The former Soviet Union's decaying nuclear archipelago could yield more deadly secrets along the lines of the huge cache of weapons-grade uranium shipped from Kazakhstan to the United States earlier this week, a Western nuclear expert said Friday.


John Large, an independent British expert on the Russian nuclear industry, said it was impossible to pinpoint where all the nuclear materials from the Soviet Union had ended up because there had been no proper inventory system for them for more than 20 years.


As a result, he said, the announcement earlier this week that more than 600 kilograms of weapons-grade highly enriched uranium had been shipped from a poorly guarded factory in eastern Kazakhstan to the United States had come as a complete surprise.


"We thought we understood where they had everything," Large said. "Now we're understanding we're completely wrong"


American nuclear experts said there was enough high-quality material in the Kazakh shipment, which was flown to the United States in a top secret operation worthy of Cold War spy fiction, to make at least 20 nuclear weapons.


Kazakhstan has declared its intention to become a non-nuclear state and was paid an undisclosed sum for the cache. Western experts said they were worried that Iran might have been interested in buying the uranium, if the United States had not got there first.


The question now arises as to how many other insecure nuclear caches like the one in Kazakhstan lie undiscovered around the territory of the former Soviet Union. Russian officials have played down the episode and said it was an isolated instance.


Georgy Kaurov, spokesman for the Nuclear Power Ministry, said in a telephone interview Friday that the Kazakh cache was a one-off find because the factory at Ulba, where the uranium was produced to power nuclear warships, had been the only one of its type in the Soviet Union.


He said that his ministry had been aware of the cache "since the collapse of the Soviet Union" and rejected the U.S. assertion that it was weapons-grade.


He said there were no more such caches in the former Soviet republics.


But Large said there was no guarantee of this. He cited a report published by the Russian nuclear watchdog Gosatomnadzor in May which said that the last inventory of nuclear materials in the Soviet Union was carried out only in the 1970s.


"It has to be stated there is no state system of control and accounting of nuclear materials in Russia," the report stated bluntly.


Large said the closed nuclear city of Arzamas-16, where the first Soviet nuclear weapons were developed, had sent out specifications to nuclear weapons-making plants and nuclear reactors.


If the nuclear components the plants produced did not match up to the specifications they would simply reject them, creating huge stockpiles of nuclear materials at or near the factories.


But two Russian experts differed in their opinion with Large as to how widely scattered and how large these stockpiles were.


Vladimir Kuznetsov, former chief inspector for Gosatomnadzor, said that "we can't say that with complete assurance" that there was no high-grade radioactive material left in the ex-republics. But, he added that the quantities involved were likely to be small and the places to investigate were the nuclear research centers in each republic.


Large said it was also possible that some "shadow factories" had been involved in nuclear weapons production in the republics outside Russia.


One likely suspect he said was a military plant in Tashkent, which is now the capital of independent Uzbekistan.


Officially, factories making parts for the Soviet nuclear weapons program were situated only inside the borders of Russia.


Alexei Yablokov, the independent-minded environmental aide to President Boris Yeltsin's Security Council, said attention should be focused on Russia and not on the bordering republics.


In recent months the whole Russian nuclear system has come under close scrutiny, with the discovery in Germany of caches of weapons-grade radioactive material, apparently from Russia. Meanwhile, nuclear scientists are poorly paid and security has declined at nuclear bases.


One American official, who declined to be named, said the only thing guarding the Kazakh uranium when the American team arrived was a "large padlock."


Large said the speed and urgency with which Washington carried out its mission in Kazakhstan indicated just how seriously the Americans were taking the problem. He said that it was a symptom that things were worse than he had supposed them to be a month ago.


"What we are seeing is yet another degree of the collapse going on in the former Soviet Union," he said.