Russia's once mighty top-secret nuclear industry has become porous and over-stretched, allowing dangerous nuclear material to slip through its borders for illegal sale abroad, nuclear analysts said Wednesday in dismissing assurances to the contrary by top Russian officials.


Workers in the closed nuclear cities of Siberia and the Urals have gone unpaid for four months, while the Nuclear Power Ministry can no longer keep track of the amount of plutonium in Russia even as a plant in Siberia continues processing the highly toxic material as fuel for a nuclear reactor.


International alarm bells have begun ringing since the discovery of the largest-ever quantity of weapons-grade plutonium at Munich airport last week on a flight from Moscow.


A German government spokesman said Wednesday that President Boris Yeltsin had written to Chancellor Helmut Kohl pledging Moscow's help in fighting the trade in nuclear contraband, Reuters reported.


Kohl, in an interview on SAT-1 television, said he wants strong guarantees from Yeltsin that Moscow will intensify efforts to stop thefts, according to The Associated Press. He also said he would telephone the Russian president in the next few days to discuss nuclear smuggling from Russia.


Yet top Russian officials in the Nuclear Power Ministry and the Federal Counterintelligence Service have persisted in denying that there is any proof that the Munich material came from Russia, or that any weapons-grade plutonium had gone missing from Russian facilities.


Non-Russian experts questioned were in no doubt, however, saying the plutonium-239 could not have originated anywhere else.


A diplomatic source in Moscow who declined to be named painted a dismal picture Wednesday of the situation in the closed nuclear cities, Krasnoyarsk-26, Tomsk-7 and Chelyabinsk-65. He said that within the former Soviet Union only those three cities, home to some of the top Russian scientists, had plutonium separation plants sophisticated enough to produce the sample discovered in Munich.


Krasnoyarsk-26 is still turning out weapons-grade plutonium because it has no alternative fuel with which to heat the city through the Siberian winter Before, residents of these cities were known as the "chocolate eaters" because of the privileged lives they led. Now they are suffering the same chaos as the rest of the country.


"People aren't getting paid, living standards are going down the tubes," the diplomat said, estimating the average salary of a top scientist at 300,000 rubles ($143) a month.


In addition, the rigid control systems that once operated in the Soviet Union have broken down. Scientists who spent virtually their whole lives in the closed cities can now travel much more freely.


"The system of terror which used to operate in the old days is gone," the diplomat said.


Yury Vishnevsky, head of the small atomic energy inspectorate Gosatomnadzor, told a press conference in February that, despite presidential decrees, the Defense and Nuclear Power ministries had refused to submit their facilities to outside inspection.


The Nuclear Power Ministry still employs 1 million workers, the same as it did at the height of the Cold War.


The fear is that some employees are now turning to crime to make money.


Greenpeace in Moscow said Wednesday that they had logged 21 thefts of radioactive material in 1993 and 1994 in Russia, relying on official sources.


The amounts that have come to light so far have fallen well short of the 8 kilograms of plutonium the International Atomic Energy Authority estimates is needed to manufacture a small nuclear bomb.


"Even in the German case, with one exception, the amount has been very, very small," said David Kyd of the IAEA in an interview on Wednesday. He said the thefts looked like the work of petty thieves, who stole the plutonium in powder form from laboratories because it was "easier to pilfer."


But the trend is upwards. Last week's find uncovered the biggest quantity yet of the material, more than 300 grams. The Colombian caught carrying the Munich sample, Justiniano Torres, was reportedly offering to sell 4 kilograms of plutonium-239 for $250 million.


The center of attention at the moment is Germany. John Large, an independent British nuclear expert, said Germany was probably "the center of the spider's web" of the illegal trade because of its international metal markets.


In addition, it is the only Western country of which part used to come within the Soviet sphere of influence.


"There is a nuclear mafia which is now largely situated in the German-speaking area," an anonymous undercover police agent told German television news, according to Reuters.


But after Germany the trail goes dead. The most likely buyers, analysts said, were nations such as North Korea, Libya, Iraq and Iran, which are suspected of trying to develop nuclear weapons, but there is no proof of this.


Large said the thefts were effectively undermining international effort to monitor illegal nuclear programs.


No one knows either how much radioactive material may be being stolen and sold undetected.


"There's no way I could put a statistic on that," the diplomat said, adding that anyone's guess was as good as his.