More than three years after the nation's worst nuclear accident, scientists late today got their first televised look inside the damaged Three Mile Island reactor and concluded that an almost complete meltdown had taken place in the center of the core.

William Hamilton, head of the technology assessment and advisory group that planned today's inspection, said the two-hour look at the central region of the core showed that "the uranium oxide fuel had melted." The discovery suggests that the temperature inside the reactor may have been higher than previously believed.

"The fuel elements were badly damaged, and the rubble was badly broken up in small particles," Hamilton said. "Some of it appeared to be congealed together. The control rods were damaged also. The center of the core was very badly damaged."

The televised look inside the highly radioactive reactor was the first hard evidence as to how much damage took place when a loss-of-coolant accident was compounded by operator error on March 28, 1979, and left the core uncovered, allowing temperatures to rise dangerously.

Today's discovery of the severe damage to the center of the core appeared to reinforce worst-case estimates, based on "inferential data" such as water samples from the reactor coolant system, that as much as 90 percent of the nearly 37,000 fuel rods in the core had been damaged by the excessive heat.

It generally had been thought before today that temperatures in the upper part of the core exceeded 4,000 degrees Fahrenheit, well above the 3,362 degrees at which the zirconium alloy cladding of the fuel rods melts. It had been thought less likely that temperatures reached 5,080 degrees, the melting point of the uranium oxide fuel contained in the fuel rods.

But the melting of the fuel suggests the level of danger at the time may have been greater than previously believed, since molten fuel could have melted through the reactor pressure vessel and released great quantities of radioactive materials into the containment building.

The report of the Kemeny Commission, which studied the accident, concluded, however, that even if a meltdown had taken place there was a "high probability" that the containment building and the rock on which it was built would have been sufficient to have prevented escape of "a large amount of radioactivity."

Today's survey of the reactor would appear to indicate that a huge pile of zirconimum and uranium melted during the accident and is fused into a large pile of rubble at the bottom of the core.

This suggests that the cleanup task scheduled to last another five years and cost another $1 billion may prove even more difficult than expected.

The televised look at the core today began at 4:35 p.m., when a five-man team standing above the reactor lowered a camera 1 1/4 inches in diameter to a point just above the fuel rods.

About 60 experts watched on monitors as the remote-controlled camera snaked through the underwater labyrinth of the fuel assembly.

The water still covering the fuel elements was quite cloudy, Hamilton said, "but we really did see a great deal inside the vessel."

The owner of the nuclear power plant, General Public Utilities, refused to permit even a pool of reporters to observe the closed-circuit showing of the videotape recorded today, and also refused tonight to make available any information about the results of the inspection.

General Public Utilities has scheduled a media briefing for 10 a.m. Thursday, when it plans to make an edited videotape of the inspection available for public viewing.

Hamilton said today's inspection focused on an area about 10 inches in diameter "right in the center" of the core. He said additional probes would probably be undertaken in the weeks ahead in an effort to determine the extent of damage toward the outside of the core.

Dubbed "Quick Look," today's inspection was the second phase of tests leading to removal of the 40-foot reactor's 160-ton dome next year and removal of 133 tons of damaged nuclear fuel.

While investigators concluded after the accident that health effects on residents of the greater Harrisburg area surrounding the plant were negligible, thousands of people spent a week living in fear of imminent disaster.

The crisis ended about a week after the accident when officials concluded that the threat of a hydrogen explosion had eased and that the long process of shutting down the reactor was under control.