With a bang and a whoosh of escaping steam, a major nuclear power plant "accident" finally occurred last night.

It was greeted here with applause and a beer-and-pizza celebration by the scientists and engineers who watched it happen -- who, in fact, had staged the whole thing as the first test ever of emergency cooling systems on a working power plant reactor.

Those systems are designed to prevent soaring temperatures of radioactive material that could melt metal and concrete safety barriers and lead to a fatal release of radiation into the environment.

"I pronounce this a successful test. Very good," said Thomas E. Murley, director of reactor safety research with the Nuclear Regulatory Commission.

His was an understated assessment of how fast the emergency cooling systems went to work.

"Unbelievable!" was one of the more common descriptions while two kegs of beer flowed at the Gay Nineties Pizza Parlor in nearby Idaho Falls.

Test participants saw a vindication of nuclear power plants safety standards, which are under attack by environmental organizations and some communites.

However, critics say the $10 million effort actually proved nothing or, worse, indicated an inability of nuclear advocates to accurately predict what would happen in the event of trouble.

"I will say before the test is conucted that it won't prove one way or the other whether reactors are safe," Jim Cubie, of the Union of Concerned Scientists, had said.

He and others say the small scale of this test facility -- it generates about one-sixtieth the heat of a commercial power plant -- makes last night's results inapplicable to the plants now producing electricity across the nation.

"It's like testing a tire one-third of an inch high and saying it's safe to ride on in your car," said Cubie.

Also, he said today, if the test was intended to verify scientists' ability to predict what would happen in an accident, it may have been unsuccessful. "If," he said, "it's a test of their ability to predict events, it's as bad being too high as being too low."

Indeed, the Department of Energy had billed this simulated "accident" as a means of testing complex computer projections of what would happen during an accident, not as a test of whether the colling systems would actually work.

In reality, last night's test showed substantial variations from what was predicted. The actual events were much "safer" than the predictions.

Reactor temperatures never reose as high as predicted -- 900 degrees compared to an expected 1,375 degrees. Nor did it take as long as expected for emergency colling water to coll the radioactive core -- perhaps seven seconds compared to an anticipated 90 seconds.

These test expectations were well below federal safety regulations, which set a maximum radioactive core temperature of 2,200 degrees (protective tubes would melt at 3,300 degrees).

Despite 21 years of nuclear generation of electricity in the United States, no emergency cooling system has ever been called into action after the failure -- planned or unplanned -- of its main cooling system, until last night.

About 40 of the 72 nuclear power plants now in operation are of the design tested last night. Another 110 nuclear plants are in various stages of planning, approval or construction.

This test was originally scheduled for Friday night, but brutally cold winds from the north -- sweeping over a desolate and snow-covered Idaho desert -- froze an outdoor device that helps remove heat from the radioactive reactor core.

DOE officals her maintained that the freeze-up had nothing to do with the proper operation of the backup cooling system and would only keep them from conducting the test at high operating power. They said there would be no danger to the community if the cooling system didn't kick in; the reactor had yet another backup system.

Finally, last night, under adstarry sky, the 90-minute countdown began. After minor problems wer worked out, a d switch was thrown. In less than the blink of an eye, two valves opened, creating a break in the cooling pipes. Temperatures around the radioactive core began rising from the normal 650 degrees.

Even before the backup system brought the temperatures down to 415 degrees, the DOE employes, private contractors and representatives of five foreign nations were applauding.

At least two major questions remain:

What would have happened if this were a full-size plant and not just a one-sixtieth mockup? DOE officials here concede that results might be different at higher temperatures in commercial plants, but they also note that heat was being generated here at the same rate as in a power plant.

Why had the nation waited 20 years to test nuclear plants, rather than testing before building the plants? "That's a good question I can't answer," said Murley. "You'll have to ask my predecessors."