Embarrassed Canadian and U.S. officials heading up the search here for remnants of a runaway Soviet satellite carrying a nuclear reactor reversed themselves yesterday for the third time in as many days.

They announced that highly radioactive pieces of the satellite may in fact be spewed along a 300-mile path through far northern Canada.

On Friday, Adm. R. H. Falls, chief of Canada's defense staff, announced in Ottawa that the satellite, which was carrying about 100 pounds of uranimum 235 when it plunged back into the earth's atmosphere Tuesday, had apparently burned itself up before reaching the ground.

Pockets of high radioactivity spotted by searchers along the satellite's path after the reentry were due to equipment malfunctions or natural radioactive sources such as uranium deposits in the area, Falls said.

But in a news conference at the operations center for the satellite search here yesterday morning, officials said they rechecked their instruments and could find no problem.

In a cautious assessment of the situation, William E. Nelson, a chief U.S. scientific adviser to the search, said it appeared radioactive waste from the satellite had come to earth intact.

"As far as I'm concerned what we are seeing on our instruments are from an artificial source," said Nelson, a nuclear engineer from the Lawrence Livermore Laboratory in Berkeley, Calif.

That is precisely what officials had said before Falls' announcement to the contrary on Friday.

Nearly 120 U.S. scientific experts have been flown in to the Canadian military base here and to other search centers at Baker Lake and Yellow-Knife in the Canadian Northwest Territory.

Four large Canadian C-130 transports equipped with detection gear, along with a pair of Chinook helicopters and several smaller planes, have been crisscrossing the area since Tuesday. So far, no one has reported actually seeing any wreckage from the satellite.

Daylight lasts only six hours in the area and there is a fair chance, Nelson said, that if debris from the Soviet craft is scattered across the Canadian North it has already sunk into ice or frozen tundra in the treeless area.

If that is the case, experts here said, the search would be extraordinarily difficult. In effect, a sunken radioactive satellite particle would throw a beam of radioactivity straight up like flashlight beam.

The search planes, which have been flying at 1500 feet, would have to go through what could be at most only a 500 foot wide beam of radioactivity to spot it on their instruments as they try to cover the 15,000-square-mile area.

"We've never looked for a downed satellite before," said Maj. Vic Keating, a Canadian forces spokesman here. "Naturally, we've made mistakes and we're probably going to make more of them."

The planes are concentrating their search on three areas: Great Slave Lake, 700 miles north of Edmonton. Baker Lake, about 600 miles to the northeast of Great Slave Lake, and a site on the vast uninhabited tundra midway between the two. All three areas have recorded indications of unusual radioactive activity on sensing devices installed on the planes.

Nelson said three of the four C-130s are equipped with extremely sensitive detection instruments flown in from U.S. nuclear facilities at Los Alamos, N.M., and the Lawrence Livermore Laboratory U.S. officials have barred reporters from the planes because they say some of the devices are classified. The fourth plane carriers Canadian mineral detection gear which reporters have been allowed to view.

In the planes with U.S. devices, magnetic tapes take five radiation readings per second. The tape is brought back to a room here at the Canadian base where it is fed through a series of electronic devices crammed into a third-floor room. The tape reading is converted to a slow-scan video image and sent back for computer analysis to Los Alamos and Berkeley.

The computers "read" gamma ray emissions recorded on the tapes from background and any other sources and filter out the background "hits." What is left, the scientists hope, are indications of manmade radioactive emissions indicating the presence in certain areas of the satellite parts.

Part of the problem in the contradicting assessments of the situation during the last few days, said Nelson, is that while officials here and in Ottawa were trying to give up-to-the-minute reports on the findings on the tapes, the computers were running behind.

The computers, Nelson said, take four hours to study one hour of magnetic tape and so far scientists here have sent back 250 hours of tape for analysis.

Nelson and other officials here said that while the radioactive parts of the satellite are dangerous at close range there is little likelihood anyone will be harmed by them at present. The weather in the area of the search is some of tha harshest on the continent, with temperatures dropping to 40 degrees below zero and few of the Indians and Eskimos in the region of the satellite path venture into the road less tundra at this time of year.

The satellite has proved to be something of a boon to the Indians of Baker Lake, a fur-trading station with about 1,000 inhabitants. Canadian troops and scientists from the search party have been paying $63 a day each to sleep six in a room in the town's only accommodations, the Indian-owned Igloo Hotel.

"On a scale of one to 10 I'd say the difficulty in this search would be a nine and the danger to the population would be around one," Nelson said. In fact, he said, if the radioactive parts of the satellite are found to have melted into the tundra they may not be removed at all.

"We could just pour concrete over them," Nelson said, "put up a fence and declare them national parts."