The United States would have been wiser not to let 143 tons of ''heavy water'' be sold to Argentina. The action may somewhat improve strained US relations with Buenos Aires. But it may well set back the cause of nuclear nonproliferation which previous administrations had tried so hard to support.
Heavy water is used in many nuclear reactors including Argentina's to make atomic energy, either for peaceful or military purposes. What went to Argentina last week actually was owned by West Germany. But because it was made in the US, Washington had to give permission for the sale.
Argentina has an advanced nuclear industry which most experts believe soon will be capable of producing atomic weapons if the nation's leaders decide to move in this direction. Even if they did, it is thought unlikely that Argentina would quickly use or threaten to use them in warfare or other hostile action; there would be much graver concern along these lines had a component of nuclear fission been provided Pakistan or Iraq.
It is more likely that Argentina, like India before it, would detonate a nuclear device primarily to restore its sense of national self-confidence, so badly battered by economic woes and defeat by Britain in the Falklands war. But such action would have another - and negative - impact on the world's nonnuclear nations: Seeing that one more country had acquired the bomb could spur others to move faster to make their own.
Actually, before Washington agreed to permit the transfer of heavy water, Argentina promised it would be used only for reactors that make electricity and not for military purposes. We certainly hope that will be the case. But it is not reassuring that Argentina has refused to sign both the nuclear Non-Proliferation Treaty and the agreement which declares Latin America a nuclear-free area.
Therefore the Reagan administration ought to have insisted that Argentina permit the International Atomic Energy Agency (IAEA) to inspect its atomic facilities and keep track of its stockpile of material, such as heavy water, which can be used to produce nuclear weapons. It is one important, albeit imperfect, way of slowing the increase in the number of nations that have nuclear weapons.
Even if the Argentine reassurance is taken at face value it leaves a major loophole: Earmarking the 143 tons of US-made heavy water for electricity manufacture would free heavy water obtained elsewhere to be used for military purposes.
It is understandable that the US desires to be seen as a consistent supplier by nations trying to develop nuclear energy for peaceful purposes. But it is important that in return for US assistance they agree to IAEA inspections and inventories, or to be signatories of the Non-Proliferation Treaty.