NUKES ON THE LOOSE

The uranium is then sold to members of the latest of Japan's "new religions," and taken to the cult's secret underground laboratories in the mountains near Niigata. There the group's "ministry of science," assisted by two renegade Russian weapons technicians, fashion a crude nuclear bomb similar to the one dropped on Hiroshima. They load it into a pizza delivery van, and park it in the crowded Ginza district. Before a policeman can order the vehicle to move, a conventional charge goes off and in a nano-second the uranium forms a critical mass, destroying one square kilometer of downtown Tokyo.

Some people believe it is only a matter of time before such a scenario is played out in a city somewhere in the world. It wasn't supposed to work out that way. The threat of nuclear Armageddon was thought to have subsided with the collapse of the Berlin Wall and the end of the Cold War. Superpowers Russia and America have begun to retire some of their vast numbers of missiles and nuclear warheads. Strategic bombers no longer routinely patrol the skies loaded with megatonnage capable of destroying enemy targets at a moment's warning. Despite the last-minute flurry of testing by China and France, the nuclear-weapons states are poised to sign a treaty banning further explosions, possibly next year. So why is nuclear terror creeping back out of the closet?

The end of the Cold War has made it more - not less - likely that one of these fearsome weapons will go off someday, killing hundreds of thousands of innocent people. The superpower stand-off at least achieved a balance of terror that discouraged both sides from launching a nuclear war. Today, more countries have acquired nuclear bombs, or are getting close to building them. Proliferation has increased the possibility that freelance terrorists might get their hands on weapons-grade nuclear material. Adding to the danger, the breakup of the old Soviet Union has put the world's largest nuclear arsenal at risk.

Last week Moscow sent troops to an electric power station in the Kola Peninsula to stop authorities from cutting off electricity to a submarine base because the bills were unpaid. In August the crew of a Russian nuclear submarine refused to go to sea unless it was paid. In Washington, experts recently told the U.S. Senate Committee on Foreign Relations that poorly guarded nuclear weapons in Russia - "loose nukes" - was one of America's most serious security threats. In April, President Bill Clinton plans to visit Moscow for an unprecedented nuclear summit meeting on safeguarding fissionable material and preventing nuclear weapons proliferation.

Japan's Aum Shinrikyo sect is the type of organization that might be tempted to try a low-level nuclear assault. The criminal cult is believed responsible for the sarin nerve gas attacks in March that killed 12 people in Tokyo and injured more than 5,000. It was the first time terrorists have used a weapon of mass destruction. "If you have a sect that is reasonably rich and wants to cause mischief, they just might be able to buy a nuclear weapon," says Fred Ikle, at the Center for Strategic and International Studies in Washington.

The sect's propaganda was filled with references to a nuclear holocaust that guru Asahara Shokou believed would occur between 1999 and 2003. Their sophisticated laboratory near Mt. Fuji was part bomb shelter, part weapons factory. Written in the notebook of the cult's "defense minister," Kibe Tetsuya, were such terrifying notations as "nuclear head - how much?" followed by "$200,000 for a second-hand and $1 million for a brand new one." Police confiscated documents concerning uranium enrichment technology when they raided the sect's headquarters.

The group also had connections with Russia, where it claimed to have 30,000 adherents, supposedly including scientists, police and soldiers. The cult owned a Russian-made helicopter and had made vague references to buying Russian weapons or brokering arms deals. Moscow officials claim these membership figures are exaggerated and insist that there is no evidence linking the Tokyo gas attacks to any of the group's activities in Russia. The ingredients for sarin gas were legally available in Japan.

Still, while the technology is sophisticated and the material restricted, it is not impossible for a terrorist group to assemble a one-kiloton nuclear bomb that could fit in a van like the one used to destroy the Oklahoma City Federal Building earlier this year. A fission bomb can be made using either plutonium-239 or uranium- 235 as the explosive charge. For an explosion to occur, there must be enough material to form a critical mass. The first A-bomb dropped on Hiroshima consisted of two lumps of uranium, each below critical mass, that were brought together by high explosives. The bomb that destroyed Nagasaki held a single lump of plutonium. It was surrounded by high explosives, which compress the plutonium uniformly, creating the critical mass.

The greatest obstacle facing a would-be bombmaker is obtaining enough pure uranium-235 or plutonium-239. Their best option would be to steal the stuff, which is why many worry about chaotic conditions in the former Soviet Union. While nuclear weapons are closely guarded, there are other potential sources, such as laboratories, not so well maintained. For example, the eight nuclear-powered icebreakers in Russia's Northern Fleet use uranium fuel enriched up to 90% - weapons-grade level - as do some other naval reactors.

Russian authorities deny that their country leaks weapons materials from all sides. They claim that all of the cases of nuclear smuggling were schemes by the German Federal Intelligence Service intended to discredit Russia's nuclear power industry. "No source has been identified for the smuggled plutonium," said Georgy Kaurov, a spokesman for the Russian Nuclear Power Ministry, referring to the 363 kg found in the back seat of a car in Munich last year. But he agrees that security in power reactors has been found wanting in some cases. Others are more blunt: "Potatoes are guarded better," claims William Potter of the Monterey Institute of International Studies.

Once you have the raw material, what do you do with it? For the amateur it is easier to make a bomb out of uranium, but it takes as much as 30 kg. Plutonium bombs are harder to make work, but require less material. The International Atomic Energy Agency claims 8 kg is the minimum, but some experts think this is deliberate disinformation designed to deter would-be proliferators, and that as little as 3 kg will suffice. The addition of sophisticated enhancers such as beryllium neutron reflectors can boost the yield and make it more certain that the bomb will go off.

Western experts also fret that Russia's nuclear scientists may be prime recruiting targets for countries seeking a short cut to nuclear weapons. The U.S. government has targeted Iran, Iraq, North Korea, Syria and Libya as rogue states that have attempted to go nuclear. So far there is no hard evidence that any Russian technician has gone abroad as a nuclear adviser. More than three years ago, the Russian secret police did block a group of engineers from fulfilling a contract with Pyongyang, but their work appeared to be related to rocket and missile design, not nuclear weapons.

Some experts downplay the threat of nuclear terrorism. "I think that most analysts would admit privately that they don't believe that a terrorist group could build a bomb even if they got the material for it," says Bruce Blair, Senior Fellow at the Washington-based Brookings Institute. The bigger worry, he says "are the 10,000 nuclear war heads that are still sitting on the tops of ballistic missiles." Known smuggling cases are few, he maintains.

Indeed, after a spate of worrisome smuggling cases last year, the flow of nuclear contraband from the former Soviet Union seemed to have halted. Criminals bent on making money might choose to smuggle other profitable contraband, such as narcotics, rather than one sure to arouse intense scrutiny from Western security services. Still, the greatest danger is not from the known smuggling attempts, but from the plot that remains concealed until that horrible day when the innocent-looking van parked in the city center turns out to be a doomsday weapon.