It is tempting to think of the Three Mile Island accident a year ago today as a kind of Tet offensive, an unexpected assault by a power that we said we had well under control.

Just as the nationwide Vietcong attack in South Vietnam was beaten back in February 1968, so the uprising in the Metropolitan Edison Co. nuclear power plant in Pennsylvania was successfully contained in March 1979.

Both the generals and the technicians claimed victory, but the public perception became just the opposite. In fact, the common view of these now infamous episodes as failures has had far more impact than the bald events themselves would ever have predicted.

Like a pebble tossed into a pond that somehow produces a tidal wave at the edge, the Tet offensive washed away any illusions that Americans really controlled anything in Vietnam. As a result, new strength flowed to those who had been saying all along that we never should have tried to control the country in the first place.

At Middletown, Pa., where the silent TMI reactor dominates the psychological landscape, the lines are drawn as sharply now over nuclear power as they ever were anywhere over Vietnam.

A year and a day ago, Middletown mirrored the nation in basically accepting nuclear power. There were some concerns, however, and reactor orders nationwide had begun to falter because of declining electricity demand, inflation and soaring interest rates on the billions that they cost. Only two reactors were ordered in 1978, but 90 were under construction.

The federal Nuclear Regulatory Commission had already acquired a backlog of 133 "generic safety concerns," that affected all 70 of the country's operating reactors. The stuck valves and fuzzy instructions that would spell major errors 24 hours later had been notified during minor accidents at other plants. But little work had been done and no effort made to spread the word.

There were internal critics at the NRC even then, complaining that evacuation plans were almost nonexistent, that dangerous radioactive material was sloppily policed, that decisions got made with the speed of the continental drift. But the critics were ignored or transferred, just as some dissenters had been in Vietnam, and electric utility officials echoed the generals in saying everything was just fine.

Anyone who followed the postmortems of TMI knows the sequence of what happened there in the predawn hours a year ago. An air pump line hiccupped as maintenance crews were going through their 4 a.m. motions, and a minor device that removes minerals from water shut down. A series of valves closed, the main pumps stopped, the power-generating turbine shut down. An auxiliary pump started up right away, but (goof number one) somebody had left the valves shut and the water couldn't get in.

The reactor, with less water than it needed, began to heat up. A safety valve popped to relieve the pressure and (goof number two) failed to shut. The steam discharged into a drainage tank; the reactor, too hot, shut down automatically. Eight seconds had elapsed.

Lights and buzzers were going off everywhere. Trained to stay calm, the operators guessed from their ambiguous indicators (goof number three) that water pressure was building in the reactor. They shut down what pumps were still going (goof number four) and the reactor heated up further.

Tons of water than had poured out through the stuck-open valve overflowed into an auxiliary building through a valve that should have been shut (goof number five). Radiation alarms howled. A site emergency was declared and the world began to learn about this small island in the middle of the Susquehanna River.

That was the technical sequence. Until the goofs were discovered and corrected, there occurred the most dreaded event in nuclear engineering -- an uncovering of the reactor core. That was supposed to mean meltdown, collapse through the molten floor of the building, a steam explosion, catastrophe: the China Syndrome.

But it didn't happen. There was core damage, and an NRC probe said meltdown might have been an hour away. Some radiation was released.

But the kinds of radiation released, the unexpectedly low levels of it now in the containment building and the exhaustive what-ifs of the past year tell a lot of very respectable scientists and engineers that they probably have been wrong all these years about the danger an uncovered core really poses.

"It proved you could fly one of these things right into the ground, beat hell out of the core and still not have the kind of disaster everyone was worrying about," said engineering consultant Norman M. Cole, who was intimately involved with Three Mile Island recovery.

Even if the worst had happened and there had been a meltdown, wrote President Carter's investigating commission headed by John G. Kemeny, "there is a high probability that the containment building and the hard rock [underneath] . . . would have been able to prevent the escape of a large amount of radioactivity."

If in fact there had been a hydrogen bubble, as the NRC told the world there was (another goof), and if it had blown up, "the pressure loads are calculated to be somewhat less than the strength of the building," the Kemeny report said.

The territory remained secured.Three Mile Island was a victory for nuclear power.

On Feb. 25, 1968, Gen. William Westmoreland, U.S. commander in Vietnam, said the Vietcong and North Vietnamese had "suffered a military defeat" in their Tet offensive, despite "some temporary psychological advantage."

But other analysts were calling Tet a disaster that revealed frightening things about the whole war effort. Their voices grew in number, and soon very few people remembered that Tet might have been a victory in any sense at all.

Much the same thing has happened in the aftermath of Three Mile Island. Radiation was released. Group after scientific group, measuring milk, unexposed film in nearby stores, and every other counter suggested, has figured an average dose for 50 miles around of 1.5 millirem per person.

Sleeping with another person for five years gives that much. Natural gas used in cooking puts 10 times that much into the air at Middletown each year. A modern coal plant gives 2 millirem annually to anybody nearby.

Does any of that matter? Probably not. The radiation actually released will always be a question mark because some monitors were not operating and many people simply do not believe the scientists.

And why should they? Didn't Met Ed delay and then evade, insisting there was no problem? Didn't the NRC tell us nuclear power was safe? And what about all those weasel words -- "high probability," but not certainty, that a reactor building can contain "a large amount," but not all, of the radioactivity; a "minimal risk," but not no risk, of disaster; and "insignificant" releases -- but insignificant to whom?

Clearly something is being glossed over, if not covered up, say the critics.

In Middletown, where 144,000 persons temporarily fled their homes a year ago, some citizens now become almost hysterical over the idea that Three Mile Island will release more radioactivity, no matter how little, in venting the krypton gas that now blocks the start of cleanup. Massive multibillion-dollar lawsuits are seeking someone to blame for all the worry.

The utilities are putting up a determined front. After a million-dollar publicity campaign and a massive manpower investment, all reactor operators have been retained and several new monitoring and response organizations are in place.

People at the NRC talk privately, convincingly, of a fundamental shift in attitude toward more skepticism, a realizatin that it is not scientists and engineers running the reactors but ordinary working stiffs with limited technical training. The critics still argue that very little has actually changed, and they have taken to the streets to make their point a la Vietnam: with public demonstrations.

In the end it will probably fall to Wall Street to decide the fate of nuclear power.

Eleven reactor orders were canceled last year and six so far this year, while seven others have been indefinitely delayed. Nuclear power insurance rates went up 10 percent.

Utility stocks have weakened -- but so has the entire stock market. "Washington Analysis Corp. estimates the overall prospects for the domestic nuclear industry as highly unfavorable," the firm's December report to clients said.

General Public Utilities Inc., Met Ed's parent company, is struggling for its financial life, awaiting the verdict of the Pennsylvania Utilities Commission next month in a complex rate and permit case. Precedents set there will reverberate through the nuclear community and indicate something about liability for all these aftershocks.

Meanwhile, politicians seem to be waiting for the dust to settle before either bolstering or dismantling the nuclear option. It is clear that the Kemeny commission summed up the public verdict.

"Whether in this particular case we came close to a catastrophe accident or not, this accident was too serious. Accidents as serious as Three Mile Island should not be allowed to occur in the future."