A little before dawn on a recent summer morning, a convoy of three large blue lorries, a handful of police cars and a bus rumbled along the dual carriageway heading north out of the Bulgarian capital, Sofia. Even if it had not been so early, the motorcade would probably not have drawn much attention. The lorries were unmarked, the bus carrying a few sleepy policemen was old and scruffy, while the lumbering shipment was big and slow enough to explain the escort and its flashing blue lights.
     But for Bulgaria, and indirectly for the rest of us, the convoy's progress marked an important transition - the departure of the country's last remaining stockpile of High Enriched Uranium (HEU), the stuff of which nuclear bombs are made. It took two years of talks and preparatory work before the highly radioactive material - just over 6kg of spent fuel from a defunct research reactor - was fished out of the storage pools in which it had lain unused and largely forgotten for nearly 20 years. It was sealed in steel casks - custom-made by ...Skoda, the Czech car manufacturer - and lowered into the three anonymous blue trucks.
     Over the course of the morning of July 5, the convoy made its way over the mountains and down to the banks of the river Danube, where the containers were winched into a long, black barge bound for Ukraine. Ten days and a rail journey later, the HEU arrived in Russia, whence it had come nearly half a century earlier as a gift. In Chelyabinsk, just east of the Urals, it is to be reprocessed or blended down. I was permitted to witness its secret journey on condition that nothing was printed until the shipment reached its destination, and this small but massively lethal fragment of the cold war was made safe.
     The operation was the latest in a series - part of an accelerating scramble to clean up the scattered legacy of nuclear profligacy. In the 50s and 60s, the US and Soviet Union exported HEU-powered nuclear reactors to their allies for power generation and experimentation. When the cold war finally came to an end, deals were done on dismantling the redundant weapons in the former Soviet republics. Under a programme called Megatons To Megawatts, one tenth of America's electricity is generated from uranium from thousands of former Soviet warheads. The Russians blend down HEU in the warheads to Low Enriched Uranium (LEU), which cannot be used in bombs; the Americans transport it to nuclear power stations back home.
     However, that still left thousands of kilograms of weapons-grade material, mostly HEU, in civilian reactors at power stations and universities around the world, some with no more security than a watchman and a padlock.
     As the logic of deterrence and mutually assured destruction recedes from the collective memory to be supplanted by the fears evoked by the September 11 attacks, nuclear terrorism has emerged as the number one threat to western security. The suicidal extremist driving a crude nuclear device into the centre of a major city is now the ultimate nightmare. George Bush and Tony Blair went to war in Iraq with the ostensible aim of preventing Saddam's assumed nuclear stockpile falling into the hands of al-Qaida jihadists. Barack Obama has called nuclear terrorism "the greatest danger we face".
     The gap between rhetoric and effective action, however, is startling. The US has so far spent $648bn on the war in Iraq to eliminate a threat that never existed. The amount spent on removing fissile material from countries that actually do have the ingredients for a nuclear device has been paltry by comparison. The Global Threat Reduction Initiative (GTRI), launched in 2004 after previous efforts at eliminating the world's civilian stocks of fissile material had proved ineffectual, has an annual US budget of about $150m, or roughly what the US military spends in eight hours in Iraq. Britain allocates a respectable £36.5m a year to the programme, most of it on helping to secure nuclear sites in the former Soviet Union, but that still pales in comparison to the £1.7bn cost last year of keeping troops in Iraq.
     There is no question that the threat of nuclear terrorism is real. As long ago as 1998, Osama bin Laden declared it was a religious duty to acquire nuclear weapons "to terrorise the enemies of God". Just days before the September 11 attacks in 2001, the al-Qaida leader met a Pakistani delegation, including two retired nuclear scientists, in Kandahar. According to accounts of that meeting, Bin Laden expressed keen interest in how to build a bomb and was told it was technically quite simple - acquiring the fissile material was the main obstacle.
     Al-Qaida has spent years trying to overcome that problem. Western intelligence officials believe the organisation was cheated several times by middlemen claiming to have weapons-grade material for sale. An al-Qaida defector, Jamal Ahmed al-Fadl, has described helping Bin Laden clinch a deal in Sudan, his home country: Bin Laden paid $1.5m for a 30-inch cylinder supposedly containing South African uranium. The cylinder is widely believed to have been a fake.
     Ivan Ivanov, a Bulgarian businessman working for a Dubai-based building contractor, claims to have met Bin Laden in Pakistan in April 2001, and to have been approached the day after the meeting by an al-Qaida scientist who proposed a scheme for buying Bulgarian nuclear material. Ivanov says he turned down the deal, but Bulgaria has remained a source of concern for anyone worried about proliferation. The country's post-Soviet nuclear industry has been hit by one safety scandal after another, and the EU has taken the unusual step of cutting off funds because of the hold that corruption and organised crime have on the country's economy.
     The removal of Bulgaria's HEU last month was therefore more than a technicality. It represented the elimination of a significant threat. Officials at Sofia's Institute for Nuclear Research and Nuclear Energy, where the spent fuel had been stored since its reactor was shut down in 1989, claimed security had been beefed up in recent years, but the Institute was clearly suffering from years of neglect. Feral dogs chased each other through the bushes in the grounds around the reactor.
     The removal operation was overseen by a two-man American team, who represent the business end of the Global Threat Reduction Initiative. Their job is to fly around the world trying to persuade governments that it is in their long-term interests to part with their stocks of fissile material.
     The US duo are both immigrants. Andrew Bieniawski, a 41-year-old from South Africa, is in charge of the programme at the National Nuclear Security Administration (NNSA), the part of the US department of energy that is responsible for looking after the US nuclear arsenal, making sure the bombs still work and do not blow up unexpectedly. His deputy, Igor Bolshinsky, a former mine technician from the Ukrainian coal city of Donetsk, has responsibility for repatriating the Soviet-origin material and does most of the travelling and persuading - "Schmoozing for world peace," as Bolshinsky put it.
     "This is what keeps us on the road 70% of the time," he said. "We see different reactors in different conditions, and we know this removal has to be done very fast. We don't want to give more time to people who are preparing to steal this material."
     In Sofia, Bolshinsky had been in fine form, cajoling, joking and flattering Bulgarian officials in a mix of Russian and English. He was consequently stunned on the designated day of the uranium's departure when Bulgarian security - somewhat prickly over the arrival of this ebullient American employee in the nation's inner nuclear sanctum - ordered the uranium convoy to leave the institute an hour earlier than agreed, without bothering to tell Bolshinsky.
     In the past few years, Bolshinsky has overseen uranium removals from Libya to Vietnam, and has been tailed in the course of his duties by some of the world's most persistent secret policemen, but this stunt stretched his sense of humour to breaking point. "Not good, not good at all," he said emphatically when he turned up at the reactor at dawn to find the car park empty. He jumped into a car for a high-speed chase along a Bulgarian motorway, in pursuit of the missing uranium casks.
     He finally caught up with the convoy about 32km north of Sofia, parked on the hard shoulder of the motorway. Ivan Gorinov, the head of 'physical protection' at Bulgaria's Nuclear Regulatory Agency who appeared to have taken the decision to leave early, was unapologetic. "I don't care about the American taxpayer," Gorinov said defiantly, and a touch sulkily, when he was reminded that the exercise had been US-funded.