The doomsday scenario of Soviet nukes falling into the hands of rogue states or terrorists has, as far as is known, remained fiction, thanks to a massive U.S.-Russian effort to lock the weaponry up safely after the Soviet Union fell apart.

The vast nuclear arsenal, scattered among several newly independent nations, was secured because Russian military officers acted with professionalism and honesty, Moscow and Washington shared clear priorities, and the U.S. taxpayer coughed up billions of dollars, former top officials who dealt with the Soviet nuclear legacy say.

Even so, as the world marks the 20th anniversary of the Soviet demise at the end of 1991, occasional doubts surface about whether the system was airtight. There's the Russian scientist who perhaps went to work for Iran's nuclear program, an old claim that portable nuclear devices went astray, the seizures of smuggled fissile material in the 1990s.

But difficult though it is to prove a nuclear negative, U.S. and Russian officials insist in interviews that the fears of the 1990s have not become a reality, even though the challenges of safeguarding Soviet nukes were daunting at the time.

"Twenty years on it's pretty hard to believe that not a single nuclear weapon has shown up loose," said Graham Allison, who played a key role in the effort as an assistant secretary of defense under President Bill Clinton and now heads Harvard's Belfer Center for Science and International Affairs.

A quick U.S.-sponsored deal had Belarus, Ukraine and Kazakhstan handing all their nukes over to Russia, and American cash helped safeguard the weapons at a time when the new governments couldn't even afford to pay military wages on time.
Additional U.S. incentives offered jobs to disgruntled nuclear scientists from the former Soviet Union, many of whom were courted by nations like Iran.

There have been gnawing fears that a few Soviet nukes still might have gone missing, but experts with inside knowledge say that if it were true the world would already know.

"If somebody or a terrorist group got hold of a nuclear weapon, they would probably use it as quickly as possible," said Steven Pifer, who served as U.S. ambassador to Ukraine, held other senior State Department posts and is now director of the Brookings Institute's Arms Control Initiative.

"So the fact that you haven't seen a nuclear detonation ... reflects the fact that the nuclear weapons have been maintained in a secure way."

That was no mean achievement given the enormous proliferation risks posed by the Soviet breakup.

The economic meltdown of the early 1990s forced many officers of the once-proud Soviet Army to moonlight as security guards or even cab drivers. And with the wars and ethnic clashes triggered by the Soviet collapse came strong incentives to steal weapons for the black market.

The immediate task for the Russian military was to quickly remove thousands of battlefield weapons such as nuclear artillery shells and land mines from other Soviet nations. These relatively compact arms posed the biggest proliferation risk and often were stored close to areas of conflict.

"The military officers who did the job were the unknown heroes," said Alexander Golts, a Russian independent analyst.

"It's hard to imagine what might have happened if the tactical nuclear weapons had remained on the territories of the states involved in military conflicts."

The next goal, strongly backed by Washington, was to remove strategic nuclear weapons from Belarus, Kazakhstan and Ukraine. The first two agreed quickly, but Ukraine, which had inherited enough of the Soviet arsenal to be the world's third largest nuclear power, balked at the plan, setting the stage for years of diplomatic battles.

A war for custody of nukes? "All this was quite terrifying," said Allison.

Pifer said that some Ukrainian officials longed to keep them, but around 1992 concluded their country had neither the money nor the expertise to remain a nuclear power.

Also, the world's worst nuclear disaster had happened in 1986 at Chernobyl, in Ukraine, and public opinion wasn't keen on keeping nukes.

Still, Ukraine bargained for years for compensation in tough talks that sometimes made even seasoned diplomats lose their temper.

"There was a lot of pressure, they threatened us with all kinds of economic sanctions, they wanted to get this issue over with fast," Leonid Kravchuk, Ukraine's then president, said.

Ukraine insisted the U.S. provide hundreds of millions of dollars to pay for safeguarding and dismantling the arsenal. From Russia it demanded nuclear fuel as compensation for the highly enriched uranium in the warheads. And it wanted security guarantees from all the nuclear powers.

"We didn't want to get naked for free," Kravchuk said.

Tensions over which country military officers in Ukraine should swear allegiance to - Russia or Ukraine - also stoked tensions. In February, 1992 an entire squadron of combat jets flew from Ukraine to Russia after their pilots refused to take the oath.

Ukraine eventually got the money and security guarantees it was seeking, but the Russians had other obstacles to overcome. For instance, the economy was so bad that the military struggled to pay wages on time, and top brass were reduced to struggling to give the strategic nuclear forces personnel better rations, said Maj.-Gen. (Ret.) Vladimir Dvorkin, a nuclear weapons expert in the Russian Defense Ministry in the early 1990s.

Control over the security of nuclear weapons never slackened, Dvorkin said. "People realised their responsibility because they were fully aware of the dangers."

Nuclear arsenals surrendered by former Soviet republics had to be safely transported long distances to centralised storage sites and secured. Dismantling missiles, bombers and submarines as required by the 1991 START treaty with the U.S. also required huge funds.

"Russia badly needed assistance," Dvorkin said, and the U.S. responded quickly with the Cooperative Threat Reduction Program initiated by Sens. Sam Nunn and Richard Lugar, which provided billions of dollars in equipment and know-how to help Russia and its neighbors deal with the Soviet nuclear legacy.

"It seems to me that Nunn-Lugar was one of the smartest uses of defense dollars we ever made," Pifer said.

Under the program, the U.S. provided reinforced rail cars to carry nuclear warheads, high-tech security systems for storage sites and dismantling mothballed nuclear subs.

"The program provided colossal support," Dvorkin said.