The United States and the International Atomic Energy Agency (IAEA) say between five and eight kg of nuclear material is needed to make a bomb, a threshold environmentalists describe as dangerously out of date.
Asked whether one kg would be enough to manufacture a nuclear device, an IAEA spokesman said: "Yes, theoretically, if you were extremely experienced and wanted only a small bang, and had sophisticated technology. But that amount is very small indeed," he said.
Fears that a non-nuclear country could acquire bombs-grade radioactive material have rocketed following the seizure of two batches of deadly plutonium-239 in Germany. Bonn says they came from Russia.
The Natural Resources Defence Council said the IAEA and other nuclear authorities were overestimating the amount of plutonium needed to make a bomb.
"From a technical standpoint, I guess there are people in the United States in weapons laboratories, who, if they were told they had to make some form of weapon with far less (nuclear material), could produce it," the IAEA spokesman said. "But that's a different kettle of fish from suggesting that a country or a group with an ambition to have a first nuclear weapon could do so with such a small quantity," he added. "It's not something you can do in a basement laboratory."
He said attempts by Iraqi President Saddam Hussein to build nuclear weapons were a good example of how difficult it was for a state without top nuclear know-how to produce a deliverable bomb.
Saddam had resources and cash to pour into a nuclear programme, but even after 10 years failed gather all three crucial elements needed for a bomb -- enough fissile material, a delivery system and detonation technology.
In its report, the Natural Resources Defence Council said a one-kiloton bomb, powerful enough to cause severe blast damage over a 40-block urban area and kill thousands with fire and radiation, could be made with as little as one kg of plutonium.
Christopher Paine, co-author of the report, said experts had known for 40 years that a bomb could be made with little material, and official thresholds were too high. "The criteria now in use are out of date, technically erroneous and clearly dangerous in light of the recent seizures of stolen Russian nuclear materials for sale on the black market," the report said.
But the IAEA spokesman said less developed states without a huge technical backup were estimated to need at least eight kg of plutonium or 25 kg of enriched uranium. "Non-proliferation efforts and the whole question of trying to head off anybody with nuclear ambitions from getting a quantity of material are predicated on the fact that these people don't have 30 years of nuclear experience under their belt," he said.
That is why Vienna-based IAEA inspectors set the benchmark at eight kg. Only the 120 governments which belong to the U.N. nuclear agency could change the limit, he added. The IAEA is mandated to inspect civil nuclear production but lacks powers to oversee military production in the five declared nuclear states -- the United States, Russia, China, Britain and France.
Monitoring gram amounts of nuclear material would be very expensive in terms of manpower and would involve inspections more intrusive than some countries would put up with, the IAEA spokesman said.