A mysterious power failure yesterday triggered an emergency shutdown of Florida's Crystal River nuclear plant and flooded the concrete containment building with as much as 60,000 gallons of radioactive water.

On the strength of preliminary reports, the accident seemed much less serious than the one at Three Mile Island 11 months ago. There was no measured escape of radiation outside the plant and, according to Florida Power Corp. and the Nuclear Regulatory Commission, no danger to the public.

"There was no damage to the reactor, no damage to the fuel rods and no radiation released anywhere outside," Florida Power spokesman Bob South said last night at a briefing outside the plant north of Tampa. "There was nothing disturbed in the plant, no core meltdown or anything like that."

But the size of the radioactive spill inside the containment meant the 825,000-kilowatt Crystal River plant would stay shut down for cleaning at least a few weeks. Florida residents will likely pay more for imported electricity in this period, and may even suffer some shortages.

Also, the Crystal River plant was built by Babcock & Wilcox, builder of the Three Mile Island plant shut down by an accident in Middletown, Pa., last March 28. That accident not only closed the two plants at Three Mile Island idenfinitely, but led regulators to shut down seven other Babcock & Wilcox plants for months while the Three Mile Island investigation went on.

"It's too early to determine if there is any generic reason for shutting down the plants again," the NRC spokesman said. "However, the commission will certainly look into it to see if there are generic implications."

The accident began at 2:23 p.m. EST when power was lost to instruments and controls, automatically shutting the plant down. "The rods that slow down the plant's nuclear chain reaction immediately fell into place, and emergency cooling pumps began flooding the reactor with extra water.

The Nuclear Regulatory Commission told Flordia state officals early last night that the situation at Crystal River was "in hand." But the cause of the accident was still unknown, and radioactive water was still escaping from the stainless steel reactor housing into the dome-like concrete containment surrounding the reactor.

Instruments inside the containment measured radioactivity at the roof of the dome at 50 roentgens at just after 3 p.m., causing the Florida Power Corp. to declare an emergency. At the same time, the NRC opened up its Incident Response Center in Bethesda, and dispatched five inspectors by chartered jet to the plant.

NRC Chairman John Ahearne and NRC Commissioner Joseph Hendrie moved from their Washington headquarters to the IRC in Bethesda. There Hendrie took charge of the emergency response team of engineers and technicians who worked on the Three Mile Island accident near Middletown, Pa., last March and April.

The reading of 50 roentgens on the roof of the dome is about 200 times normal. And it is higher than engineers expect to get from radioactive water that has not been at least a little contaiminaed by fission products escaping from a partly damaged core.

NRC sources said between 30,000 and 50,000 gallons of radioactive cooling water had spilled through a valve that had stuck open. That is more than half the water which normally covers the hot uranium core where the nuclear reaction in a power plant takes place.

The fear among nuclear engineers when so much leaks out of the reactor is that the top part of the uranium core is no longer covered by cooling water. This is what happened at Three Mile Island, causing the core to overheat and its stainless steel fuel bundles to crack open and release huge amounts of radioactivity to the containment building.

A little less than three years old, the Crystal River nuclear plant lies on the Gulf of Mexico between a cattle ranch and a large stretch of orange groves in Citrus County about 60 miles north of Tampa. The land around it is almost uninhabited for five miles and Crystal River the nearest city, is 7.5 miles southeast of the plant.

The accident came just after Chairman Ahearne told Congress the NRC was ready to begin licensing nuclear plants for the first time since the Pennsylvania accident.

Ahearne said the almost year-long moratorium on licensing -- he called it a "pause" -- should end in the next few days when the NRC considers an interim operating license for the Sequoyah nuclear power plant to be run by the Tennessee Valley Authority near Chattanooga.

"We now feel we're at the point that we have given the new requirements out to the plants that are operating," Ahearne said in an interview after testifying yesterday before the House Appropriations subcommittee on energy, "and we are now ready to look at operating licenses."

Congressional sources said last night that at least one of the new requirements for nuclear power plants since the Three Mile Island accident had been waived at the Crystal River plant. That was a requirement for each plant to install an instrument to monitor the water level inside the reactor core to make sure it never fell below the top of the core.

Sources said the Crystal River plant was granted a one-month delay on installing the water-level monitor, in part because Florida officials feared an electrical shortage when the plant shut down to install the instrument.

What puzzled NRCS officials last night was the power failure that triggered the shutdown and how such a large volume of radioactive water escaped from the reactor vessel into the containment building.

Sources said engineers suspected a valve leading from the reactor vessel had stuck open when it should have closed, releasing thousands of gallons of radioactive water into the containment. There were no signs that a pipe had broken and caused the flood.

"Out continuing concern has to do with the inability of the NRC to identify the cause of this leak," Sen. Gary Hart (D-Colo.), chairman of the Senate subcommittee on nuclear regulation, said last night. "That means the accident may still be going on."

By 8 o'clock last night, Florida Gov. Robert Graham was flying to Tallahassee after the National Governors Association meeting here. Graham said he and his staff "will monitor the situation through the night."