Iran has rejected most of the IAEA material on weaponisation as forgeries, but has admitted carrying out tests on multiple high-explosive detonations synchronised to within a microsecond. Tehran has told the agency that there is a civilian application for such tests, but has so far not provided any evidence for them.
Western weapons experts say there are no such civilian applications, but the use of co-ordinated detonations in nuclear warheads is well known. They compress the fissile core, or pit, of the warhead until it reaches critical mass.
A US national intelligence estimate two years ago said that Iran had explored nuclear warhead design for several years but had probably stopped in 2003. British, French and German officials have said they believe weaponisation continued after that date and may still be continuing.
In September, a German court found a German-Iranian businessman, Mohsen Vanaki, guilty of brokering the sale of dual-use equipment with possible applications in developing nuclear weapons. The equipment included specialised high-speed cameras, of the sort used to develop implosion devices, as well as radiation detectors. According to a report by the Institute for Science and International Security, the German foreign intelligence service, the Bundesnachrichtendienst, testified at the trial that there was evidence that Iran’s weapons development was continuing.
The IAEA is seeking to find out what the scientists and the institutions involved in the experiments are doing now, but has so far not been given a response. The agency’s repeated requests to interview Mohsen Fakhrizadeh, whose name features heavily in the IAEA’s documentation and who is widely seen as the father of the Iranian nuclear programme, have been turned down.
The agency has also asked Iran to explain evidence that a Russian weapons expert helped Iranian technicians to master synchronised high-explosive detonations.
The first implosion devices, like the “Fat Man” bomb dropped on Nagasaki on 9 August 1945, used 32 high-explosive hexagons and pentagons arrayed around a plutonium core like the panels of a football. The IAEA has a five-page document describing experimentation on such a hemispherical array of explosives.
According to a diplomat familiar with the IAEA documentation, the evidence also points to experiments with a two-point detonation system that represents “a more elegant solution” to the challenges of making a nuclear warhead, but it is much harder to achieve. It is used in conjunction with a non-spherical pit, in the shape of a rugby ball, or explosives in that shape wrapped around a spherical pit, and it works by compressing the pit from both ends.The IAEA has expressed “serious concern” about Iran’s failure to give an account of the research its scientists have carried out.
Descriptions of “two-point implosion” warheads designs have occasionally appeared in the public domain (there are extensive descriptions on Wikipedia) and they were first developed by US scientists in the 1950s, but it remains an offence for American officials or even non-governmental nuclear experts with security clearance to discuss them.