Radioactive gases continued to escape yesterday from the shut-down Three Mile Island nuclear power plant near Harrisburg, Pa., after the accident that federal officials conceded was the worst civilian nuclear mishap the United States has had.

"I don't think we were anywhere near a fuel melt-down but we suffered damage to the reactor core, which is significant," Joseph M. Hendrie, chairman of the Nuclear Regulatory Commission, told the House subcommittee on energy and the environment yesterday. "This is the first time we've had this level of core damage at all in the history of the civilian program."

The precise sequence of events that led to the accident Wednesday morning that released radiation as far as 20 miles from Three Mile Island still is not fully understood, NRC officials said yesterday. Apparently, a number of mechanical failures, each feeding the other, took place to trigger the accident.

"I'll speculate, even though I don't know for sure, that there were multiple failures," Hendrie said. "The result was overheating that caused some core damage to the fuel rods, which in turn released gaseous radioactivity."

Measurable radioactivity ranged from 30 millirems per hour at one of the plant's gates to one-tenth of that three miles away to one- hundredth of that 20 miles away. Inside the concrete dome that houses the reactor, levels were 10 rems on the floor and a starting 20,000 rems on the ceiling.

Hendrie explained that a human would have to be exposed to the radiation outside the plant for as long as 34 years for cancer to be caused but that the dose measured on the dome of the reactor building, if accurate, was a lethal one many times over. There was some question about the reliability of the measurement, since it seemed to conflict with others. Hendrie said the Environmental Protection Agency requires evacuation of a site where outside radiation levels exceed 1 rem per hour, which is 30 times the levels reached on Wednesday at Three Mile Island.

"The radiation we see is not a level I would take causally, however," Hendrie said. "We regulate on the basis that any exposure is to be avoided."

The radioactivity released by the accident is in the form of three gases: xenon, krypton and iodine. All are reactor fission products, meaning they were produced by chain reaction in the fuel and meaning they escaped from the fuel rods broken open in the overheating caused by the accident.

Some of the radioactive iodine found outside the reactor building apparently escaped through vents and filters in the concrete dome of the containment structure. Most of the radioactivity was seeping from an estimated 12,000 gallons of coolant water used to flood the nuclear core after the accident; the water had become contaminated and then was pumped from the core to an outside sump that did not have the shielding the containment provides.

"At this time, the danger is over for people off-site," said an NRC inspector who has been monitoring the plant since the accident. "Our readings show radiation levels have dropped significantly," he said, adding that contamination remains a problem at the plant.

Hendrie was asked by several subcommittee members how he could be sure that overheating had only expanded and broken open the fuel rods and not melted them. He was asked if he had seen the film "The China Syndrome," whose plot is about a possible accident in which a nuclear power plant core melts down so completely that it eats its way through the Earth to China.

"I can't be absolutely sure that parts of the fuel haven't melted but we think we'd see more and different fission products if the fuel had melted," Hendrie said. "As for The China Syndrome, we were nowhere near it, nowhere near it."

Early in the day, Hendrie said he thought some of the iodine gas that escaped from the plant may have found its way into the milk of some dairy cows not too far away but later in the day the NRC denied that any radioactive iodine had been found in cow's milk.

The overheated reactor had cooled down by last night to less than 200 degrees, allowing operators to turn off the emergency core cooling system that flooded the reactor with water. Technicians for the first time were able to begin using a more conventional cooling method to remove heat from the shutdown reactor.

Specifically how long the reactor will stay shut down and when technicians can begin to clean it up was an open question, but it will be weeks before a cleanup can begin and even longer before the broken fuel rods can be removed from the reactor core. Most NRC officials figured the plant will be out of operation for months.

It costs Metropolitan Edison Co. $500,000 each day the plant is shut down to buy electricity from neighboring utilities to make up the shortage caused by the shutdown. Whether or not Metropolitan's customers will pay part of that bill through fuel adjustment charges is unclear.

The federal Price-Anderson Act allows the government to pay up to $560 million in insurance claims in case of a nuclear accident but only for personal injuries. Price-Anderson covers none of the damage suffered by Metropolitan Edison Co. as a result of the accident, including cleanup cost.