Intelligence provided to U.N. nuclear officials shows that Iran’s government has mastered the critical steps needed to build a nuclear weapon, receiving assistance from foreign scientists to overcome key technical hurdles, according to Western diplomats and nuclear experts briefed on the findings.

Documents and other records provide new details on the role played by a former Soviet weapons scientist who allegedly tutored Iranians over several years on building high-precision detonators of the kind used to trigger a nuclear chain reaction, the officials and experts said. Crucial technology linked to experts in Pakistan and North Korea also helped propel Iran to the threshold of nuclear capability, they added…

According to Albright, one key breakthrough that has not been publicly described was Iran’s success in obtaining design information for a device known as a R265 generator. The device is a hemispherical aluminum shell that is lined with pellets of high explosives and electrically wired so the detonations occur in split-second precision. The explosions compress a small sphere of enriched uranium or plutonium to trigger a nuclear chain reaction.

Creating such a device is a formidable technical challenge, and Iran needed outside assistance in designing the generator and testing its performance, Albright said.