she will leave with neighbors if Thornburgh gives the word.

Glenn Seiders is one of the men-and he is not alone-who provides diversity of thought in Middletown. He works at the Three Mile Island plant, in radioactive waste disposal, and he thinks his company is taking a bad rap.

"It's being blown out of proportion. A lot of people don't know this plant, and they don't know what they're saying," Seiders said. "I respect it, but I don't fear it."

As he talked, he walked along with a Geiger radiation counter in front of him, checking the atmosphere-negative in downtown Middletown-for excessive radiation.

"In seven years I've had two overdoses of radiation, but they took care of me. I checked out fine at the hospital. The way I see it, when my time comes, I'll go-car accident, gas stove, nuclear plant."

As the watch and wait proceed, there are eerie little flashes of nuclear cinema. Helicopters and small planes buzz up and down the Susquehanna River valley, monitoring air for radiation levels. Men with the bright yellow Geiger counters walk the streets. Ambulances line up-16 in a row-to carry the elderly away from Frey Village. Television crews stumble over each other, residents keep count of the number of interviews they've had. A wire service provides its reporters with breathing masks and air tanks. Just in case.

Chiseled in the stone beside the steps to the ornate Capitol in Harrisburg, 15 miles north of here, is another thought. It says: "In God We Trust."