MR. BREZHNEV'S newly stated willingness to authorize some kind of temporary moratorium on peaceful nuclear explosions and an indispensable explosion at that, one uniquely suited to digging canals and moving mountains, should have gone out with the tailfin. But this is, nevertheless, good news of a very modest, belated kind.

Why is that? Well, for one thing, this superpower gavotte over a comprehensive test-ban treaty has been going on for the better part of two decades now. In that time many of the weapons developments that a comprehensive ban on nuclear testing might have averted in fact have taken place. We are living with new generations of weapons and consequently, new strategic problems and threats that could not have come into existence under a strictly observed comprehensive test han. The gains to be achieved now are real but marginal: If the superpowers themselves forswear PNEs, a certain amount of pressure - not a lot, but some - will be brought against currently non-nuclear countries' taking the Indian "peaceful-device" route to acquire weapons. And if such a comprehensive ban on testing is worked out, and if it works equally and fairly to inhibit both sides, then it could have a kind of anti-nuclear impact - slowing the climb to new levels of nuclear weapons and diminishing confidence, or at least overconfidence, in those already in our arsenals.

We think these are prospective gains worth achieving. But no one should think that even these will come easily just because of Mr. Brezhnev's altered view. It is far from certain that any comprehensive nuclear-test ban can be sustained so long as the French and the Chinese insist on remaining outside its terms. And the offer of Mr. Brezhnev itself leaves much yet to be settled and spelled out. The Soviet leader's apparent insistence on separating a PNE "moratorium" from a weapons-test "ban," for instance, preserves the assumption that there is something different (and legitimate) about conducting nuclear explosions in the name of peace. And how, in any event, would such a moratorium be terminated? Would it be drafted and agreed to in language allowing it to end with a bang, as the last joint moratorium did - an unexpected supertest by the Soviets? What kind of verification would be authorized to assure each side that the other was not cheating?

We can almost hear the groans of some of our friends if the arms-control community, those people for whom recollections of certain footdragging by the United States have become controlling and who tend to regard all talk of tightened terms and verification as pointless, time-wasting, anti-arms-control stuff. But we would insist on the point. Events like that sudden, brutal one-sided termination of the moratorium in 1961 create exactly the kinds of pressures and dangers and needs to respond in kind that serious, stable arms control is meant to diminish. You can't prevent such occurences with 100 per cent certainty, but you can write terms that make them less likely. And you can also insist on verification measures that are not intrusive on the other side, but that provide a tolerable level of assurance that violations on an important scale are not taking place.

In any comprehensive test-ban agreement, as in any prospective SALT accord, there needs to be enough clarity of language and assurance that violations on a certain scale can be detected to make that agreement politically manageable in this country. Any agreement that is not proof against wild charges of Soviet cheating or that divides our intelligence agencies on the subject of whether such cheating has occurred is little more than a request for domestic and international trouble. That is why Mr. Brezhnev's change of position on underground testing, welcome as it is, is not the end of the test-ban story.