Pennsylvania Gov. Richard Thornburgh decided last night not to order evacuation of the estimated 165,000 persons in a 10-mile radius of Three Mile Island nuclear power plant near Harrisburg, Pa., even though there were dangers of a gas explosion or a meltdown of the atomic core.

"No evacuation order is necessary at this time," Thornburgh told a press conference at 10 p.m. in Harrisburg. "But my advice to pregnant women and preschool children in a 5-mile radius stays." His advice earlier in the day was that they leave the area around the plant.

Thornburg explained that he was not calling for an evacuation because he felt the reactor was stabilized and presented no immediate danger to the surrounding area."We anticipate re-evaluating this continuously. We'll make the determinationa of what to do each day," he said.

There is no danger of a nuclear explosion occuring like the detonation of an atomic bomb. Even if the entire fuel assembly were to suffer a meltdown, the uranium in the fuel rods is not the type that is used to create a nuclear explosion.

The State Emergency Management Agency said "a significant number" of people had evacuated the area in which the Three Mile Island nuclear plant is located. Tmeporary shelters received 187 pregnanat woment and preschool children. Fifteen mass-care centers had been set up in the four counties of York, Cumberland, Lancaster and Dauphin. Many of those who left have moved in with relatives or taken early vacations.

In a press conference late llast night, officals of the Nuclear Regulatory Commission told reporters that temperatures inside the reactor had not cooled down the way they had hoped. The reactor has been overheating since an accident Wednesday morning, and there been a number of releases of radioactivity.

A giant bubble of hydrogen gas was believed to be blocking cooling water from reaching some of the 36,000 uranium fuel rods in the reactor. Temperatures of some of these rods were reported to be as high as 700 degrees, raising the possibility that they could overheat and bring on a meltdown of at least part of the core.

Potentially even more alarming than a meltdown was the possiblity that hydrogen gas might leak out of the reactor and explode, breaking the reactor and carrying tons of radioactive debris with it inot the 4-foot-thick containment building around the reactor. There is a chance that the gas explosion could be strong enought to break the containment building, releasing the radioactivity.

"With a meltdown, you get a warning of four to five hours that it has begun," said one source at the Nuclear Regulatory Commission who asked not to be identified. "There's no warning at all of an explosion of hydrogen gas."

Pennsylvania state officials had put together an evacuation plan to move the residents of the small towns in a 10-mile radius of the plant within one hour and the residents of Harrisbug, the state capital, in two hours. Presumably, they would be moved upwind of the plant in case an explosion or a meltdown breached the containment vessel and spread radioactivity into the air.

Earlier in the day, Thornburgh had advised the evacuatioon of pregnant woment and preschool children living within 5 miles of the plant. He also ordered the 23 schools in the 5-miles radius closed and told residents within a 10-mile radius to stay indoors.

The governor's actions came after the company unexpectedly released large amounts of radiation into the atmosphere - without telling state or federal officials they planned to do so.

Three Mile Island power plant, operated by the Metropolitan Edison Co., has been a growing problem since Wednesday morning when its reactor started overheating and radiation was emitted and detected at last 20 miles way.

At The White House yesterday, President Carter made no direst comment on the developments at Three Mile Island. But in an interview with a group of editors, the president said the incident "will make all of us reasses our present safety regulations...and will probably lead inexorably toward even more strigent safety design mechanisms and standards."

Implicit in the comments, White House press secretary Jody Powell said later, was a suggestion that Carter will order a review of Nuclear safety standards after the emergency in Pennsylvania is over.

Meanwhile, the senior NRC staff official on hand at Harrisburg, said, "I Denton said that "the core is extremely damaged, the coolant is very hot and the bubble is there to deal with."

Denton said last night it would be several days before the bubble would be dealt with, declaring officials wanted to choose the best of at least three options to collapse the bubble without releasing more radioactivity to the atmosphere.

"We can cool the reactor in this mode we're in now for quite awhile," Denton said. "We've assured ourselves there is no immiment danger to the public."

Denton said the NRC was disturbed that radioactive xenon and krypton were still being "outgassed" from waste water being pumped from the reactor at 10 gallons a minute. Water is pumped into the reactor to keep it as cool as possible, then pumped to sump tanks outside where the radioactive gases picked up inside the reactor bubble out and into the aire.

"I hope there's a change in this released radioactivity in the next 24 hours," Denton said. "Our goal is to keep all radiation down as low as possible."

The bubble was created when coolant water came in direct contact with damaged and overheated fuel rods, decompsoing the water into hydrogen and oxygen. It was not known how big the bubble is but it is of sufficient size to prevent cooling water form reaching at least some of the rods.

