Small amounts of uranium and plutonium of suspected Soviet origin have cropped up on the black market in at least four European countries in recent months, investigators say.

In these cases, the sellers tried to pass off virtually useless radioactive material as weapons-grade goods at astronomical prices.Some experts say the cases are black-market scams by hustlers capitalizing on what they believe is a booming market for former Soviet nuclear technology.

But other European officials fear they represent the first tiny cracks in the former Soviet Union's nuclear security, the greatest danger posed by the superpower's breakup.

"We don't like it. There seems to be a somewhat higher frequency," said Wilhelm Gmelin, director of safeguards for Euratom, the European Community's nuclear watchdog treaty agency. "We are concerned that with the situation in the former Soviet Union, this situation may get worse."

On March 5, German police arrested two former Soviet emigres who tried to sell about 2 pounds of slightly enriched uranium pellets for $1.16 million in the Bavarian city of Augsburg.

"Although the confiscated material is relatively harmless from a weapons standpoint, it is still an incident seen with great concern," German Environment Minister Klaus Toepfer said.

The uranium was found in a lead-lined container in the trunk of a Mercedes, capping an undercover sting operation by Bavarian police.

The material itself was fairly typical of the recent cases. Only 2 percent to 3 percent was weapons-grade uranium-235, said Wolfgang Thomas, a German physicist and nuclear expert.

Uranium of 90 percent enrichment - 20 pounds of it - is needed to make a nuclear bomb, said Thomas, who is assisting authorities in the case.

He said the seized material appears to be impure "scrap" that had been used in fuel processing at a light-water reactor.

Wilma Resenscheck, head of the Bavarian prosecutor's office in Augsburg,said it is believed to have come from Russia or Kazakhstan.

She said the two suspects have given very little information and remain in custody pending further investigation.

Germany and the United States have discussed ways to keep nuclear hardware and expertise from leaking out of the unstable Soviet states.

The Vienna-based International Atomic Energy Agency says it has been monitoring the various incidents of radioactive peddling.

"There have been many more cases since the change of political situation in eastern Europe," said agency spokesman Hans Friedrich Meyer.

Meyer stressed, however, that the uranium in each case is of low radiation and the plutonium is of tiny quantities.

"It was (peddled by) very unprofessional people - in most cases they didn't have a clue as to the value of the material and demanded enormously high prices," he said. "In all cases we have seen the material has proven completely worthless for a nuclear explosion."

But Gmelin said there is "clearly a great contamination potential."

In Zurich, Switzerland, police on March 23 arrested two Austrians carrying an attache case holding a pound of material containing very small amounts of plutonium.

Meyer said the plutonium is believed to have come from an old Soviet smoke detector and was potent enough only to power a pacemaker.

He said Hungarian authorities last September reported confiscating about a pound of defective uranium dioxide pellets that had been stolen from a fuel fabrication plant in Romania.

Also, the agency said a tiny amount of plutonium - .0000105 of an ounce - of Russian origin was seized last December in Austria. Meyer said that, too, came from a Soviet smoke detector.

In October 1991, a similar amount of plutonium with Russian markings was seized by customs officials in the Italian province of Como, Meyer said.

On Nov. 11, Zurich police arrested seven people carrying 66 pounds of slightly enriched uranium. Among the arrested was the Honduran consul, Friedrich Renfer.

Police said Renfer had been sitting on the material for several years. Its origin has not been determined, although police suspect its source is the East.

"You couldn't use this for anything," said Beat Kuenzli, a Zurich district attorney. "They came up with fantasy numbers, saying they wanted to sell it for $90 million. But it's actually worth about 1,000 Swiss francs ($670)."