Nuclear arms technology continues to spread quietly. Growing number of `threshold nations' have latent capabilities

A trial in Israel and the arrest of a Pakistani native in the United States have renewed concern about the dangers of creeping nuclear proliferation. And in another sign of anxiety about the spread of nuclear arms technology, Congress on Tuesday let new US aid to Pakistan expire until it takes up the issue in mid-November.

In the early 1960s, politicians predicted that the 1970s and '80s would be haunted by 15 to 25 nations possessing nuclear weapons.

This nightmare has not occurred. Today, only five states admit to having nuclear weapons: China, France, Britain, the US, and the Soviet Union.

India's so-called ``peaceful nuclear explosion'' in 1974 added another government with the demonstrated capability - but not the declared intention - of fabricating nuclear weapons. And at least one state, Israel, may possess a modest clandestine arsenal.

All things considered, this record seems remarkably positive. But a few realities rule out complacency:

First, the technology and infrastructure required to develop nuclear weapons has spread significantly in the past two decades. Roughly 35 additional countries now have the financial base, technical know-how, industry, and resources for a nuclear weapons program, according to proliferation experts.

Second, most experts agree that the actual testing of a traditionally designed nuclear weapon is no longer mandatory, meaning that the most detectable phase of a nuclear weapons program is no longer essential, making it extremely difficult to verify nonproliferation.

And third, the capabilities required for a peaceful nuclear explosion do not differ markedly from those needed to produce weapons. Nor are specialized delivery systems essential, since most states likely to contemplate a nuclear program have advanced military aircraft.

In short, the secret creation of a small nuclear arsenal has become difficult to detect. And countries that have detonated an explosion for nonmilitary purposes can easily adapt the technology to a weapons program. Governments now have an option, in other words, of not disclosing the extent of their achievements.

So-called ``threshold'' or states can make a strong case for maintaining ambiguity about their nuclear capabilities. Such a policy avoids international antiproliferation sanctions. It is less likely than a declared nuclear capability to panic enemies into creating their own nuclear forces. But at the same time, it is sufficiently threatening to force those enemies to think twice before adopting provocative policies.

Who are the potential proliferators, the so-called threshold or ``de facto'' states? Six countries are normally cited: Argentina, Brazil, India, Israel, Pakistan, and South Africa. All have some degree of indigenous enrichment or reprocessing capacity - the two methods of obtaining material for weapons production.

Of this list, Argentina and Brazil are currently the least threatening. Military regimes have given way to more moderate civilian governments, and bilateral talks have largely defused an inchoate nuclear arms race between the two.

Israel has long been suspected of passing the nuclear threshold, a reputation Israeli officials neither confirm nor discourage. Its status as a nonnuclear state has been further blurred by the trial of former Israeli nuclear technician Mordechai Vanunu, who claims his country has created an arsenal of between 100 and 200 weapons.

Pakistan has been under the microscope for a number of years. The arrest of a Pakistani native in the US in July for the attempted illegal procurement of nuclear-related items, although disavowed by Islamabad, is merely the last in a long series of such incidents. In the wake of the July arrest, Pakistan announced it had started an investigation into the alleged attempt.

Reports about reprocessing facilities and successful enrichment strengthen a conclusion that the Pakistanis are close to a nuclear capability - and deterred from actually testing or assembling a device primarily by American pressure.

India appears to have taken no steps toward fabricating weapons, even though it has convincingly demonstrated its mastery of nuclear technology. Little would be needed for New Delhi to use its know-how for military purposes - especially given India's large Air Force and consequent capability for weapons delivery.

South Africa's nuclear research facilities and atomic energy plants render it a strong candidate to create a weapons capability, should the situation dictate. A new enrichment plant is expected to produce weapons-grade uranium.

On the plus side, a number of states previously thought to have nuclear ambitions no longer present an imminent threat. Under American pressure, South Korea did not purchase a reprocessing capability in the early 1970s. Similarly, Taiwan did not follow through on an initial decision to ``go nuclear.''

Iraq's Osirak reactor, which could have been used to produce fissile material, was destroyed by Israel before completion and has not been rebuilt. Iran no longer has the organization or resources. Libya's attempts to purchase a weapon have thus far failed. And despite the widespread presence of fissile material or actual weapons in power plants, military installations, and academic programs, no terrorist organization is known to have obtained a usable nuclear device or weapons-grade material.

The proliferation record, in short, is mixed. While a near-hiatus in the spread of openly declared nuclear arsenals is certainly a positive sign, the incessant spread of latent capabilities to produce weapons is not. And a few states are poised to take the plunge into a full nuclear program, should their interests be served by such an action.

The writer was a government official for two decades before becoming a consultant on international affairs.