U.S. officials today said the risk of a dangerous gas explosion within the reactor of the Three Mile Island nuclear power plant had been eliminated, and they expressed hope that the lives of the 630,000 persons who live in the area can rapidly return to normal.

"I think the danger point is considerably down from where it was a few days ago," said Dr. Harold Denton of the Nuclear Regulatory Commission.

At the same time, however, U.S. officials revealed that "very small amounts" of radioactive iodine have been detected for the first time in samples of milk, taken from farms in the area around the crippled nuclear plant.

This news, coupled with the fact that small releases of radioactive gases are continuing to take place, persuaded Pennsylvania Gov. Richard Thornburgh that his 5-day-old advisory that pregnant woman and young children leave the area around the atomic plant should remain in effect.

"I have placed, and I will continue to place, the public safety as my paramount consideration in this period," Thornburgh told a news conference. He also recommended that the schools in the immediate area around the plant remain closed.

State and federal officials projected a cumulative radiation dose level to local residents only slightly higher than normal background radiation from the sun and other regular sources in the Harrisburg area.

The projected dosage from the plant is less than 100 millirems. The level from other sources is 75 millirems. An additional 70 millirems could be expected in average dosage from medical X-rays in the course of a year.

Thornburgh announced yesterday that extremely low levels of iodine 131, a radioactive byproduct of fission within the reactor, had been found in milk sampled Saturday and Sunday from 22 dairies as far as 18 miles from the accident site.

Those levels were from 11 to 46 picocuries per liter, far below the 12,000 picocurries per liter considered dangerous.

The governor said that monitoring of milk, water and other products would continue and hat he was "concerned about sensational reports" playing up dangers that did not exist.

The Washington Post, meanwhile, has learned that the trouble that launched the nation's worst nuclear crisis last Wednesday apparently began at about 3 a.m., about an hour earlier than the time claimed by the plant's operator, the Metropolitan Edison Co.

Glenn A. Seiders, a 'Metropolitan Edison nuclear waste disposal worker employed at the atomic plant, said he was summoned to Three Mile Island shortly after 3 a.m., when alarms in the control room indicated trouble.

Seiders said that plant officials realized by 4 a.m., that a serious situation had developed. By then, he said, other off-duty workers had been awakened and called to the plant.

The disclosure that the trouble apparently began at least an hour earlier than previously reported seemed certain to add to the controversy over the fact that Metropolitan Edison officials did not notify any state, local or federal officials of the situation until after 7 a.m.

According to the official logs of the state and Dauphin County emergency preparedness offices, a plant foreman made the first calls at 7:02 a.m. A second call at 7:20 from another plant foreman indicated that the situation was more serious than first reported.

Denton was asked today if he had any indications that the trouble at the plant started earlier. He said a detailed account of the accident would be presented at a public briefing to the Nuclear Regulatory Commission in Washington "in a few day."

U.S. officials today emphasized that there was no cause for public concern over the levels of radioactive iodine found in the milk samples.

Denton said the highest iodine level discovered by the Food and Drug Administration was about 31 picocuries per liter, and that the level in most of the samples tested was 10 to 20 picocuries.

"I don't consider radiation levels of 10 to 20 picocuries per liter any cause for alarm with regard to milk," he said. "The FDA action level is 12,000 picocuries per liter, and during the Chinese fallout period levels of 100 picocuries and in, some instances up to 300 picocuries were reached."

U.S. officials said that some small amounts of iodine were continuing to escape into the atmosphere from the Three Mile Island plant, but that their best estimate of the total amount released from this accident to date is 1 curie.

Denton also said that the NRC and the Pennsylvania state government had instructed Metropolitan Edison to cease discharging industrial waste water from the plant into the Susquehanna River after traces of iodine and cobalt were discovered.

While the discharge were halted until the NRC could make further analyses, Denton said he thought the plant would be allowed to resume discharging the water.

The report of the discovery of iodine leakage tended to put a bit of a damper on what otherwise was a second day of generally encouraging news for residents of this area.

U.S. officials, who yesterday seemed almost unwilling to believe indications that the gas bubble that had built up in the nuclear reactor had virtually disappeared, today were cheerfully agreeing that the bubble had "for all practical purposes" been eliminated.

"There's no bubble any longer in the top of the core," Denton said. He added that officials were still uncertain which of several possibilities was primarily responsible for the bubble's disappearance.

"I suspect it was a little bit because of our actions and maybe a bit of serendipity," he said with a smile. "A little bit of luck and little bit of fore-thought."

With the bubble, and the risk of a possible explosion that it posed, now gone, officials said the primary problem was to figure out how best to bring the nuclear reactor to a "cold shutdown" without any major new release of radioactivity.

As of noon today, the temperature in the core was reportedly 281 degrees, with the pressure in the reactor being maintained at 1,100 pounds per square inch.

The major objective now are to reduce the pressure in the reactor and bring the temperature down below the boiling point.

"As long as the system is pressurized there is a potential that some loss of cooling mechanism might still cause the core to become uncovered," Denton said. That could permit heat to build up and possibly result in a meltdown.

Plant officials also will not be able to use the residual heat removal (RHR) system-the big pumps probably needed to bring the temperature to cold shutdown-until the pressure has been reduced to no more than 400 pounds per square inch.

Officials are planning to continue using their present method of cooling the reactor for the time being because putting the RHR system, located in the auxiliary building, into operation poses major risks and problems.

"Since the RHR brings contaminated water from the containment structure out (into the auxiliary building) and cools it and returns it, it's very critical that we not turn it on until we have all the leaks and potential leaks of that system isolated," Denton said.

He said officials will want to put filters on compartments that might leak, and try to install special sheilding. The problem in doing this, however, is that the auxiliary building was contaminated by radioactivity early in the accident, when waste tanks were overfilled.

A Department of Energy team made a radiation survey of the auxiliary building Monday and Denton said they found it "quite hot."

Denton also reported that the radioactivity level in the containment structure, contaminated with xenon, radium, iodine, barium and cesium, remains extremely high. He said he thought it would be "a month or longer" before workers would be able to enter the contaminated structure.

Denton also reported that the hydrogen concentration in the containment structure was 2.1 percent, but said there was no significant chance of a hydrogen explosion.

"There probably are still some small bubbles in the containment, but they're not the type that are up at the top of the dome, and they post no further significant safety problems," he said. "The recombiner is working in removing hydrogen from the containment." CAPTION: Picture 1, A helicopter, taking radiation samples, flies over the crippled reactor at the Three Mile Island nuclear plant. AP; Picture 2, Dr. Harold Denton of the Nuclear Regulatory Commission tells reporters about conditions at Three Mile Island. AP