Officials had said Thursday that the fuel cooled to less than 200 degrees, but last night temperatures of some of the rods were as high as 600 degrees. There was fear that they would pass on their excess heat to undamaged fuel rods and break them, releasing more radioactivity and making it harder for the water to keep the reactor from overheating.

As much as 50 percent of the fuel rods are damaged. Denton said last night there might even be melted pellets inside some of the rods.

So hot was the core last night that it was superheating the water flooded into the reactor to cool it down. The inlet temperature of the water under pressure was 280 degrees, 100 degrees higher than technicians wanted it, and was under such pressure that it could not turn to steam.

The fear was the water temperature would climb even higher, which might tend to expand the gas bubble. If that happened, the bubble could grow to a size that it could completely block the coolant water from entering the reactor and allow the reactor to built up more heat.

"As long as the reactor just sits there without changing, there's no chance of a meltdown," one NRC source said. "But on the other hand, you don't want to leave the reactor in the state it's in now because there's so much radioactivity in there."

Last night NRC engineers met with engineers from Metropolian Edison and the Babcock & Wilcox Co., the operators and builders, respectively, of the Three Mile Island plant. They discussed ways to break the bubble or draw it out of the reactor in a safe enough way that the gas would not explode.

There is a device in the containment room called the "recombiner," put there in case of an emergency. It provides a controlled burning of the hydrogen gas-not as a solution to the problem but to prevent an explosion which is far worse than a fire.

One plan under consideration last night was to raise the pressure inside the reactor in an attempt to dissolve the bubble into the cooling water flooding the reactor room. If the bubble dissolved, it could be pumped out with the coolant into the waster water tanks outside the reactor.

Another possibility was to lower the pressure inside the reactor, which could collapse the bubble. A third was to "sink" the bubble by dropping the water level, then pouring in fresh water from pipes at the top of the reactor. Still a fourth option was to restart the reactor, which is shut down, to create so much heat that water would flash into steam to saturate the bubble and break it up.

The trouble with all four options is that each carries some risk of making even more mischief. If the reactor is restarted, there is no guarantee that after pulling the control rods out of the core to begin the chain reaction the control rods can be put back in to stop the chain reaction.

"There's a chance the rods are damaged and won't fit back in," one NRC source said. "There's also the chance the rods might scrape on something and start a spark that could ignite that hydrogen."

So much radioactivity is believed loose in the reactor that the NRC believes as many as 10,000 of the 36,000 fuel rods have been damaged and broken open. There are speculation that some rods might have melted, a more pressing danger than if they had just been damaged.

"We don't know if any of the fuel had melted," one NRC source said. "There is a chance that a small protion of the rods suffered a meltdown." The significance of that would be that far more dangerous radiation would be loose in the reactor than was previously thought.

If none of the procedures work to get rid of the bubble and cool the fuel, there is an outside possibility that the overheated rods could damage the overheated rods and raise the temperature of the core high enough to bring on at least a partial meltdown of the entire core.

If even a partial meltdown took place, that part of the core where it occurred would be so hot it would melt through the floor of the building and not come to rest until it was so far underground that tearth has acted as a remover of heat. This is what is referred to in a current film as "The China Syndrome."

Even worse, the melted fual could trigger steam explosions of whatever water and hydrogen was still in the reactor room. These explosions could carry radioactive drbris outside the reactor and even through the containment vessel, if the explosions were numerous and strong enough.

"It's unlikely we'll get to the point of a meltdown," said Victor Stello, director of NRC's division of operating reactors. "But I can't say for certian. Anytime you change the status of the core you change the risk."

Before discovering that they faced the poential of a meltdown, Metropolitan Edison technicians and engineers aggered Thornburg and most members of the state's congressional delegation when they dumped some radioactive waste water into the Susquehanna River Thursday night and vented radioactive gas into the air Friday morning without notifying officials.

The water dumped into the river was described as "slightly radioactive" and was dumped to make room in storage tanks for the more radioactive coolant water being pumped out of the reactor room. By late yesterday, there had been no reading of the river water to see how much it had been contaminated.

The gases vented into the atmosphere apparently were blown there by mistake, when technicians were cycling radioactive water from one tank to another. Radioactive xenon and krypton gases "bubble out" of the water and escaped into the air.

Radioactivity 600 feet above the plant was measured at 8 yesterday morning at 1-200 millirems per hour, which violates NRC rules and is almost one-fourth the annual exporsure allowed to nuclear workers. At least one worker at Three Mile Island was contaminated by the venting, bringing to nine the number of workers exposed to radiation at the plant since Wednesday.