Vienna — In one of the most crucial battles of our time, there have been some encouraging signs: - No expansion of the nuclear club since the Chinese mushroom cloud in 1964.
-Indications that the United States and the Soviet Union might be moving toward reducing nuclear stockpiles by striking a deal to limit medium-range nuclear missiles.
- Acceptance by Pakistan of another surveillance camera and more international safeguards on its atomic reactor near Karachi.
Yet there are some worrying signs as well:
- New aid to Pakistan from China, confirmed by Washington officials, may mean Pakistan is nonetheless closer to acquiring a nuclear device.
- Brazil has taken a deliberate though long-term step to produce its own plutonium without international inspection.
- ''Backs to the wall'' nations Israel and South Africa are of continuing concern because it is generally agreed that they already have nuclear weapons.
- Argentina, defeated by Britain in the Falklands last year, has an unsafeguarded nuclear cycle and watches Brazil like a hawk.
This is how officials in Vienna and Washington summarize recent developments in the battle to keep nuclear weapons from spreading to unstable corners of the earth.
Officials in the steel-and-glass headquarters of the UN watchdog group here in Vienna, the International Atomic Energy Agency (IAEA), deliberately stress the positive signs.
''The international system to block proliferation works,'' a senior official says. ''The agency and its members put the spotlight on any country, like Pakistan, which tries to get around the rules. The word gets round.''
Sources in Washington with access to intelligence material are considerably more cautious, especially about Pakistan, Brazil, and Argentina.
''I don't see any real progress over the last 18 months,'' said a Capitol Hill source. ''The jury is still out on Pakistan. The Brazilians have defied at least the spirit of international rules. Argentina has been defeated in the Falklands by Britain and is only prevented from faster progress towards a nuclear device by lack of money.''
Another Washington expert, on the other hand, is more optimistic. Although no friend of the Reagan administration, he is impressed with the Reagan commitment to trying to alleviate individual nations' fears about their own security.
In an interview, IAEA director general Hans Blix indicated there is a general concern about Pakistan, South Africa, Israel, India, and Argentina.
''We need more inspectors and more equipment,'' said Mr. Blix, who is a former foreign minister of Sweden. ''But people tend not to recognize the agency's achievements.
''That nations are prepared to allow agency safeguard inspections of reactors on their own soil is still a new development in world history. Besides, the agency is only one of many ways governments have of forestalling proliferation.
''We are a signaling system, not a nuclear detective.''
Mr. Blix told the UN General Assembly last year that ''we may be at a crossroads'' on proliferation. It remains possible, he says, to ensure that no more nations go nuclear, but ''significant progress toward nuclear arms-control and disarmament'' would be important to reassure developing nations.
Or nuclear bombs could spread, having ''incalculable consequences for escalation and international security.''
In Washington, sources say Pakistan has encountered trouble in its bomb plans. Information in Vienna is that 5,000 ''cascades'' - barrel-like containers that spin at constant speeds to enrich uranium 238 by extracting from it fissionable U-235 - have been secretly bought by Islamabad but have been impounded by Swiss authorities in Zurich.
The enrichment process is extremely complex and expensive. Pakistani progress has been slow, but it continues.
Among the positive signs ticked off by officials here:
* The only detonation since China's in 1964 was India's in 1974, but India has not moved into a nuclear weapons program. New Delhi has agreed to accept ''bundle counters'' - which count the fuel rods in a reactor core - in its CANDU reactors, opening the way for rival Pakistan to do the same.
The counters are important. Both India and Pakistan can fabricate their own fuel rods, and CANDU reactors can be fueled and unloaded while operating. As the rods ''burn'' they produce plutonium, which can later be extracted and used for bombs. Counters can detect suspicious use.
An IAEA mission to Pakistan Jan. 25 to 28 won approval to prepare a bundle counter in the Kanupp reactor near Karachi for tests and operation, as well as for a second camera system and agency seals on an emergency airlock.
Pakistan apparently continues to refuse other requests, such as allowing film from the cameras to be flown to Vienna for processing. But the agency is relieved it has gone as far as it did in January, since the struggle to safeguard Kanupp has gone on for two years.
* Inside the atomic energy agency, the budgets for technical assistance and for safeguard inspection systems have been rising sharply. They stand at roughly
* The number of nations signing the 1968 Nonproliferation Treaty (NPT) reached 119 with the signatures of Egypt and Vietnam. Neither France nor China has signed. Nor has Israel, Pakistan, India, South Africa, Brazil, or Argentina.
* The apparent possibility of a compromise on medium-range nuclear missiles in Europe between the United States and the Soviet Union is a boost for arms-control hopes. The superpowers could use an agreement to try to dispel the developing world's fear that the superpowers plan to continue their own proliferation of nuclear stockpiles ad infinitum.
* Moscow has repeated its June 1981 announcement that it is prepared to accept IAEA inspectors at some of its peaceful atomic installations. The IAEA is one world forum where Moscow and Washington usually see eye-to-eye.
