SUNDAY U.N. AMBASSADOR Andrew Young told a television interviewer that the United States was not about to end its longstanding policy of cooperation with South Africa in the development of that country's peaceful nuclear technology. Mr. Young said that while he might personally favor such a cutoff, he regarded it as impractical in the extreme. That is because South Africa, if it is determined to so so, could doubtless develop nuclear explosives without our help, given the state of its current nuclear technology and plant, its access to other exporters and its own natural resources. "To cut things off now," Mr. Young said in reference to U.S. export to South Africa of nuclear materials for peaceful purposes, "would only encourage separate development of South Africa's own nuclear potential."

That might or might not be the case. Certainly the ambassador was right in adding that, in any event, such a gesture would reduce to zero our chance of persuading the South Africans to go in the other direction - i.e., to sign her nuclear nonproliferation treaty and accept stringent monitoring of their peaceful nuclear installations. Such influence as we have would vanish. But there is another and, if anything, even more compelling reason not to terminate our peaceful nuclear cooperation with South Africa by way of registering our disapproval of its government's cruel and reckless racial policies. It is that such a cutoff of assistance - especially, a cutoff of nuclear fuel, as has been suggested - would run directly counter to a set of assurances to foreign countries that are the bedrock of Mr. Carter's nuclear nonproliferation policy.

The point is this: The United States is trying to get other nations around the world to defer the reprocessing and use of plutonium on grounds that the technologies and material involved lead too readily to a bomb-making capacity. We want them at least to delay moving toward a plutonium economy while less dangerous alternatives are explored. To the objection raised by many of these countries that plutonium offers them needed energy independence and a reliable source of nuclear fuel, we have replied that we are and intend to remain a reliable source of the far less dangerous low-enriched uranium fuel. We have also pledged that we will not use it - turning a country's heat on and off, as it were - to further our own political objectives. Even though our current exports to South Africa involve nothing so dramatic, it is essential that this assurance not be undermined - especially at this point in the negotiations over a host of countries' nuclear plans.

The standard that should guide U.S. decisions concerning peaceful nuclear exports is a nuclear standard. No matter how abhorrent an importing country's policies and politics may be, the American government needs to indicate that it will remain a steady and reliable supplier of any peaceful uses nuclear material it has contracted to export - provided that the importing country meets a properly tough and comprehensive set of safeguards against the misuse of that material and provided that it does not appear to be moving in the direction of developing nuclear explosives. Such considerations should be the only kinds of criteria we use in deciding whether to end peaceful nuclear cooperation with importing countries.

There has been more than a little confusion on precisely this score with the South African government. Did it or did it not give Jimmy Carter firm assurances this summer that its nuclear intentions were entirely and irrevocably peaceful?The South Africans now seem to want to fedge the issue. The matter is worth pursuing because that , as distinct from other Johannesburg policies, is the one that must be the touchstone of our own intentions as a nuclear exporter. The irony of a decision to end peaceful nuclear cooperation with the South Africans on any other grounds is that it would not just give South Africa a push toward nuclear bomb-making in all probability: It would also deal a devastating boomerang-like blow to Mr. Carter's efforts to curb the spread of nuclear weapons elsewhere.