Thanks to the Reagan administration, the unthinkable is now thinkable, which means a phenomenal amount of thinking needs to get done.

Up until now, most of us have stuck the possibility of nuclear war away in the fourth dimension and left the ultimate horror up to the politicians, the scientists and engineers, the intelligence community and the military planners. The result has been the evolution of a nuclear-ignorant population and the rapid development of massive weapons of destruction on both sides. The result has also been peace, so far.

But it is a peace imperiled by everything from a malfunctioning microchip in a computer, which we have had, to a malfunctioning world leader, which we have also had. Richard W. Lyman, president of the Rockefeller Foundation, recently told a symposium on nuclear war: "We are faced with an unprecedented, all-but-indescribable power of destructiveness in the hands of a species with an all-too-familiar capacity for aggressive, not to mention self-destructive, behavior.

"Treating nuclear war, and even the nuclear arms race, as 'insanity' too readily becomes a way of oversimplifying the problems," he said. "It suggests that all that is necessary to be rid of this nightmare is to put power into the hands of people more rational than those who have been wielding it."

Lyman went on to suggest that the academic world should work to reduce the politicization of arms control and to educate students about arms control issues. The rest of the population must educate itself, as well.

That is already beginning to happen. A nuclear freeze movement is sweeping the country and has overtaken Congress.

Next week, Ground Zero, a bipartisan movement, will hold nuclear education week in 650 cities across the country. In Washington, Ground Zero week kicks off today with a descriptive tour of the aftermath of a 1 megaton bomb hitting Lafayette Park. Roger Molander, former nuclear strategist for the National Security Council under Presidents Nixon, Ford and Carter, and now executive director of Ground Zero, tells what would happen:

There would be a crater 1,000 feet in diameter and 200 feet deep, and out to a distance of 6/10ths of a mile nothing would be recognizable. Within a radius of two miles everything would be flattened. At a distance of five miles out, all concrete and frame buildings would be destroyed. Within a radius of three miles almost everyone would be killed. At 10 miles, most people would survive the blast but face extreme dangers from fire.

But in the event of war, he says, strategists expect Washington and its surrounding towns to be hit by a number of bombs. Fort Belvoir, Andrews Air Force Base, the National Security Agency at Fort Meade, the communications equipment in Annapolis, and possibly the CIA headquarters in McLean, would be hit by separate bombs, he says. "We don't have the Soviet war plans . . . We assume they would do it the way we would do theirs."

Ground Zero has put out a paperback book, "Nuclear War, What's In It For You," a primer on nuclear weapons, nuclear war and nuclear issues. This is a book that ought to be read by every citizen of every country that has nuclear weapons. It doesn't give any answers, but it lays out the issues so that we, the people, can help decide our fate. It describes the Soviet arsenals and it shows how we could get into nuclear war. It tells how in 35 years we came from having one bomb with the explosive power of 15 kilotons of TNT to having a world arsenal whose destructive power represents "10 tons of TNT for every man, woman and child on the face of the earth."

Ground Zero can educate us. The next step is to sustain the momentum. We have to institutionalize our concern and our understanding of nuclear issues the way we institutionalize our knowledge of English and math. Nuclear issues are certainly more fundamental to survival of the species. From that, we may begin to find ways toward a less perilous peace, the way the English and the French did after centuries of war.

Throughout history, men have sent boys out to battle. But in the kind of war that is now becoming thinkable, men would be sending out entire populations of cities and nations of people. In nuclear jargon, Ground Zero is the point where a nuclear weapon is detonated. In survival jargon, it marks a beginning. We should, we must, make it the first step on the way back from the edge.