What if the nuclear club is enlarged?

Here is the question nobody wants to ask. What happens 8 or 10 years hence if a new group in the nuclear club comes forward and says it has the atomic bomb? What would we do?

The question is almost too disagreeable to discuss. Hitherto it has been the story of the superpowers; they continue their ritual dance over disarmament. There are compensations. Fear of the bomb has helped keep the peace for 30 years. But irrational accumulation of hardware continues with the increased militarization of foreign affairs. More and more money goes into the military; technical improvement in the labs goes on, too.

But suppose there is a new entry in the plot - a political, a patriotic, or a piratical group. We are as much at a loss as was Harry Truman until half an hour after he became President.

Mr. Truman succeeded to the presidency April 12, 1945. His first Cabinet meeting was hastily summoned and at the end Secretary of War Stimson tensely asked him to stay behind, he had something to reveal. It was about the atomic bomb. Mr. Truman had not heard of it before. (Margaret Truman recalled the story at a conference last week at Hofstra University, Long Island, held in commemoration of her father.) Colonel Stimson said there was an extraordinary new weapon that would be ready ''around the first of August'' and that it would be the equivalent of 1,000 to 1,200 tons of TNT. (Actually 20,000 tons.) What would Truman do?

Now the problem in sight is what we shall do. It could be infinitely more complex. So far the nuclear club is rather small: the United States, the Soviet Union, Britain, France, and China. India made a nuclear test but held back from producing weapons. Other nations are close to the secret or may have it; Israel, for example. Theoretically the International Atomic Energy Agency polices production of nuclear power: There are some 840 facilities engaged in power and research; the agency is supposed to see that no one in the group is secretly making atomic weapons. It becomes harder and harder to police.

Sen. William Proxmire (D) of Wisconsin reviewed the situation at a recent breakfast here. He recalled a college physics student who a few years ago worked out a formula for making a bomb. Some professors looked over the student's formula with a measure of awe: Yes, this was a possible solution to the bomb construction problem. All a private group needed was a laboratory, a leader of superior technical skill, and some stolen plutonium: 10 or 20 pounds would make an atomic device.

It was a stunning thought. The world had never faced it before. Journalists are still writing about it; they casually drop phrases about blowing up a city or destroying civilization. People shiver and shrug and go on as before, but the fact is the threat is true. Arguments rise over the nuclear freeze. President Reagan notes that the freeze could be used against him in the intricate bargaining process. (Moscow, of course, doesn't allow dissidents to parade for the freeze.) The argument goes on.

But can we hope that the nuclear club will stay small? Fanatics are not easy to restrain as the bomb at the US Embassy in Beirut grimly illustrates. What happens if a third party gets a nuclear explosive in the time just ahead - a patriotic, a nationalistic, or a marauding group? Could we preserve civil liberties if we felt that a nuclear threat walked the streets? Not very long perhaps. What happens then? It is a question nobody wants to ask.