THE BELGIAN government says it is considering a proposal to sell $1 billion worth of nuclear equipment to Libya. It is difficult to think of a worse idea -- more irresponsible, more wantonly dangerous, more likely to lead to spectacular violation of the world's fragile rules against the proliferation of nuclear weapons. The Belgians can argue that they have very high unemployment and need to export. But high unemployment doesn't begin to justify nuclear sales to Col. Muammar Qaddafi.

The Belgians say, defensively, that they would sell only civilian power equipment and only for peaceful purposes. That's transparent. Once the equipment is delivered, neither the Belgians nor anyone else outside Tripoli will have much control over the purposes to which it is put.

Ever since rising oil prices made them rich in the early 1970s, the Libyans have been trying to buy nuclear weapons. Frustrated in that endeavor, they now appear to be seeking another route to the same goal. To extract plutonium from a power reactor's spent fuel requires a reprocessing plant and, as the Belgians will point out, the Libyans do not have one. Not yet. But there have been reports that Libya has been contributing funds, for purposes that seem self-evident, to Pakistan's attempt to build reprocessing capability.

Libya has no shortage of energy. It is up to its ears in oil. It is a thinly populated country with almost no industry outside the oil fields. Its need for huge and expensive new power sources is not immediately obvious. What do the Belgians suppose the Libyans want reactors for?

Harry Truman once said that, as president, he spent most of his time trying to persuade people to do things that they ought to have the sense to do without being asked. The diplomats who try to preserve the nonproliferation code can say the same thing. Among other governments, the United States has been remonstrating with Belgium.

But, unfortunately, it has not been remonstrating as effectively as it might. The American protests against nuclear sales to Libya come at a moment when the U.S. Department of Defense -- which frequently seems to carry on its own independent foreign policy -- is embroiled with Belgium over sales of machine tools to the Soviet Union. The machine tools are neither unique nor crucial. They are merely a target in the Defense Department's continuing campaign to deny to the Soviets all imports of any strategic significance whatever -- except, of course, American grain.

By even contemplating nuclear sales to Libya, the Belgian government betrays a dismaying confusion in its sense of what's truly important. As for the United States, the disproportionate vehemence of its attacks on the machine tools shipments shows that it is not exempt from the same charge.