In a surprise twist almost as bizarre as the events that plunged the nation into its worst nuclear crisis, officials abruptly announced today that the threat of a potential disaster at the Three Mile Island atomic power plant had greatly eased.

Leading U.S. nuclear officials said the large gas bubble that had formed at the top of the reactor pressure vessel-hindering efforts to coll the fuel and posing the risk of a major explosion and possible nuclear meltdown-had shown a "dramatic drop" in seze.

Officials of the Metropolitan Edison Co., operator of the atomic plant, and Babcock and Wilcox, its builder, went even further, claiming that the bubble had virtually disappeared.

Nuclear Regulatory Commission officials remained more cautious, but NRC estimates of the size of the bubble tonight ranged from 47 to 150 cubic feet-down from 850 cubic feet a few days ago.

"I think there certainly is reason for optimism," declared Dr. Harold Denton, chief of reactor operations for the NRC. Whatever the exact size, Denton agreed that the bubble was "going down," that the situation was much "safer," and that prospects had greatly improved for bringing the reactor to a "cold shutdown without death or serious contamination of the surrounding area. Few officials will feel that the current crisis is really over, however, until the temperatures in the reactor have been brought down another 200 degrees to the point of cold shutdown-a condition not expected to be accomplished before the end of the week.

And with hydrogen gas continuing to build up outside the reactor in the containment structure, and more than 250,000 gallons of radioactive water slopping about on the floor, the prospect remains for new alarms in the days ahead.

A new potential problem emerged today when the Three Mile Island plant suffered its first instrument failure due to the high radiation levels inside the containment structure.

"This particular failure itself doesn't affect our ability to monitor or control the core," Denton said, but he added that the failure was "something you have to be concerned about."

"This indicates we should anticipate there may be other occasional instrument failures with regard to instruments of this type," he said.

But Denton said he had conferred last night and this morning with Pennsylvania Gov. Richard Thornburgh, who has been supervising plans for a possible general evacuation of about 630,000 residents from a 20-mile area surrounding the plant. "I don't believe it's warranted," Denton declared.

Nevertheless, Thornburgh said today that his recommendation that pregnant women, pre-school children and mothers of pre-school children stay outside the area within a five-mile radius of the plant remains until further notice. He also said that a liberal leave policy for state employes implemented today will stay in effect for the time being.

Denton said the general radiation levels monitored by instruments placed at 37 locations around the nuclear plant were continuing to drop, and that the amount of radiation being vented from the power station had dropped dramatically.

The highest radiation reading during the past 24-hour period at a monitoring station about five miles northeast of the plant was 1.1 millirems per hour-about 1/20 of the exposure a person would get from a medical Xiray. Denton said the instruments atmost of the locations, however, "read extremely low in the range of .05 millirems for that 24-hour period"-not much above the normal background level.

U.S. officials stressed that while the situation at the Three Mile Island plant seems to have been brought under control, extreme care will need to be taken to keep the highly toxic radioactivity from escaping from the four-foot-thick reinforced concrete containment structure around the reactor.

Officials said an instrument this morning had measured the radiation level near the dome of the containment structure at "about 30m000 rems per hour." Medical sources generally agree that a human exposed to 400 rems would die in a short time.

NRC officials seemed confident, however, that the radioaction gases and liquids can be safely confined to the containment structure until processes can be devised for getting rid of them safely.

"The containment is performing as it was intended to function," Denton said.

In order to be sure that no further accidents take place that might allow leakage, Denton indicated that NRC officials felt there should be no rush no rush now to try to get the reactor totally shut down, but rather that the process should move slowly.

"If we are satisfied that the hydrogen explosion potential has diminished," he said, "there is no timeframe pressure to cold shutdown.

"If there is no bubble in the vessel, the temperature of the core can be reduced throughj the present cooling mode. I would empect to continue to cool the core in this method. You can go indefinitely in this mode providing that the equipment that is operating in this mode holds up," Denton said.

Officials said the fuel temperatures in the core continued to drop today, and that now only two fuel bundles remain over 400 degrees. Sunday there were four. Denton said he would like to see the temperature be brought down eventually to a "cold shutdown temperature-normally thought of in the 170 to 180 degree range." NRC sources said they expected this to be accomplished by the end of the week.

Officials also said that on eof two hydrogen recombiners, which remove hydrogen gas from the containment structure and then pump it back into the building as waste water, started operation late this afernoon with the other on standby.

They said use of the recombiners had been delayed while officials checked seals to be absolutely sure that no radioactivity would leak during this process.

Officials also conceded today that they may have taken a somewhat too alarmist view of the imminence of the threat of a gas explosion when they warned that the bubble problem had to be solved within five days.

This warning, they said, had been predicated on the belief that oxygen gas was building up in dangerous proximity to the hydrogen gas in the bubble at a rate of 1 percent a day, rapidly creating a highly explosive mix.

The actual rate at which oxygen was increasing was in fact "much lower than that," Denton said today. He said he no longer considered the five-day deadline to be "a critical determination point,"

"It's much further out in time," he said, and "if we can decide on the bubble issue, the time-frame is mooted."

All the way around, a new mood seemed to prevail tonight in the trailer camp command post set up across the Susquehanna River from the fog-shrouded cooling towers of the Three Mile Island plant. Remarked John Hilbish, a Metropolitan Edison engineer, "It looks now like we're well on track."

President Carter has ordered a federal inquiry into all aspects of the nuclear accident at Three Mile Island, White House press secretary Jody Powell said.

Powell declined to discuss details of the probe, saying it was "not appropriate to be making public judgements on this matter" while efforts are still under way to control the plant.But he said Carter has ordered a federal task force to start looking into the situation.

Meanwhile, a White House-sponsored task force is sending Pennsylvania 1 million doses of a chemical intended to reduce the effects of radiation on human thyroids, a White House officeal said.

The chemical, potassium iodide, would be used if persons living and working near the crippled plant receive significant doses of radiation. CAPTION: Illustration, Three Mile Island Nuclear Power Plant, By Robert Barkin-The Washington Post; Picture 1, A concerned Mary Loranzo, a Middletown resident, listens at left, By John McDonnell-The Washington Post; Picture 2, as Harold Denton, chief of reactor operations for the NRC, briefs the press with the assistance of Roger Mattson, far right. By Frank Johnston-The Washington Post