The town of Hypothetica lies a few miles from a sophisticated research center where scientists have been studying new types of viruses. A serious accident occurs at the center, and later it is discovered that some viruses have been released into a stream that supplies drinking water to the people of Hypothetica.

In an atmosphere of crisis, federal experts are called in. They find that the viruses are all of a type that can infect only the rare African armadillo. Not only that, but this particular virus is commonly found in a dormant state in humans, where it has no detectable effect.

With relief, the federal experts and the scientists announce that the viruses can do no harm to people. But the announcement only raises the fears of the townspeople. "Cover-up," is the general response. "After all, the federal government licensed that research center in the first place, and everyone knows that viruses are dangerous." Experts from the state government -- which has no direct involvement with the center -- are then called in. They reach the identical conclusion, which elicits the same disbelief.

By now the people of Hypothetica are reporting nightmares among their children, and sleeplessness, constipation, rashes and a variety of other symptoms of stress among the adults. There is endless debate and controversy and soon, talk of evacuation. In desperation, the governor turns to yet another set of experts, scientists trained to study viruses, who are also leaders of a group whose aim is to stop the kind of research being done at the center. If these guys say the viruses aren't dangerous, thinks the governor, surely the local people will believe them.

The anti-virus scientists soon report that the government experts were correct: the viruses pose no danger to human health. Nevertheless, because of the local concern, they recommend building a high dam upstream from the town, with a special filter that can reduce the number of viruses from their current, admittedly harmless, level to, perhaps, one-millionth as many.

At this point we can drop the fantasy and admit that this is really a story about Three Mile Island, where a controversy is raging over whether radioactive krypton gas can be released from the reactor containment building so that crews can begin to clean up the crippled reactor.

The essential points are the same. The utility, the Nuclear Regulatory Commission and Pennsylvania officials -- and now the anti-nuclear Union of Concerned Scientists -- have all agreed that there is no health risk associated with simply venting the gas. Yet the bigger the number of experts that agree, the more local opinion disagrees.

The terrible irony is that this is one of those rare times when there really is no risk. Krypton -- despite its ominous name -- is chemically inert. Its only danger comes from its radioactivity. A radioactive substance is one whose atoms spontaneously rearrange themselves, emitting energy in the process. Depending on the amount of energy released, the radioactivity is given different names -- alpha particles, beta rays, gamma rays, I rays, and so on. Krypton happens to give off almost entirely beta rays and a small percentage of gammas. Beta rays are not energetic enough to penetrate more than the human skin, while gamma rays can even pass through lead.

The point of this is not that everyone should know his betas and gammas, but that, although radioactivity cannot be seen or felt, it can be measured with certainty. There is nothing mysterious about it. In the case of the radioactivity at Three Mile Island, its nature and its exact amount are known, and therefore the dose that people in the area would receive from venting is also known. And the dose is tiny.

In their report, the USC concluded that in the worst case, the dose from gamma radiation would be the same as the dose everyone receives from naturally occurring background radiation in three hours. The beta dose would be about 100 times as big, but, because of its lower energy, would still be only one ten-thousandth of the dose needed to cause health effects.

Nevertheless, the controversy over venting has lasted for months and the just-released UCS report will probably make it worse. If there is really no risk from direct venting, Pennsylvanians are going to ask, why does the UCS recommend venting the gas through a 2000-foot tube suspended from a tethered hot-air balloon? The USC thinks its recommendation will help reduce the local stress and confusion. In fact it will almost certainly do the opposite.

In a more rational world, the krypton would have been vented months ago and all the stress prevented. The controversy seems like the ultimate example not of a technology gone wild, but of society's coping machanisms having broken down. There is almost nothing most of us to these days out of our own certain knowledge. Whether it's building a house, eating packaged food or living with man-made chemicals, we depend on experts to determine what is safe, what will work and what can't be done. While society may choose to accept very different risks from different kinds of activities, and though there may often be disagreements among the experts, so long as we trust the people who are supposed to know what they are doing, the system works.

But when the experts have been wrong too many times, or when their too carefully worded statements confuse the public -- as with saccharin, nitrites in bacon, carcinogens in Scotch, and all the rest -- credibility evaporates and the system breaks down. What has happened at Three Mile Island, and what may be happening to nuclear power in general, is that that essential trust may have fallen below the minimum threshold. How -- or even whether -- the NRC and the nuclear industry can now buy back their credibility is an open question.