Iran almost certainly knows how to build a miniaturized nuclear device. China provided Pakistan with information about a nuclear weapon small enough to fit on a DF-2 missile, which the Pakistanis further miniaturized and tested in 1998. Authorities breaking up the Khan network found copies of Pakistani designs in Libya and Switzerland. As customers of the Khan network, Iran and North Korea almost certainly have access to this information. (Iran has already provided the IAEA with one weaponization-related document supplied through the Khan network.) Iran also has access to former Soviet nuclear weapons scientists, including Vyacheslav Danilenko, who seems to have described a rather slim detonator arrangement that is useful for a variety of industrial applications involving shock implosion, such as producing nanodiamonds and digging canals.
There is no fundamental secret to building a miniaturized device. Most of the important innovations are well documented in the public literature. I am sure there are Farsi translations of the Los Alamos Primer and the Smyth Report, Chuck Hansen's U.S. Nuclear Weapons: The Secret History, John McPhee's Curve of Binding Energy, and one or two other choice publications on bookshelves in leafy suburbs of Tehran like Lavizan. North Korea doesn't need to put Mohsen Fakhrizadeh up at the Koryo to explain levitated pits. We don't know the status of Iran's design program at the time the program was paused in 2003, but they are surely familiar with the major approaches to building missile-deliverable warheads.
That brings us to the purpose of testing. Why test at all?
Much of the U.S. testing program was about exploring, understanding, and validating design approaches. The United States explored all sorts of interesting design approaches, including a few dead ends. The advantage to Iran and North Korea is that the United States has already demonstrated through testing that certain approaches work in principle. Moreover, they probably have access to actual designs that have been validated in tests.
The problem for Iran and North Korea is how to execute those approaches in practice. That presents some interesting engineering challenges, but hardly insurmountable ones. In 2002, the National Academies argued that states like Iran and North Korea could build miniaturized nuclear devices, though these weapons would not be reliable without testing. "A single full yield test," they explained, "would validate both the legitimacy of a blueprint and success in reproducing the object." North Korea learned about the importance of conducting a test the hard way -- with a disappointing 2006 test of what was probably a miniaturized warhead.
One may get a sense of the purpose of testing from the nuclear cooperation between the United States and the United Kingdom. The United States provided significant support to the U.K. nuclear weapons program -- U.K. warheads are "Anglicized" versions of U.S. ones like the W76. There are some differences, of course, between U.S. warheads and their British cousins. Brits will tell you that they have more stringent safety requirements when it comes to high explosives and the like, while Americans will tell you that British manufacturing gave the world the Austin Allegro. Either way, everyone agreed that Britain needed nuclear tests to confirm they had successfully reproduced U.S. warheads.
After 1962, the United Kingdom conducted its nuclear weapons tests at the Nevada Test Site in the United States. Joint tests involved a U.K. device, diagnostic hardware, and British personnel. (The United States provided other equipment, leading to some interesting interface issues, and the test was conducted jointly under a U.S. test director, who had the final say, and a British trial superintendent.) Although the United States provided an off-the-shelf design with an exquisite test pedigree, the United Kingdom still needed to ensure that changes introduced in manufacturing the device did not have adverse effects. Simply attending U.S. tests as a VIP guest might be enjoyable, but it does not substitute for testing one of your own devices.
Now consider the case of Iran. Iran probably has plenty of information on previously tested warheads from China, Pakistan, and the former Soviet Union, as well as North Korea. The real question is whether they can successfully reproduce the object. If Iran wishes to build a miniaturized nuclear warhead, Tehran ought to want to test it. So, unless the device test in North Korean was Iranian manufactured, the question remains.
Now, of course, the Iranians might still learn other things from a North Korean nuclear test. There are also important lessons for digging and instrumenting the test tunnel, particularly with regard to containing the explosions. Someone from Kimia Madan might enjoy hanging out at Chongjin University of Metal Mining. But unless the Iranians manufacture their own device and detonate it in North Korea, the Iranians still do not know if they have a missile-deliverable weapon. Now of course, the Iranians might be overconfident or simply not care that future warheads are unreliable. But then, what's the point of attending the North Korean test other than to be churlish?
Iran's interest in North Korea's nuclear program is something worth watching, but VIP visits to test sites simply do not alter the current policy choices regarding Iran. Fakhrizadeh's nuclear program is still paused, even if he got a ringside seat for the juche bomb. I wrote last year regarding allegations of yet another reorganization of his tawdry little empire: "Oh, this is interesting all right, and worth considering, but hardly the sort of thing one puts in a PowerPoint to U.N. Security Council. Um, moving on."