The outlook for stopping the quick spread of nuclear weapons around the world has, for the moment, brightened. According to the director general of the United Nations International Atomic Energy Agency here, Hans Blix, this is emphatically good news - though he readily granted in an interview that clouds still lie on the horizon.
As seen from the circular, aluminum-and-glass towers of the IAEA headquarters in Vienna, progress is evident on several fronts.
The threat that Pakistan might soon become the sixth nation to explode a nuclear device appears to have eased because of pressure from Washington as well as the agency.
A United States intelligence report that Argentina may be about to take secret steps toward a bomb is dismissed here as speculative and ill-informed.
Three more developments are seen as strengthening the IAEA's own prestige and cohesion:
* China has been admitted to full membership. It is the last of the nuclear-weapons states to join.
Although Peking refuses to sign the Non-Proliferation Treaty (NPT), seeing it as discriminating against nonnuclear states, China has decided to start generating electricity from nuclear plants. It accepts the need for IAEA safeguards against diversion of fissionable material as a way to ensure that the US and other nuclear powers will sell it technology. It wants the benefits of agency knowledge on safety and waste management.
* The political storm that broke over the IAEA in 1982 over Israel abated this year. Last year Arab and other states refused to accept Israeli credentials to the annual conference, and the US left the agency altogether in protest. This year, with the US back in, Israeli credentials were accepted. Arab displeasure at the Israeli raid on an Iraqi reactor in 1981 was limited to a rap on the knuckles, with little practical effect.
* The Soviet Union has reached substantial agreement in two meetings with the agency on a voluntary offer to open some of its nuclear plants to agency inspection. (The US, France, and Britain already do so.)
The IAEA is one forum in which Moscow and Washington see eye to eye regardless of the ups and downs of detente. As superpower tensions increase elsewhere, Moscow has just offered an extra 1 million rubles ($1.3 million) for technical assistance projects. Mr. Blix is delighted.
The clouds on the horizon include world tensions that fuel the uncertainties both of nations that can already build bombs, such as Israel and South Africa, and of nations that are well advanced toward a bomb capacity, such as Pakistan and Argentina. Maverick states such as Libya remain objects of suspicion.
Many nonnuclear countries, especially in the third world, continue to strongly criticize the US and the Soviet Union for ''vertical'' proliferation - constantly enlarging their own stockpiles of nuclear weapons.
The nonproliferation treaty requires nuclear nations to dismantle their weapons as fast as practicable. At the same time, the third world accuses the nuclear club of breaking the NPT in another way: by withholding technical assistance for peaceful nuclear uses.
Both criticisms will surface when the NPT is reviewed for the third time in 1985, probably in Geneva.
Meanwhile, Pakistan and Argentina are less pressing problems today than newspaper headlines might suggest. After several years of vigorous backstage prodding by the IAEA and the US government, Pakistan has installed extra surveillance cameras and automatic fuel-rod counting machines at its Kanupp reactor outside Karachi.
Few in Vienna or Washington doubt that President Zia ul-Haq is maintaining Pakistan's massive, clandestine, decades-long campaign to achieve the ability to detonate a nuclear device. India exploded one in 1974.
But the feeling here is that progress is slow and that President Zia knows only too well that any detonation would cause the US Congress swiftly to halt the US military aid given because of the Soviet invasion into nextdoor Afghanistan.
That aid includes F-16 fighters that Zia badly wants for national and personal prestige.
Mr. Blix now says that ''arrangements at Kanupp are satisfactory.'' The IAEA can say nothing about other nuclear facilities in Pakistan, which are not under agency safeguards.
Meanwhile, experts here dismiss a reported CIA intelligence estimate that Argentina may be about to divert one ton of uranium from a treatment plant, put it into fuel rods, irradiate them in a reactor, extract plutonium, and use the plutonium for bombs.
''Nothing in our relations with Argentina justifies such suspicions,'' Mr. Blix said.
Privately, other senior experts said they found the US reports speculative and highly theoretical. US State Department and nuclear energy sources also throw cold water on the idea.
''I don't think they're aiming to build a bomb,'' a knowledgeable agency source said, ''but no Argentine general or civilian is going to foreclose a policy option to leaders 30 years from now. . . .''
In an interview in Vienna, the head of Argentina's nuclear power program, Adm. Carlos Castro Madero, said the US reports were ''misguided.'' He interpreted them as public American pressure to warn Buenos Aires to stay away from any temptation to make a nuclear device as a way of regaining prestige lost in the vain battle to take over the Falkland Islands from Britain.
Argentina, as he acknowledged, attracts suspicion because it has refused to sign the Non-Proliferation Treaty and refuses to acknowledge full IAEA safeguards on nuclear facilities built by Argentines themselves.
cl11.5 But the two nuclear reactors now operating in Argentina are fully covered by IAEA safeguards included in agreements with the nations that provided them under contract: West Germany and Canada.
It would be ''madness'' for Argentina to break safeguards and try for a nuclear device, Admiral Castro Madero said. Such a move would make Argentina a pariah in Latin America. It would force arch rival Brazil to build a bomb as well, end all technical help from Canada, West Germany, and Switzerland, alienate the US, and make agreement with the United Kingdom over the Malvinas (the Argentine name for the Falkland Islands) even more remote.