Most A-Bomb Parts Are Easily Available : Weapons: Assembling components is harder. But activists warn that only the difficulty of getting plutonium or uranium slows proliferation.

On the face of it, building a nuclear bomb does not seem that difficult.

Most of the components are readily available on the commercial market, similar to those used in photocopying machines, lasers and even some ambulance lights. Much of the technology can be researched right in the public library.

But as Thursday’s indictments of four Iraqis and a Frenchwoman show, assembling a nuclear weapon still is far from easy, and it requires skillful deception to evade an array of regulations and international police surveillance.

Pakistan tried in 1984, and was caught. Israel followed the next year, and had to return half the 800 high-technology switching devices that it had bought. This week it was the Iraqis’ turn to face an international sting effort.

Even so, anti-nuclear activists warn that it is mainly the difficulty of obtaining the nuclear material itself--the plutonium or highly enriched uranium that creates the actual explosion--that stands between would-be bomb-makers and the nuclear devastation that everyone fears.

Everything else is relatively easy to get, either by smuggling or by falsifying export applications. West Germany, France and other European countries often look the other way at illegal shipments, and even U.S. authorities rely mainly on tips from manufacturers.

So far, the world has been very lucky that terrorists have not been successful in building and detonating a nuclear bomb, said Milton Hoenig, scientific director of the Nuclear Control Institute in New York. “There’s sufficient information in the open literature,” Hoenig said.

And David Albright, a physicist with the Foundation of American Scientists in Washington, said that even the nuclear material might be vulnerable to theft. “The safeguards system is so weak that it’s possible that the plutonium could be stolen and no one would know it,” fretted Albright.

Except for the uranium or plutonium, a nuclear bomb is considered simple enough for those with sufficient technical ability: Start with a neutron initiator--to fire neutrons and start the chain reaction. Add high explosives and a tamper to hold the material together. Put it all in a casing, and it’s ready to go.

The snag is in how sophisticated the nuclear device is intended to be. To deliver the payload accurately and obtain a high-yield explosion, it is necessary to have higher-technology parts that are more difficult to buy.

In theory, the Western industrial nations restrict the export of nuclear materials and technology. The International Atomic Energy Agency keeps tabs on uranium and plutonium. A 17-country coordinating committee, known as Cocom, sets rules limiting sales of technology abroad.

Albright of the Scientists’ Foundation says the countries that have the bomb generally have been careful not to provide plutonium and uranium to those that do not. But the export of igniter explosives and components has not been that closely guarded.

Dan Hirsch, a nuclear weapons expert at the Committee to Bridge the Gap, a Los Angeles anti-nuclear organization, said the University of California press even has published books containing calculations of scientists at the Los Alamos, N.M., nuclear weapons facility, providing specifications for the explosives.

Indeed, much of the United States’ enforcement depends on tips from manufacturers--as was the case in this week’s Iraqi sting--and on electronic eavesdropping of telephone calls and telex messages.

“Random checks by agents just don’t do the job,” said Paul Freedenberg, a former undersecretary of commerce for export administration.

But Hirsch said that even the supply of plutonium and weapons-grade uranium is becoming more readily available. The United States has shipped some 24 metric tons of it worldwide, ostensibly for nuclear reactors.

And Hirsch fears that “emerging suppliers” such as Brazil and Argentina are beginning to become a force.

Gary Milhollin, director of the Washington-based Wisconsin Project on Nuclear Arms Control, said the Iraqi incident underscores the need for the industrialized nations to take new steps to crack down on abuses in the nuclear weapons field.

“All we have is our own export laws, and the often-inadequate laws of our allies,” Milhollin said. “If we’d trade in some of our B-2 bombers for more detectives and agents, we’d get a lot more security for ourselves.”