* The so-called ''Zanger committee'' continues to refine its list of sensitive items industrial nations are forbidden to export to nonnuclear states.
* While South Korea and Taiwan are on the threshold of a bomb, experts believe that neither will detonate one as long as their security shield remains the US.
Having said all that, however, some setbacks that have occurred in the last 18 months cause considerable concern.
Pakistan and Brazil are cases in point.
Confirming that China has apparently given some aid to Pakistan, Washington sources tell this newspaper that it is ''hard to pin down'' exactly what that help has been. A Washington Post report indicated it was designs for a bomb, and that it could help Pakistan assemble a device without the need for an early test.
Such a test would cause Congress at once to suspend the massive, $3.2 billion military aid package now flowing to Pakistan over a period of five years. The aid includes sophisticated F-16 fighter aircraft, the first of which touched down at Pakistan's Sargodha Air Force Base Jan. 16.
Critics of President Zia ul-Haq in Washington say congressional strings attached to the aid may prove ineffective. ''All Zia has to do is to wait for five years, take his planes and his aid, and do what he likes with his nuclear device,'' a Senate source says.
Asked whether detonating a device would not jeopardize spare parts from the US, the source said: ''All he has to do is fly half the F-16s and use the other half for spares. . . .''
Muslim Pakistan is a key state. It has close Arab friends. It is next door to Soviet troops in Afghanistan and is of enormous concern to Israel.
Brazil has also raised eyebrows in Washington by using a small laboratory-scale reprocessing unit to produce plutonium from a research reactor originally supplied by the US. Only a few grams of plutonium emerged, intelligence sources say, and Brazil lacks the capacity to produce enough to make a bomb (five to six kilograms).
Yet Brazil has signaled it intends to press on with weapons research despite international concern. The research reactor is under inspection, as is US fuel for it, but the Brazilians apparently used rods they had made themselves, claiming this was legal under its various agreements.
Argentina has been alarmed by Brazil's action, especially in the wake of its own humiliating defeat by Britain in the Falklands. Buenos Aires is building a large, unsafeguarded reprocessing plant of its own.
More obstacles to effective control of nuclear weapons proliferation have emerged within the IAEA itself.
One was the walkout of the US last September, and a long wrangle about how it was to return.
The US left in protest after the annual agency conference rejected Israeli credentials in a dramatic and close series of votes. Developing-country delegates were punishing Israel, in effect, for its airstrike against the Iraqi reactor near Baghdad on June 7, 1981, and for the massacre of Palestinians in Lebanon as the conference opened.
US officials say they suspended membership and all budget payments to defend Israel, to weaken similar exclusion efforts in other UN bodies such as the General Assembly, and to try to lessen the intrusion of politics into agency affairs.
Yet the move led to more politicking within the agency, not less, and to much criticism within the agency that in the last analysis, the US subordinates support of nonproliferation to political backing for Israel.
Since the US provides about 30 percent of all agency money and resources, its action slowed safeguard research and raised a threat to the agency's future.
At this writing the State Department had said it wanted to return to membership. Details, under a congressional resolution instigated by Sen. Robert Kasten (R) of Wisconsin, were unclear. Considerable damage had been done to the US image in third-world eyes.
Another difficulty inside the agency: the growing conviction among the so-called Group of 77 countries (the developing world) that the big industrial nations were failing to keep their promises under the NPT.
The NPT pledges nuclear ''have'' countries to share technical aid with others and to negotiate a reduction in stockpiles of nuclear arms. In return, the ''have-nots'' agree to forgo nuclear weapons and to accept IAEA safeguards.
''But you big countries don't give us much valuable technical assistance,'' a third-world ambassador to the IAEA said. ''You say it might help us have a bomb. You call our governments unstable. . . . But what could be more unstable than Italy, for instance?'
The Group of 77 wants more of its own men in key agency positions, much more technical assistance, and genuine superpower arms control. It fought hard in 1981 to have its own candidate as director general and will do so again in 1985.
The question remains: What can best be done now to promote nonproliferation?
Mr. Blix listed some things in his interview:
''Alleviate security concerns of individual states. . . . Take steps towards nuclear arms control and disarmament. . . . Strengthen the IAEA itself. . . .
''And I would urge that more attention be given to the concept of nuclear-free-zones, especially in the Middle East. . . . It may be unacceptable unless there is an evolution towards a peaceful climate there, but I think we must use our imaginations and not just give up. . . .''
The most urgent task for diplomats and politicians is to ease the sense of insecurity in world trouble spots. It is insecurity, coupled with national pride and ambition, that spurs countries to develop nuclear programs, diplomats agree.
The Carter administration tried to block sensitive exports and to penalize countries that did not accept safeguards. The Reagan view, welcomed within the IAEA, is to ease this policy somewhat while giving arms and other conventional aid as a way of promoting security in countries such as Pakistan.
Much more, however, needs to be done - and quickly.