WASHINGTON -- Since the collapse of the Soviet Union in 1991, the West has watched nervously for signs that Moscow's formidable nuclear stockpile was leaking into outlaw hands.

Now, the nightmare is beginning to come true. Since May, German authorities have made four seizures of smuggled nuclear material. The latest -- a 0.07 ounces of plutonium -- was seized Saturday with the arrest of a German.

But the most alarming seizure was 10.5 ounces of weapons-grade plutonium that authorities said was brought by a Colombian last Wednesday on a flight to Munich from Moscow.

Experts say that four to 13 pounds are needed to make a small nuclear bomb.

Nuclear weapons aren't the only threat posed by the smuggled plutonium. Powdered plutonium cannot penetrate skin, but the tiniest grain can cause cancer if swallowed or inhaled.

This makes its sale doubly threatening -- as a bomb-making material for governments with nuclear ambitions, such as Libya, Syria, Iraq and Iran, and as a terrorist weapon in water or air supplies.

The International Atomic Energy Agency in Vienna has logged more than 80 "significant" cases of plutonium smuggling since November 1991. But there have been dozens of other cases of fraudulent offers of fissionable material. The German authorities FTC reported 241 cases last year of people claiming to have plutonium for sale; 118 of them were scams. The other 123 cases involved small but genuine samples of fissionable materials.

The full picture is far from clear. Officials don't know whether the suspects had access to far larger quantities than they were carrying.

'We're alarmed'

"We're alarmed," a U.S. official involved in nuclear affairs acknowledged yesterday. "We're not going to soft-pedal that."

The United States and its allies have been working to secure the former Soviet Union's far-flung military and civilian nuclear operations.

The United States has paid for the dismantling and storage of weapons and for the employment of nuclear scientists in non-military jobs. Washington has also arranged visits to one another's nuclear research labs and storage sites. It recently set up an FBI office in Moscow to keep an eye out for illegal sales of nuclear materials.

The United States also has earmarked $10 million to help Russia account for and control nuclear material removed from warheads.

But bureaucracy moves slowly. The Clinton administration says Russia has far to go in improving its accounting of and control over nuclear material.

Moreover, the administration has at times encountered resistance from the secretive and proud Russian establishment.

This was evident yesterday when Russia persisted in denying that it was the source of the plutonium involved in the four recent cases. One official, Vladimir Tomarovsky, a spokesman for the counterintelligence service, accused the West of a propaganda campaign.

The smuggling is a symptom of a collapsing society, where once-prized military officers, bureaucrats and scientists find their earnings diminished as an estimated 5,700 criminal groups spread bribes and fear.

Nuclear weapons sites themselves are guarded by elite troops. Manufacturing plants, where much nuclear fuel is stored, generally also have good security, according to Leonard S. Spector, a proliferation expert at the Carnegie Endowment for International Peace.

The most likely source of smuggled nuclear fuel is the dozens of research facilities scattered around the former Soviet Union, some of which have enough material to produce a bomb.

The research sites generally have been protected only by local militia, Mr. Spector said. Security at one Moscow site, the All-Union Research Institute, was so poor that the Russian government closed it this year.

"The Russians certainly have many years of experience of protecting these materials, but because there are so many materials, and the amounts are so great, and because the society is in a state of turmoil, it might be more of a problem there these days," said Oleg Bukharin, a Russian physicist who is a visiting fellow at Princeton University.

Mr. Bukharin said the danger could be solved by "stabilizing" Russia's nuclear facilities by paying the workers decent wages and using technology, such as personnel monitors at laboratory exits. He also called for regulatory controls and training to monitor fissionable materials.

From civilian labs

Most of the smuggled plutonium has been in the form of oxide powder, suggesting that it is from civil research labs rather than from weapons programs, according to David Kyd of the International Atomic Energy Authority. Russia, France and Japan are the only countries that use plutonium as fuel in civilian nuclear programs.

Most of the cases have involved tiny amounts of weapons-grade plutonium, with promises of more later.

"That is the traditional come-on: 'Give me good money for this tiny sample, and I will be back next week with more,' " Mr. Kyd of the IAEA said. "Normally, you never see the guy again."

Germany has been the most frequent interception point. Cases also have cropped up in former Czechoslovakia, Scandinavia, and Turkey.

Why Germany? One theory is that the German police, with its intelligence connections to the former East Bloc, is superior at detecting smuggling.

Another is that by indulging in so many "sting" operations involving nuclear contraband, the German authorities are actually stimulating efforts to sell it. Germany also is a transportation hub within easy reach of Eastern Europe.

German media also speculate that officers of the former East Germany's Stasi secret police are in cahoots with their one-time brethren in Russia's security services.

The most serious recent case involved the 10.5 ounces of plutonium 239, which is used for building bombs. There is dispute over how much plutonium it takes to build a bomb -- ranging from about four pounds to 13 pounds.

40,000 casualties

But according to Christopher Paine, a senior research associate at the Natural Resources Defense Council and a former congressional expert on nuclear proliferation, the smallest bomb could spread a plutonium cloud, knock down New York's World Trade Center and kill about 40,000 people.

The powdered plutonium found in Germany last week was transported in a lead container in the suitcase of a Colombian resident of Moscow. Plutonium emits mainly alpha radiation, which cannot be detected by a Geiger counter. The small amount of radiation it emits would be blocked by the lead container, making it difficult to detect. But the lead should have been picked up by airport security screening in Moscow. It apparently was not.

German police, tipped off about the shipment by German intelligence, arranged with two Spanish go-betweens to examine an initial sample of a fraction of an ounce of the plutonium. The Spaniards told the Germans that the Colombian, Justiniano Torres, would be arriving on a flight from Moscow with 1.1 pounds of the fuel.

A team of German specialists was at the airport when Mr. Torres landed. His suitcase was emitting a small amount of gamma radiation, enough for detection but insufficient to cause harm to passengers on the flight, according to German authorities. The lead container contained a mixture of oxide of plutonium 239 and uranium, suggesting that it was from a breeder-reactor research program.