In what federal regulators called one of the nation's most serious nuclear accidents, a cooling system failure early yesterday at Pennsylvania's Three Mile Island power plant spread radiation as far as Harrisburg, 10 miles northwest of the plant.

The Nuclear Regulatory Commission described the radiation over Harrisburg as "quite low" and not dangerous to humans, but it was detected by instruments carried above Harrisburg by helicopter last night, more than 12 hours after the accident that triggered the radioactive release.

At midnight, the NRC said it believed the radiation found over Harrisburg had come from an auxiliary water shed where radioactive waste water had been pumped from the plant after the mishap. The NCR's Harold Denton said the debris sampled over Harrisburg showed radioactive kryto, xenon and tritium, all gases that are readily dissolved in waste water.

"We don't think," Denton said, "there were any detectable fission products from damaged fuel elements that had blown that far from the plant."

Whatever the source of the radiation, Denton said it was measured at one-third of a milliroentgen per hour over Harrisburg, 3,000 times less than the level at which the Environmental Protection Agency recommends protective action. The levels inside the reactor building were quite different, described as a lethal dose of "thousands of roentgens" inside the reactor containment along the ceiling.

"We have a serious contamination problem on site," said Charles Gallina, an NRC investigator who spent all day at the plant site. "Nothing critical failed, but it's a dirty problem. it's going to take some time to clean up."

At midnight, the reactor was still in an overheated state that had federal and state officials still at their desks and telephones. It was not clear, however, whether the uranium fuel rods that broke up earlier in the day were continuing through the night to release radiation through the 4-foot-thick walls surrounding the reactor.

"We are continuing to monitor the area with helicopters," said Charles Blaisdell of Pennysylvania's Emergency Management Agency, which did not tell people to stay indoors and had no plans to evacuate any region in or around Harrisburg. "Our word is that people have nothing to worry about. The radiation level is what people would get if they played golf in the sunshine."

Officials of Metropolitan Edison Co., part owner of the plant, said some radioactive steam is also escaped during efforts to cool the overheated fuel rods with water. They conceded that as many as eight employes had been exposed to radiation but there was no evidence of injury.

The radioactive release apparently occurred when an operator mistakenly turned off the pumps driving the plant's emergency water cooling system, allowing the uranium fuel to overheat and rupturing an unkown number of the 36,000 fuel rods that generate heat for the plant.

By last night, the uranium fuel was still so hot that the plant's managers had to vent radioactive steam into the atmosphere to prevent an explosion inside the thick concrete dome protecting the nuclear reactor.

"The situation is still not stabilized," Edson Case, deputy director of the Nuclear Regulatory Commission, said last night. "The reactor core is not cooling down as fast as we'd like."

Case said that more than 60,000 gallons of water had been flooded into the room housing the reactor, which was still as hot as 550 degrees 12 hours after being shut down. Some of the water was flashing into steam, The reason the radioactive steam had to be periodically vented to the atmosphere.

Radioactive iodine was found on the ground around the plant, suggesting it had escaped as a gas from the broken fuel bundles in the reactor. Gamma and beta radiation wad detected as far as a mile from the plant in the nearby townships of Middletown and Londonderry.

the NRC's Case described this as "direct radiation" escaping from the broken fuel bundles through the 4-foot-thick concrete walls surrounding the reactor building. This radiation was measured at three illiroentgens per hour, a measure of radiation less than the exposure a patient receives from an x-ray.

The sequence of events that led to the accident was unclear but what happened was an event that the nuclear power industry long has said was unlikely or even impossible.

A pump driving cooling water into the reactor broke down at 4 a.m. yesterday, causing an automatic shutdown of the plant and turning the job of keeping the reactor from overheating over to an emergency cooling system.

At some point in the transition from prime cooling system to the emergency cooling system, Case said last night, an operator for the Metropolitan Edison Co. which runs the plant turned off the emergency core cooling system, then turned it back on again.

"We don't know how long a time elapsed while both cooling systems were off," Case said, "but that's when we believe the fission products were released from the damaged fuel rods."

Case said the NRC does not think the emergency coolant was out long enough to cause a partial meltdown of the fuel rods. He said that sensors placed inside the reactor indicated the fuel rods were ruptured and not melted.

"We're not absolutely certain that some of the fuels rods didn't melt but we should know that bytomorrow," Case said last night. "There are ways to analyze the radioactive decay products escaping from the reactor to tell the condition of the fuel."

The NCR's main concern last night was the slow loss of heat from the shutdown reactor, whose chain reaction was stopped automatically at 4 a.m. yesterday at the first sign of trouble. The part of the reactor first exposed to the cooling water was still as hot as 250 degrees; the parts close to the water outlet were as hot as 550 degrees.

"It's only losing heat at a rate of 3 degrees an hour," Case said. "It concerns us that the temperature is not coming down faster than that".

Case said the reactor temperature must fall to 400 degrees before the plant's conventional cooling system can be turned on to remove heat at a more rapid rate. He said the emergency cooling system still in operation last night appeared to be in a struggle to cool down the reactor.

"There might be a steam bubble in the core that's preventing water from reaching that part of it," Case said. "There is some concern that what we're doing now to cool it down it not enough."

While the chain of events that led to the release of radiation began at 4 a.m., officials of Metropolitan Edison Co. did not notify state and county aides of the accident until 7:30 a.m., when they declared an "emergency" at the plant site. Apparently, it took utility officials more than three hours to discover that radioactive iodine gas had escaped through safety vents in the containment building to the outside.